Home » Chapter 12: Tobacco Products » 12.4 Emissions from tobacco products

12.4 Emissions from tobacco products

Last updated: January 2022
Suggested citation: Winnall, WR. 12.4 Emissions from tobacco products. In Greenhalgh EM, Scollo, MM and Winstanley, MH [editors]. Tobacco in Australia: Facts and issues. Melbourne: Cancer Council Victoria; 2022. Available from https://www.tobaccoinaustralia.org.au/chapter-12-tobacco-products/12-4-emissions-from-tobacco-products

We are grateful to Prof Thomas Eissenberg and Prof David Ashley for their review of this section.

Tobacco product emissions are all the chemicals released from a tobacco product when used as intended. These include the chemicals in smoke from cigarettes and other burned products, the chemicals emitted from heat-not-burn products and those released during chewing, sucking or nasal use of smokeless tobacco products.¹

Studying the chemicals in tobacco products (before use, such as before burning) is important in understanding potential harm, as described in Section 12.3. However, many chemicals are produced anew during the burning of tobacco, some of which have carcinogenic (cancer-causing) or other toxic activity. It is the emissions from tobacco products that people are exposed to that are the cause of its many health effects. Tobacco emissions are the cause of innumerable health problems, as well as addiction to the use of tobacco products and ultimately the deaths of millions of people each year. The chemicals that make up these emissions are therefore the subject of intense research, industry reporting and regulation by governments that are attempting to reduce the adverse individual and societal consequences of tobacco use.

This section discusses the nature of tobacco smoke and other tobacco product emissions; the processes that occur during the burning of tobacco products and the chemicals that constitute tobacco product emissions. With over 7,000 different chemicals found in tobacco emissions, the emphasis of this section is on those chemicals that are of most concern to tobacco product regulation—those that cause disease and addiction and that reduce the intrinsic aversiveness of tobacco smoke or otherwise increase the attractiveness of tobacco products. The methods used for measuring emissions and the use of biomarkers to understand human exposure are described in Section 12.5. The chemicals that are added to tobacco and those that increase the attractiveness of tobacco products are discussed further in Sections 12.6 (Additives and flavours) and 12.7 (Menthol).

The scope of this section includes smoke from factory-made cigarettes, roll-your-own tobacco, as well as cigars, pipes, waterpipes, kreteks and bidis, where information is available. Also discussed are emissions from heated tobacco products and smokeless tobacco. Emissions from e-cigarettes, which do not contain tobacco and are not generally defined as tobacco emissions in Australia, are discussed in Chapter 18.

12.4.1 What is tobacco smoke?

12.4.1.1 Particles and gases in tobacco smoke

Tobacco smoke is an aerosol; it consists of small particles that are floating in a mix of gases.² ^,³ The gases in tobacco smoke are primarily nitrogen, oxygen, carbon dioxide and carbon monoxide. However, low levels of many other gases are also present, including toxic acrolein and hydrogen cyanide. These gases are produced during the heating and burning of tobacco in the processes described below. Some chemicals that exist in the gaseous part of smoke also exist as liquids or solids in the aerosol particles.

The particles in tobacco smoke are tiny droplets made up of liquid or solid chemicals. They float in the gases of smoke because they are too light to fall to the ground. Thousands of different chemicals make up the tobacco smoke aerosol, with one study estimating that cigarette smoke contains at least 7,357 different chemicals (in both the gases and particles).³ These include chemicals that were present in the unburned tobacco product as well as those produced during the burning of the tobacco and other parts of the product, such as the paper. Many of the toxic compounds in smoke are not present in the tobacco before it burns, but are created in chemical reactions when the tobacco is heated or burned.⁴

There are billions of individual aerosol particles in a cubic centimetre of fresh smoke.² In fresh smoke these are spherical in form and range from 0.15 mm to 1.3 mm in diameter.⁴ The nature of these aerosol particles changes rapidly with time. As the smoke ages, the aerosol particles stick together, increasing in size and decreasing in number. One study has shown that within five seconds of ageing of fresh cigarette smoke there is a 10-fold reduction in the number of individual particles, and at least a 3-fold increase in the average diameter, as the particles stick together (coagulation).² The heavier particles may eventually settle on surfaces, becoming thirdhand smoke, as described in Section 4.3.

Most research that has informed the understanding of tobacco smoke has examined smoke from cigarettes. Data from other types of tobacco products are limited. Notably, emissions from a waterpipe also contain a wide range of toxic chemicals, despite being passed through water before inhalation.⁵

The overall chemical composition of tobacco smoke is influenced by many factors, including the type of tobacco product that is burned, the varieties of tobacco plant used, the parts of the plant used, the non-tobacco additives, the type of filter, the degree of filter ventilation (see Section 12.8) and differences in smoking habits.² ^,⁴

12.4.1.2 Mainstream smoke, sidestream smoke and secondhand smoke

Smoke from a cigarette is created under two different conditions: 1) during puffing on a cigarette (mainstream smoke) and 2) during free smouldering from the lit end, that occurs between puffs (sidestream smoke). Puffing by the user pulls air into the lit end, heating the burning contents to about 900°C. The smoker draws the mainstream smoke from the lit end through the filter and into their mouth.³ ^,⁴

Sidestream smoke is generated at lower average temperatures than mainstream smoke, at approximately 400°C. The burning conditions, and the chemical reactions occurring, are different at this temperature. Sidestream smoke, therefore, differs in chemical content to mainstream smoke, with more toxic chemicals produced in sidestream smoke. These chemicals are diluted in the air as the smoke travels away from the burning cigarette.⁴ ^,⁶ More information on the differences between sidestream and mainstream smoke can be found in Section 12.4.2.3.

Secondhand tobacco smoke (also called environmental tobacco smoke) is inhaled by people when breathing air contaminated by tobacco smoke. Secondhand smoke mainly consists of sidestream smoke and exhaled mainstream smoke—that which is breathed out by smokers. It may also contain mainstream smoke that has dissipated from the mouth-end of the tobacco product, rather than being inhaled by the smoker, as well as smoke gases that have diffused across the paper wrapper of a cigarette.³ ^,^6-8 A smoker will likely be puffing on the mainstream smoke from their own tobacco product as well as breathing in secondhand smoke from that product. The health effects of secondhand smoke are addressed in Chapter 4.

The definitions of mainstream, sidestream and secondhand smoke are essentially the same for roll-your-own cigarettes, cigars and pipes as they are for cigarettes.⁴ ^,⁷ ^,⁹ Waterpipes also emit both mainstream and sidestream smoke containing toxic chemicals, however, sidestream smoke mostly comes from the burning coals used in a waterpipe (see Section 12.2.5).⁵

12.4.2 What happens when tobacco products are burned?

When a cigarette is lit and burns, all the tobacco, added chemicals and paper wrap, but not the filter, are converted into smoke and ash. This occurs via numerous overlapping processes, including pyrolysis, pyrosynthesis, combustion, distillation, sublimation and condensation—each explained below.

Combustion and pyrolysis are different types of chemical reactions that occur in different regions of the burning cigarette. These regions are called the combustion zone, on the periphery of the lit end, and the pyrolysis zone immediately interior (see Figure 12.4.1).³ ^,^10-12 Heat is produced in the combustion zone. The majority of the wide range of chemicals in smoke are produced in the pyrolysis zone.⁴ ^,¹¹

Figure 12.4.1 Production of mainstream and sidestream smoke during puffing (upper image), and sidestream smoke during free smouldering (lower image). Adapted from McAdam, et al, 2016 ¹²

Most of the information on tobacco smoke comes from studies of smoking cigarettes, with considerably less known about smoke from other tobacco products.

12.4.2.1 Pyrolysis

Pyrolysis of tobacco products is the decomposition of the chemicals that make up these products, which is driven by heat. Pyrolysis is a chemical reaction. It is the breaking of chemical bonds and formation of new chemical compounds as breakdown products.¹³ Pyrolysis of tobacco products generates gases and vapours — gaseous chemicals that can be cooled into liquids. These vapours become the aerosol particles in the smoke once they cool. Pyrolysis may be considered as a preliminary step for combustion, as described below.²

Pyrolysis of tobacco mostly occurs between 100°C and 500°C.¹⁰ ^,¹⁴ Within these temperatures, most organic materials break down. The majority of volatile products are produced between 200°C and 350°C.¹⁵ Carbohydrates break down into smaller molecules by pyrolysis. These include sugars (a common additive in the manufacture of many tobacco products) and cellulose (that makes up much of the tobacco and the cigarette paper).¹⁵ Pyrolysis releases water, carbon dioxide and carbon monoxide as gases, as well as thousands of different toxic compounds. Volatile products from pyrolysis contribute to the aroma of burning tobacco products.

12.4.2.2 Combustion

Combustion of a tobacco product is the process of burning. It is a chemical reaction between the fuel (tobacco), and the oxygen in the air surrounding a smoker. Combustion, therefore, differs from pyrolysis, which occurs without oxygen as part of the reaction. During combustion, oxygen molecules (O ₂) are split and added to other molecules, producing water (H ₂O) and carbon dioxide (CO ₂).¹¹ Tobacco products burn in a process called incomplete combustion, where many other products are also produced. Some of these, such as carbon monoxide (CO), are highly toxic.³

Combustion of a cigarette or other tobacco product requires ignition to start the reaction— the fire that lights the tobacco prior to smoking.¹⁶ Pyrolysis can be considered as a preliminary step for combustion;¹³ it produces chemicals and gases that fuel combustion, which supplies the heat required for pyrolysis. The heat from combustion can produce enough energy to make the reaction self-sustaining, i.e. the lit tobacco does not go out.¹²

The combustion of a cigarette is referred to as smouldering. It is a slow, flameless form of combustion on the surface of a solid fuel. Smouldering, which produces both sidestream and mainstream smoke,⁸ is an incomplete combustion reaction that emits toxic gases, such as carbon monoxide, and leaves behind ash. Tobacco smouldering requires an influx of oxygen into the cigarette, either through the lit end or through the porous paper.¹²

Combustion in a cigarette occurs at varying temperatures. During the drawing in of air, combustion that produces mainstream smoke occurs at approximately 900°C, when the tip of the cigarette glows red. Sidestream smoke is produced without puffing, where combustion occurs at about 400°C.³

12.4.2.3 Formation of smoke in cigarettes

During the formation of smoke, combustion (burning) and pyrolysis (breakdown of chemical bonds) occur in parallel with processes called pyrosynthesis, distillation, sublimation and condensation, with each explained below.²

After a cigarette is lit, burning occurs in the peripheral combustion zone, producing heat and the chemical products of combustion. In the interior pyrolytic zone, under cooler temperatures, pyrolysis breaks down the chemicals that make up the tobacco. New chemicals may form during chemical reactions between the breakdown products of pyrolysis, a process called pyrosynthesis. Most of the thousands of chemicals that are found in smoke are produced in the pyrolysis zone.

Newly formed volatile chemicals in the pyrolysis zone undergo distillation. Distillation involves the separation of a mix of chemicals under heat. As heating occurs, these chemicals undergo sublimation—turning from a solid into a gas. As the gases are drawn out of the pyrolysis zone they quickly cool and condensate— they change from gas to liquid. This creates an aerosol, whereby tiny particles of liquid or solid chemicals are suspended in the gases that emanate from the cigarette.² ^,³ ^,¹²

Sidestream smoke is produced during free smouldering. This is facilitated by the porous nature of the paper wrapper that allows oxygen to enter the combustion zone. Air enters through the lit end and through the paper, close to the pyrolysis zone, and smoke emanates from the lit end (Figure 12.4.1).¹²

During puffing, air is quickly drawn in from the lit end of the cigarette, producing mainstream smoke. This fast influx of oxygen increases the combustion rate, producing a hotter red glow at the burning end, where the tobacco heats to approximately 900°C.¹² During the puff, the flow of air is disrupted by this heat. Air is then drawn in through the paper, closer to the filter (Figure 12.4.1).¹² Mainstream smoke passes through the cigarette filter before inhalation by the user.

Sidestream smoke contains a different range of chemicals than mainstream smoke. Sidestream smoke is not filtered and is produced under different temperature and oxygen flow conditions to mainstream smoke, as described above. Chemicals with a lower boiling point will be enriched in the sidestream smoke compared to the mainstream smoke, and vice versa for those with a higher boiling point.³ Other factors that affect the chemical makeup of sidestream and mainstream smoke include the tobacco variety and blend, the additives and curing process, the dimensions of the cigarette, the weight of the tobacco rod, the porosity of the paper and the type of filter.³ Compared to mainstream smoke, sidestream smoke contains higher levels of polycyclic aromatic hydrocarbons (PAHs), N-nitrosamines, aromatic amines, carbon monoxide, nicotine, ammonia, pyridine and the gases 1,3-butadiene, acrolein, isoprene, benzene, and toluene.³ More details about these chemicals can be found in Section 12.4.3 .

12.4.2.4 Formation of smoke by other types of combusted tobacco products

Sections 12.3.2.1 to 12.4.2.3 relate to the formation of smoke only in factory-made cigarettes. The formation of smoke from tobacco in other types of tobacco products usually occurs though similar processes but has received less attention from researchers. There is very little information on the formation of smoke from roll-your-own cigarettes, illicit (chop-chop) tobacco, pipes and kreteks. However, each of these products involves setting alight the tobacco and produces toxic chemicals in the smoke that are likely to be products of combustion or pyrolysis.⁸ ^,^17-19

The chemicals in the emissions of tobacco products other than factory-made cigarettes are discussed below. Tobacco products that are non-combusted (i.e. not directly set alight) include waterpipes, where heated coals are placed above the tobacco, heated tobacco products and smokeless tobacco. Emissions from these products are described in Section 12.4.4.

Cigars

Cigars consist of a roll of tobacco wrapped in a tobacco leaf or material with added tobacco. See Sections 12.2.1 for information on the tobacco in cigars and their manufacture, and Section 3.27.3 for the health effects of smoking cigars.

The same processes of combustion, pyrolysis, distillation and pyrosynthesis are reported to occur in cigars in a similar manner to cigarettes. Burning of cigars, however, is highly variable as the products themselves vary considerably, as well as the manner of smoking, which varies between users. Burning cigars produces emissions containing a range of chemicals such as nicotine, carbon monoxide, aldehydes, tobacco-specific nitrosamines, benzo[a]pyrene and polycyclic aromatic hydrocarbons.^20-23 See Section 12.4.3 for more details about these chemicals.

Measurements of the temperatures inside the burning cone of a cigar vary. One study has estimated a maximal temperature between 380°C and 630°C,¹⁰ while another has estimated between 910°C and 1,290°C.²⁴

An important characteristic of cigars, particularly large ones, is the very long periods of time that they can require to be completely smoked. Burning one product for a long time may change the nature of the product and its smoke as the burning progresses. For instance, the temperature of mainstream smoke from the first puffs of a cigar is low, between 25°C and 30°C. But as the cigar is smoked, over a long period of time, the last puffs can be as high as 75°C.²⁴ The pH of smoke from cigars increases from an acidic 6.5 in the first puffs to a basic (non-acidic) 7.4 to 8.0 in the last puffs. This affects the amount of free-base nicotine in mainstream smoke, as described in Section 12.4.3.1.

The amounts of certain gases in cigar smoke, such as the ratio of oxygen to carbon monoxide, suggest that combustion is less complete in cigar smoke than it is in cigarette smoke. This may be due to the relatively low porosity of the cigar wrapper and binder leaves, and the large size of the interior of the cigar, restricting oxygen access, which is required for combustion.²⁴ Incomplete combustion means that the reactions produce products other than oxygen, water and carbon dioxide, many of which are toxic.

Sidestream smoke emanates from the burning cone of a cigar in a similar manner to that from cigarettes. The pH of cigar sidestream smoke is higher than that of cigarette sidestream smoke due to higher ammonia levels in the cigar smoke. This increases the free nicotine levels in the sidestream smoke from cigars.²⁴

How often a cigar is puffed may affect its burn rate. Puffing with insufficient frequency can allow the cigar to go out and need relighting. According to cigar smokers, frequently relighting a cigar could introduce unpleasant flavours and bitterness.²⁵

Bidis

Bidis are thin, hand-rolled cigarettes popular in Asia. Bidis are rolled in a dried tendu or temburni leaf (from plants native to Asia). These leaves are non-porous and make-up about 60% of the cigarette weight. See Section 12.2.7 for more information about bidis, and Section 3.27.7 for information on the health effects of smoking bidis.

There has been little research on the formation of smoke in bidis. However, it is believed that the non-porous leaf wrapper does not burn as readily as the paper in cigarettes. This requires more frequent and deeper puffs by the user to keep the bidi alight.²⁶ Increasing this puff frequency is predicted to increase the nicotine and tar inhalation from bidis.²⁷ Pyrolysis and combustion occur in bidis, similar to cigarettes, and a range of toxic products of these processes have been measured.²⁸

12.4.3 Chemicals that are emitted from combusted tobacco products

Over 7,000 different chemicals can be detected in the emissions of combusted tobacco products such as cigarettes. Many new chemicals are formed when components of cigarettes (the tobacco, the chemicals added to it and the paper wrapper) and other tobacco products undergo chemical reactions during burning. Some chemicals are unchanged, moving from the tobacco product into the smoke, such as nicotine and toxic metals. The chemicals in emissions that contribute to the toxicity, addictiveness or attractiveness of tobacco products are of greatest concern for tobacco product regulation.²⁹

TobLabNet (Tobacco Laboratory Network, under the World Health Organization) is a global network of government, academic, and independent laboratories that collaborate to strengthen capacity for the testing of tobacco product contents and emissions. TobLabNet has produced a list of 39 chemicals in cigarette smoke that should be measured by local administrations to monitor tobacco products.³⁰ In the US, the FDA has also produced a list of 93 harmful and potentially harmful constituents in tobacco products and smoke.³¹ These chemicals are summarised in Table 12.4.1. The focus of these lists and this section is on those chemicals that contribute most to the toxicity and addictiveness of tobacco products. Chemicals that contribute to the attractiveness of tobacco products are discussed in Sections 12.6 (Additives) and 12.7 (Menthol).

12.4.3.1 Nicotine and other alkaloids

Nicotine is the main addictive chemical found in tobacco smoke, coming from the plant itself. Nicotine is an alkaloid—a naturally occurring basic (i.e. non-acidic) compound containing nitrogen atoms.³² Similar alkaloids from tobacco plants are also found in tobacco emissions, such as anatabine, anabasine, and nornicotine. However, nicotine is by far the most common alkaloid found in smoke.³ ^,³³

Nicotine carried by tobacco smoke moves rapidly from the lungs into the bloodstream and from there into most regions of the body. The addictive effects of nicotine come from its binding to cellular receptors in the brain, triggering the dopaminergic reward system. Nicotine reaches the brain within 10 to 20 seconds of inhaling smoke.³⁴ See Chapter 6 for more details about nicotine and addiction.

Table 12.4.1 Toxic and addictive chemicals in tobacco as listed by the US Food and Drug Administration and by the World Health Organization

Chemical type	Chemical name	Toxicity/ Addictiveness	FDA list	WHO list	WHO: proposed for mandatory reduction	Being assessed for inclusion on the WHO list	WHO Standard Operation Procedure available	WHO recommended method from literature
Alkaloids	Anabasine	AD	X
	Nicotine	RDT, AD	X	X			X (content and emissions)
	Nornicotine	AD	X
Aldehydes	Acetaldehyde	CA, RT, AD	X	X	X		X (emissions)
	Acrolein	RT, CT	X	X	X		X (emissions)
	Butyraldehyde			X				X (emissions)
	Crotonaldehyde	CA	X	X				X (emissions)
	Formaldehyde	CA, RT	X	X	X		X (emissions)
	Propionaldehyde	RT, CT	X	X				X (emissions)
Aromatic amines	3-Aminobiphenyl			X				X (emissions)
	4-Aminobiphenyl	CA	X	X				X (emissions)
	1-Aminonaphthalene	CA	X	X				X (emissions)
	2-Aminonaphthalene	CA	X	X				X (emissions)
	o-Anisidine	CA	X
	2,6-Dimethylaniline	CA	X
	o-Toluidine	CA	X
Hydrocarbons	Benzene	CA, CT, RDT	X	X	X		X (emissions)
	1,3-Butadiene	CA, RT, RDT	X	X	X		X (emissions)
	Ethylbenzene	CA	X		X
	Isoprene	CA	X	X				X (emissions)
	Toluene	RT, RDT	X	X				X (emissions)
Heterocyclic aromatic amines	2-Amino-9H-pyrido[2,3-b]indole ( A-α-C)	CA	X			X
	2-Amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole ( Glu-P-1)	CA	X			X
	2-Aminodipyrido[1,2-a:3',2'-d]imidazole ( Glu-P-2)	CA	X			X
	2-Amino-3-methylimidazo[4,5-f]quinoline ( IQ)	CA	X			X
	2-Amino-3-methyl-9H-pyrido[2,3-b]indole ( MeA-α-C)	CA	X			X
	2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine ( PhIP)	CA	X
	3-Amino-1,4-dimethyl-5H-pyrido[4,3-b]indole ( Trp-P-1)	CA	X			X
	1-Methyl-3-amino-5H-pyrido[4,3-b]indole ( Trp-P-2)	CA	X			X
Polycyclic aromatic hydrocarbons	Benzo[a]pyrene	CA	X	X	X		X (emissions)
	Benz[a]anthracene	CA, CT	X			X
	Benz[j]aceanthrylene	CA	X
	Benzo[b]fluoranthene	CA, CT	X
	Benzo[k]fluoranthene	CA, CT	X
	Benzo[c]phenanthrene	CA	X
	Chrysene	CA, CT	X			X
	Cyclopenta[c,d]pyrene	CA	X
	Dibenz[a,h]anthracene	CA	X			X
	Dibenzo[a,e]pyrene	CA	X			X
	Dibenzo[a,h]pyrene	CA	X			X
	Dibenzo[a,i]pyrene	CA	X			X
	Dibenzo[a,l]pyrene	CA	X			X
	Indeno[1,2,3-cd]pyrene	CA	X			X
	5-Methylchrysene	CA	X			X
	Naphthalene	CA, RT	X			X
N-Nitrosamines	4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone ( NNK)	CA	X	X	X		X (emissions)
	N′-nitrosonornicotine ( NNN)	CA	X	X	X		X (emissions)
	N′-nitrosoanabasine ( NAB)			X
	N′-nitrosoanatabine ( NAT)			X
	N-Nitrosodiethanolamine ( NDELA)	CA	X			X
	N-Nitrosodiethylamine ( NDEA)	CA	X			X
	N-Nitrosodimethylamine ( NDMA)	CA	X			X
	N-Nitrosomethylethylamine ( NMEA)	CA	X			X
	N-Nitrosomorpholine ( NMOR)	CA	X
	N-Nitrosopiperidine ( NPIP)	CA	X			X
	N-Nitrosopyrrolidine ( NPYR)	CA	X			X
	N-Nitrososarcosine ( NSAR)	CA	X
Phenols	Catechol	CA	X	X				X (emissions)
	m- and p-cresol	CA, RT	X	X				X (emissions)
	o-cresol	CA, RT	X	X				X (emissions)
	Hydroquinone	CA		X				X (emissions)
	Phenol	RT, CT	X	X				X (emissions)
	Resorcinol			X				X (emissions)
Other organic compounds	Acetamide	CA	X			X
	Acetone	RT	X	X				X (emissions)
	Acrylamide	CA	X			X
	Acrylonitrile	CA, RT	X	X				X (emissions)
	Aflatoxin B1	CA	X
	Benzo[b]furan	CA	X
	Caffeic acid	CA	X
	Chlorinated dioxins/furans	CA, RDT	X
	Coumarin	Banned in food	X
	Ethyl carbamate (urethane)	CA, RDT	X			X
	Ethylene oxide	CA, RT, RDT	X			X
	Furan	CA	X
	Methyl ethyl ketone	RT	X			X
	Nitrobenzene	CA, RT, RDT	X
	Nitromethane	CA	X
	Propylene oxide	CA, RT	X			X
	Pyridine			X				X (emissions)
	Quinoline	CA	X	X				X (emissions)
	Styrene	CA	X			X
	Vinyl acetate	CA, RT	X			X
	Vinyl chloride	CA	X			X
Inorganics	Ammonia	RT	X	X			X (content)
	Carbon monoxide	RDT	X	X			X (emissions)
	Hydrogen cyanide	RT, CT	X	X				X (emissions)
	Nitric oxides	Toxic non-cancerous		X
	Hydrazine	CA, RT	X			X
	2-Nitropropane	CA	X
	Selenium	RT	X			X
Metals, metalloids and radionuclides	Arsenic	CA, CT, RDT	X	X				X (content and emissions)
	Beryllium	CA	X			X
	Cadmium	CA, RT, RDT	X	X				X (content and emissions)
	Chromium	CA, RT, RDT	X			X
	Cobalt	CA, CT	X			X
	Lead	CA, CT, RDT	X	X				X (content and emissions)
	Mercury	CA, RDT	X	X				X (content and emissions)
	Nickel	CA, RT	X			X
	Polonium-210	CA	X			X
	Uranium-235	CA, RT	X
	Uranium-238	CA, RT	X
Humectants (not on WHO or FDA list)	Glycerol						X (content)
	Propylene glycol						X (content)
	Triethylene glycol						X (content)

Sources: WHO list from the Expanded priority list for monitoring and regulation, COP/6/14, Report on the Scientific Basis of Tobacco Product Regulation: Seventh Report of a WHO Study Group.³⁰ FDA list from April 2012: Harmful and Potentially Harmful Constituents in Tobacco Products and Tobacco Smoke: Established List.³¹

Notes: Toxicity/addictiveness abbreviations: Carcinogen (CA); Respiratory Toxicant (RT); Cardiovascular Toxicant (CT); Reproductive or Developmental Toxicant (RDT); Addictive (AD), according to the FDA list as a default, otherwise according to the WHO list.

Nicotine exists in tobacco smoke in both the aerosol particles and as a gas.³⁵ The nicotine molecule has two nitrogen atoms to which protons (positively charged sub-atomic particles) can be added. Nicotine can therefore exist in three forms as defined by protonation status: unprotonated, monoprotonated and diprotonated. Unprotonated nicotine is often referred to as ‘ free-base nicotine’, referring to the molecule carrying no charge. Free-base nicotine is the only form that can be readily converted into a gas by vapourisation.³⁵ The pH (a measure of the acid-base balance) of smoke affects the proportion of nicotine that is free-base. The more acidic the smoke, the less nicotine is in its free form. One study of a range of cigarettes describes the proportion of free-base nicotine in the particles as ranging from 1% to 36%, with the higher particle pH resulting in a higher proportion of free-base nicotine.³⁵

Free-base nicotine is proposed to be more readily deposited into the lungs than protonated nicotine.³ Subsequently, it’s believed that decreasing the acidity of smoke increases the amount and/or the rate of nicotine entry into the body through the lungs. However, there is little biological evidence to support this hypothesis. In fact, some studies comparing free-base to protonated nicotine have shown no increase in blood levels of nicotine when a higher proportion of free-base nicotine was consumed.³⁶ ^,³⁷ However, studies of smokeless tobacco of differing pH showed that nicotine delivery to the blood was higher and faster for products with a higher pH (higher free-base nicotine).³⁸ ^,³⁹ Nicotine uptake from smokeless tobacco is absorbed through the mucosa of the mouth (buccal absorption), rather than the lungs. Increasing the pH of the smokeless tobacco has been shown to increase blood nicotine levels and nicotine-induced increases in heart rate and blood pressure.³⁸

Chemicals such as ammonia and ammonium compounds are added to tobacco products to increase the pH,⁴⁰ which may be increasing the delivery of nicotine to the blood, although this is not supported by biological studies.³⁷ Addition of ammonia increases smokers’ satisfaction,⁴⁰ perhaps by enhancing flavours,⁴¹ but the mechanisms by which it does this are not well understood. More information about the role of ammonia in tobacco can be found in Section 12.6.2.1.

Other (non-nicotine) alkaloids in tobacco smoke may also contribute to the addictiveness of tobacco smoke.³ Both nicotine and some non-nicotine alkaloids serve as precursors for the production of carcinogenic N-nitrosamines produced in tobacco and in its emissions after burning.³ ^,³³

12.4.3.2 Aldehydes

An aldehyde is a carbon-based (organic) molecule that has a carbon atom that is double-bonded to an oxygen and single-bonded to a hydrogen atom.⁴² Aldehydes of concern in tobacco smoke include acetaldehyde, acrolein (acraldehyde), formaldehyde, crotonaldehyde, propionaldehyde and butyraldehyde.³⁰ ^,³¹ Acetaldehyde, acrolein and formaldehyde are priority toxicants for regulation recommended by the WHO and standard operating procedures for their measurement have been established (see Section 12.5.3).

Aldehydes are often toxic to the respiratory system. Some are also toxic to the cardiovascular system and some damage DNA in tissues such as lung cells.³⁰ ^,³¹ Aldehydes from tobacco smoke are a likely cause of cancer in the lungs and nasal cavity.³

The smallest and simplest aldehyde molecule is formaldehyde; a compound with both cardio-toxic and carcinogenic activity. Formaldehyde is a preservative used in embalming solutions and for preserving dead tissue in laboratories.⁴³ Formaldehyde is toxic to the cardiovascular system and has a genotoxic (mutating DNA to cause cancer) effect in lung cells.³ ^,³⁰ ^,⁴⁴ Similarly, acetaldehyde has both genotoxic activity in lung cells and is a cardiovascular toxicant.³⁰ ^,⁴⁴ Acetaldehyde may also contribute to the addictiveness of tobacco by interacting with signalling processes in the brain that affect motivation, reward and stress-related responses.⁴⁵ However, more studies are needed to confirm this.

Acrolein is a volatile and highly toxic aldehyde.⁴⁶ Aside from tobacco smoke, people are exposed to acrolein due to the cooking of fatty foods and burning of fossil fuels. But acrolein from tobacco smoke is responsible for much of human exposure. In tobacco smoke, one source of acrolein is the breakdown of glycerol during combustion. Glycerol is often added to tobacco to maintain moisture, but even without this additive, tobacco smoke still contains significant levels of acrolein.⁴⁶ Compared to non-smokers, smokers have four times the level of biomarkers in their urine that indicate acrolein exposure, and this is reduced by 78% after smoking cessation.⁴⁷ Acrolein is an irritant to the human respiratory system, toxic to lung cilia and is likely to be a carcinogen in the lungs.³ Acrolein is toxic to the cardiovascular system and causes oxidative stress in the heart and increases cardiovascular disease risk.³ ^,³⁰ Animal experiments indicate that acrolein may be involved in the formation of type 2 diabetes and increase the risk of bacterial infections.³⁰

12.4.3.3 Aromatic amines

An aromatic amine is a carbon-based compound consisting of an aromatic ring (of six carbon atoms) attached to an amine (containing a nitrogen atom). Human exposure to aromatic amines has long been associated with bladder cancers and aromatic amines are considered a likely cause of bladder cancer in smokers.³ ^,⁸ ^,⁴⁸ ^,⁴⁹

Of the aromatic amines in tobacco emissions listed in Table 12.4.1, 4-aminobiphenyl is one of the most concerning for human health. One study has estimated that the average smoker consumes 36 µg of 4-aminobiphenyl, 1,309 µg of o-toluidine and 98.2 µg of β-naphthylamine each year. The effects of these three aromatic amines combined cause bladder cancer in 40 out of 100,000 smokers each year.⁴⁸

12.4.3.4 Hydrocarbons

Hydrocarbons are carbon-based compounds containing only hydrogen and carbon atoms. Hydrocarbons occur often in nature, particularly in trees and other plants, and make up much of fossil fuels.⁵⁰ There is a wide range of hydrocarbons, some of which are toxic, damaging various human organs.

Benzene is a long-established carcinogen in humans and the benzene from tobacco smoke is a likely cause of lung cancer and acute myeloid leukaemia in smokers.³ ^,⁵¹ ^,⁵² Benzene may also damage the cardiovascular system³⁰ and reproductive system.⁵³

1,3-butadiene is of high concern to tobacco regulators and is on the WHO list of chemicals in tobacco emissions proposed for mandatory regulation. 1,3-butadiene from tobacco smoke is a carcinogen that is a likely cause of lung cancer. It is also toxic to the respiratory system. A study from 2003 used data that included measures of individual chemicals in cigarette smoke to compare the potency of 158 toxicants. The contribution of 1,3-butadiene (BDE) to the cancer risk index was over twice that of acrylonitrile, the next highest contributing carcinogen.⁵⁴

Isoprene, present in the gases of tobacco emissions, causes cancer in mice and rats,⁵⁵ ^,⁵⁶ and may cause lung cancer in smokers.³ Toluene is toxic to the respiratory system, central nervous system and reproductive system.³⁰

12.4.3.5 Heterocyclic aromatic amines

Heterocyclic aromatic amines are carbon-based compounds containing more than one ring of carbon atoms with one ring containing a nitrogen atom. They are the carcinogenic chemicals that are produced during pyrolysis of tobacco (but are not present in unburned tobacco) and when cooking muscle meats such as beef and pork.³ ^,⁵⁷ ^,⁵⁸ Carcinogenesis (the formation of a cancer) currently is the only suspected toxicity of heterocyclic aromatic amines from tobacco.

Many heterocyclic aromatic amines show strong gene mutagenic activity in laboratory-grown cells.^59-61 All of the heterocyclic aromatic amines listed in Table 12.4.1 show carcinogenic activity in rodent experiments. Target organs in the rat include the liver, bladder, intestines and blood vessels.⁶⁰ ^,⁶¹ Once heterocyclic aromatic amines are broken down upon entering the body, their metabolites can form DNA adducts—they bind to regions of DNA where they can potentially cause mutations.

12.4.3.6 Polycyclic aromatic hydrocarbons (PAH)

A polycyclic aromatic hydrocarbon (PAH) is a carbon-based molecule that contains only hydrogen and carbon atoms and has multiple aromatic rings (rings of six carbon atoms). PAHs are found in fossil fuels and produced when these fuels, wood, garbage or tobacco are burned.⁶² Tobacco is a significant source of human exposure to PAHs.⁶³ PAHs are produced by pyrolysis in the burning cigarette.³ Over 500 PAHs have been detected in tobacco smoke.⁶⁴ They are mostly found in the particulate phase of tobacco smoke, but a small proportion are also detected as gases.³ One study has found that 1 to 1.6 μg of total PAHs are detected in the smoke from the average commercial cigarette under standard FTC machine-smoking conditions (see Section 12.5.1 for details about machine smoking conditions).⁶⁵ There appears to be an inverse relationship between the levels of PAHs and tobacco-specific nitrosamines (TSNAs). This may be due to the use of different varieties of tobacco, which yield different amounts of these chemicals and also be related to the amount of nitrogen present in the tobacco.³ Both TSNAs and PAHs are highly carcinogenic, so choosing tobacco with lower PAH levels will not necessarily mean a lower level of carcinogens overall.

The FDA includes 16 PAHs in its list of 93 harmful and potentially harmful constituents in tobacco. The WHO’s list only includes benzo[a]pyrene ( Table 12.4.1), which is also proposed for mandatory regulation. Benzo[a]pyrene is classified as a group 1 human carcinogen by the IARC (indicating that the evidence is strong). Ten of the PAHs on the FDA list are being considered by the WHO for addition to its list of regulated chemicals from tobacco smoke.³⁰ Each of the 16 PAHs on the FDA’s list is suspected to be a carcinogen and may be causing cancer in smokers. For most, there are animal studies that show carcinogenic effects.⁶³ PAHs are likely causes of cancer in the lungs, larynx, mouth and cervix of smokers.³ Some are also toxic to the cardiovascular system.

12.4.3.7 N-nitrosamines

N-nitrosamines are carbon-based compounds that contain two or more nitrogen atoms, and a nitrogen atom double-bonded to an oxygen atom. Most N-nitrosamines are known or suspected carcinogens. Humans are exposed to low levels of N-nitrosamines from many different sources, including foods, chlorinated water and personal care products. Tobacco emissions are a major source of human exposure to N-nitrosamines.⁶⁶

Tobacco smoke contains volatile and non-volatile N-nitrosamines. A subset of N-nitrosamines are found only in tobacco and emissions from tobacco products— called tobacco-specific nitrosamines (TSNAs).³

N-Nitrosamines are uncommon in fresh tobacco. They are produced in chemical reactions as modifications of alkaloids (such as nicotine) after the plant is harvested or during burning. TSNAs are mainly produced during the processing, curing and storage of tobacco. N-nitrosamine levels in tobacco emissions are influenced by the amount of nitrogen fertiliser used for plant growth, the curing method used, and the variety of tobacco used, with Burley tobacco containing the highest levels (see Section 12.3.3). Growing and curing conditions can therefore be manipulated to reduce the levels of TNSAs in tobacco emissions.³ It is not yet known whether these changes could lead to better health outcomes in smokers.

The IARC has classified two TNSAs as group 1 carcinogens, meaning that there is sufficient evidence to conclude they are carcinogenic to humans.⁶⁶ ^,⁶⁷ These are N-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Both are listed by the WHO as proposed for mandatory regulation (Table 12.4.1). Two tobacco N-nitrosamines are classified into group 2A (probably carcinogenic to humans): NDMA and NDEA. Six others are classified as group 2B (possibly carcinogenic to humans): NDMA, NMEA, NMOR, NPIP and NPYR (see Table 12.4.1 for chemical names).⁶⁶

NNK is a likely cause of cancer in the lungs, mouth, nose, liver, pancreas and cervix of smokers. NNN is a likely cause of cancer in the nose, mouth and oesophagus of smokers, and other N-nitrosamines are likely causes of nasal, oesophagus and liver cancers.³ Both NNN and NNK can form DNA adducts—where the chemical binds to DNA and potentially causes DNA sequence mutations. Higher levels of NNK or NNN-DNA adducts have been found in the oral cells of smokers with head-and-neck squamous cell carcinoma, indicating the importance of these DNA adducts.⁶⁸

12.4.3.8 Phenols

Phenols are carbon-based compounds containing an aromatic ring of six carbon atoms attached to one or more hydroxyl (-OH, oxygen bound to a hydrogen atom). Phenols are usually acidic, and many are highly toxic. The simplest form of these molecules is called phenol, whereas the term ‘phenols’ refers to a class of similar molecules, of which seven are listed by the FDA as harmful or potentially harmful constituents of tobacco.³¹ Phenols detected in tobacco emissions, such as phenol, catechol, m- and p-cresol are likely to have formed during the pyrolysis of common tobacco biomass components such as lignin, tyrosine and ethyl cellulose.⁶⁹ Levels of some phenols, such as resorcinol and hydroquinone, may be increased in smoke with a higher pH (less acidic).³

There is currently insufficient data to support a role for these phenols as causes of specific cancers in smokers, but their relatively high levels in tobacco smoke and their known toxicities from laboratory and animal studies are cause for concern. Phenols found in tobacco emissions that may have carcinogenic activity are catechol, cresols and hydroquinone. Cresols and phenol are also respiratory toxicants and phenol may harm the cardiovascular system.³¹ Catechol is a co-carcinogen (increases the effects of other carcinogens) that is present at relatively high levels in cigarette smoke.⁸ Hydroquinone causes? DNA mutations in animal and laboratory experiments and may form DNA adducts.⁷⁰ ^,⁷¹

12.4.3.9 Other carbon-based (organic) compounds

There are many other carbon-based chemicals found in tobacco smoke that may be damaging the health of smokers. The WHO includes acetone, acrylonitrile, pyridine and quinoline in its list of chemicals to be measured for tobacco product regulation (Table 12.4.1). The longer FDA list adds 17 more ‘other organic chemicals’ to this in their list of harmful and potentially harmful constituents of tobacco. These include furan, aflatoxin B1 and styrene.

Most of the chemicals listed in Table 12.4.1 are carcinogenic and/or respiratory toxins. Chlorinated dioxins/furans, ethyl carbamate, ethylene oxide and nitrobenzene are also classified as reproductive or developmental toxins.³¹ Ethylene oxide and ethylene carbamate are carcinogens that are likely to be a cause of lung cancer in smokers.³ Furan in cigarette smoke is a likely cause of liver cancer in smokers.³ Caffeic acid can cause kidney cancer in animal experiments, although humans are exposed to considerable levels of caffeic acid through their diet as well as smoking.⁷² Coumarin (1,2-benzopyrone) has toxic activity on the liver and kidneys and was banned as a food additive in the United States in 1954.⁷³ Acrylonitrile is a compound that is highly toxic at low doses. It does not occur naturally but is made by industrial processes. The FDA classifies acrylonitrile as a carcinogen and respiratory toxicant. A study from 2003 used data that included measures of individual chemicals in cigarette smoke to compare the potency of 158 toxicants. The contribution of acrylonitrile to the cancer risk index was the second-highest of all tested.⁵⁴ Aflatoxin B1 is a toxin produced by fungal infections of tobacco plants. It is classified as a group 1 carcinogen by the IARC.⁷⁴ Acrylamide is found in tobacco smoke and also formed when carbohydrate foods are cooked at high temperatures. Inhalation of acrylamide can irritate the respiratory system and long-term exposure may lead to nerve damage. There is data from animal experiments demonstrating the formation of cancer, nerve damage and reproductive toxicity with acrylamide exposure.⁷⁵

12.4.3.9 Inorganics

The WHO lists the inorganic gases ammonia, carbon monoxide, hydrogen cyanide and nitric oxides to be measured in tobacco smoke ( Table 12.4.1). The longer FDA list also includes hydrazine, 2-nitropropane and selenium. Of these inorganic substances, ammonia, hydrogen cyanide, hydrazine and selenium may be toxic to the respiratory tract. Hydrazine and 2-nitropropane have carcinogenic activity.³¹

Ammonium compounds are often added to tobacco to increase the pH of smoke and may be making smoking more addictive (see Section 12.6.2 for more information). Hydrogen cyanide is a toxic gas that affects the cardiovascular, respiratory and central nervous systems. Chronic exposure of smokers to hydrogen cyanide in smoke may lead to impaired wound healing, fertility issues and other conditions.³⁰

The carbon monoxide in tobacco smoke is of particular concern due to its many effects on human health. Carbon monoxide is a cardiovascular toxicant, which binds to haemoglobin in the blood, displacing oxygen. This reduces oxygen delivery through the blood. Carbon monoxide also causes damage to blood vessel walls, and promotes the progression of atherosclerosis (hardening of the arteries) and other cardiovascular diseases.³⁰

12.4.3.10 Metals, metalloids and radionuclides

Toxic metals and metalloids can be detected in the particle phase of tobacco smoke. A metalloid is an element that has properties of both metals and non-metals. The WHO recommends testing of the metalloid arsenic and metals cadmium, lead and mercury in tobacco emissions ( Table 12.4.1).³⁰ The FDA adds beryllium, chromium, cobalt and nickel to this list as harmful or potentially harmful substances, as well as radioactive polonium-210, uranium-235 and uranium-238.³¹

All of these elements are considered carcinogenic based on animal and laboratory experiments.³¹ Arsenic, beryllium, cadmium and polonium-210 are IARC group 1 carcinogens, meaning there is sufficient evidence that they are carcinogenic to humans.³ ^,⁸ ^,⁷² ^,⁷⁶ Lead, nickel and cobalt are group 2B carcinogens (possibly carcinogenic to humans).⁷⁶ Polonium is a potent carcinogen and has been detected in the lungs of smokers and former smokers.⁷⁷ Whether polonium in the lungs of smokers has impacts on human health is currently unknown due to insufficient data.⁷⁸

Lead and cadmium are of particular concern in tobacco smoke as they are at relatively higher levels than the other metals.³ Cadmium is a respiratory toxicant, promoting oxidative injury and emphysema. Cadmium, lead and mercury have toxic effects on reproduction and development. Cadmium exposure in pregnant women may also affect the development of their children.³

12.4.3.11 Additives and flavours

Tobacco manufacturers add numerous chemicals to the tobacco to control moisture, add flavour, manipulate free nicotine levels and other reasons. These additives often contribute to the toxicity, addictiveness and attractiveness of tobacco products. See Section 12.6 for more information.

12.4.4 Emission from tobacco that is not combusted (set alight)

Tobacco that is not directly lit on fire does not undergo combustion— a chemical reaction described in Section 12.4.2.2. Waterpipe tobacco and heated tobacco products are heated but not directly set alight, therefore do not undergo combustion. However, waterpipe emissions are a mix of emissions from the heated tobacco and the burning coals that heat them, which are undergoing combustion. Therefore the emissions from waterpipe contain the chemical products of combustion of the coals as well as emissions from the heated tobacco.

Heated tobacco products, not to be confused with e-cigarettes/vapes, produce emissions from pyrolysis and other chemical reactions that occur at lower temperatures. However, pyrolysis is responsible for the production of many of the toxic chemicals in tobacco emissions (see Section 12.4.2.1). Currently, there is debate as to whether the emissions from heated tobacco products should be referred to as smoke or another term.

Products collectively known as ‘smokeless tobacco’ include those used orally and nasally. The emissions from these products are all the chemicals that move out of tobacco and have the potential to be absorbed into the cells of the body. These products are not heated at all, but the emissions may be modified by saliva or other bodily excretions, before uptake into the body.

12.4.4.1 Emissions from waterpipes

A typical waterpipe consists of numerous elements that are necessary for the heating of tobacco using burning coals. The tobacco is placed in the ‘head’ and often covered with perforated foil or a screen. Heated coals are placed over the top of the tobacco and a cover (wind guard) with air holes may sit over this, which keeps the burn rate and temperature low by restricting the flow of air. With suction from the mouthpiece through the hose, emissions are pulled down through the ‘body’, moving through the water in the bowl before inhalation through the hose ( Figure 12.4.2).⁷⁹

Figure 12.4.2 Diagram of a hookah waterpipe.

Source: Smackware, Public domain, via Wikimedia Commons.

The water in a waterpipe cools the emissions and may reduce the concentration of some chemicals. Experiments comparing waterpipes used with and without the water have shown that the water reduces the concentration of nicotine in the emissions by half, but does not reduce carbon monoxide and many other toxic chemicals.⁸⁰

The tobacco in a typical waterpipe is not directly set on fire, presumably because it is so moist that it will not burn in a self-sustaining manner. However, heated tobacco produces emissions are inhaled by waterpipe users.⁸¹ The burning coals heat the tobacco unevenly, ranging from about 450°C near the heat source to 50°C furthest from it.⁸² These temperatures are generally lower than would be found in cigarettes or tobacco pipes (non-water), which may change the range of chemicals produced in waterpipe emissions. However, the temperature is sufficient for pyrolysis to occur in parts of the tobacco. Chemicals produced from the burning charcoal, which is undergoing combustion, are also present in the mainstream emissions coming from a waterpipe.⁸²

Emissions from waterpipes used with tobacco include aldehydes, carbon monoxide, nicotine, tobacco-specific nitrosamines, polycyclic aromatic hydrocarbons and lead.⁸³ ^,⁸⁴ See Section 12.4.3 for more details about these chemicals. The charcoal is the source of most of the carbon monoxide and polycyclic aromatic hydrocarbons (such as benzo[a]pyrene) found in waterpipe smoke.⁸⁵ ^,⁸⁶

Waterpipe tobacco can be heated by an electrical heating element as an alternative to the heated charcoal, which may reduce the harmful emissions from the charcoal. These electrical elements may reduce some toxicants, such as carbon monoxide and polycyclic aromatic hydrocarbons. However, nicotine and many other toxic chemicals remain at similar levels in the emissions from electrically-heated waterpipe tobacco compared to charcoal-heated.⁸⁶

See Section 12.2.5 for more information about the tobacco used in waterpipes and Section 3.27.5 for the health effects of smoking tobacco in a waterpipe.

12.4.4.2 Emissions from heated tobacco (‘heat-not-burn’) products

Rather than setting alight the tobacco, heated tobacco products apply heat to tobacco to produce emissions (described further in Chapter 18C). The heating system uses an external heat source that liberates nicotine and other chemicals from specially designed cigarette-like sticks containing reconstituted tobacco sheet or loose leaf tobacco (see Section 12.2.8).^87-89 Features of some heated tobacco products include lower- and higher-temperature variants, capacity to heat both tobacco and liquid, use of a metallic mesh punctured with tiny holes to heat a sealed liquid cap, and features that allow users to customise the temperature and manage the aerosol and flavour output.⁸⁷

Heated tobacco products should not be confused with e-cigarettes (vapes), which do not contain tobacco. Most e-cigarettes heat a liquid to produce an aerosol, which often includes nicotine that is derived from tobacco plants (see Section 18B.1).

Heated tobacco products generally heat tobacco to lower than 600°C using a battery-powered heating system. Tobacco companies often claim that their devices do not heat above 350°C.⁸⁹ This produces an aerosol that, like the smoke aerosols from combusted tobacco, contains particles that float in gases. The tobacco industry maintains that the aerosol produced by heated tobacco products is not smoke and sometimes erroneously refers to it as a “vapour”.⁹⁰ ^,⁹¹ These aerosol emissions are not accurately described as vapours, which are defined as chemicals in their gaseous state (at a temperature in which they can be liquified with added pressure). In fact, the aerosols emitted by cigarettes and other combusted tobacco products share much in common with the aerosols produced by heated tobacco products.

While some chemicals in smoke from combusted products are produced in the combustion reactions, many more are produced by pyrolysis. Pyrolysis is a chemical reaction that is best described as decomposition driven by heat (see Section 12.4.2.1). Pyrolysis of tobacco mostly occurs between 100°C and 500°C,¹⁰ ^,¹⁴ with the majority of volatile products produced between 200°C and 350°C.¹⁵ Many of the toxic chemicals produced during the use of conventional burned tobacco products are made by pyrolysis. There is evidence that pyrolysis is occurring in heated tobacco products, in that charring is consistently observed in the tobacco plugs after use⁹² and polycyclic aromatic hydrocarbons are detected, which are produced when tobacco is pyrolysed.³ It is therefore quite feasible that heated tobacco products would produce emissions containing similar toxic products to those produced by pyrolysis in combusted tobacco products.

The emissions from commonly used heated tobacco products were shown to produce similar levels of addictive nicotine as conventional cigarettes.⁸⁸ ^,⁸⁹ ^,^93-95 Harmful reactive oxygen compounds (such as hydrogen peroxide), and a range of carbonyl compounds (including toxic aldehydes such as acrolein,⁸⁹ formaldehyde, acetaldehyde and butyraldehyde),⁹³ ^,⁹⁶ pyrene, benzo[a]pyrene and other carcinogenic polycyclic aromatic hydrocarbons⁸⁹ ^,⁹⁴ have been found in emissions from heated tobacco products. However, they are usually at lower levels than in the smoke from conventional cigarettes. Carcinogenic tobacco-specific nitrosamines were found in the emissions of heated tobacco products⁹⁶ at levels 50 to 100 times higher than those in e-cigarette emissions but approximately 10 times lower than conventional cigarette emissions.⁹⁷ Gases found in heated tobacco product emissions include toxic carbon monoxide and nitric oxide⁸⁸ ^,⁹⁴ ^,⁹⁶ but carbon monoxide levels were considerably lower than in emissions from conventional cigarettes.⁸⁹ More information about the individual chemicals in these emissions can be found below in Section 12.4.3.

Emissions from common heated tobacco products, therefore, contain a broad range of the toxic chemicals found in cigarette smoke, but most at lower levels. For some of these chemicals, it has been shown that their concentrations in heated tobacco product emissions lie in between those of e-cigarettes and cigarette smoke. However, there are studies showing similar health risks from the use of heated tobacco products and factory-made cigarettes (see InDepth 18C for more information). It should therefore not be assumed that a lower amount of specific chemicals in heated tobacco product emissions will lead to a reduction in the harm to health caused by their use.

12.4.4.3 Emission from smokeless tobacco

Smokeless tobacco refers to tobacco products that are chewed, sucked or placed in the mouth or nose.⁹⁸ Chewed tobacco is a loose-leaf, plug tobacco or twist. Oral snuff is ground tobacco that may be in a bag, held in place in the mouth and sucked.⁹⁸ Snus is a type of moist snuff that is placed between the user’s cheek and gums, or behind the upper or lower lip, whereas dry or liquid snuff may be used nasally.⁶⁷ Broad generalisations about the emissions and health risks can be problematic due to the wide variety of these products and the ways they are used.⁹⁹ Sales of smokeless tobacco products have been banned in Australia since 1991.¹⁰⁰ See Section 12.2.9 for more information about the tobacco used in these products and for the prevalence of use, health effects and regulation of smokeless tobacco products.

Smokeless tobacco products are not heated and therefore do not undergo combustion, pyrolysis chemical reactions, distillation or other processes that create many of the chemicals found in smoke.⁶⁷ These products, however, do have emissions: the chemicals released from the product during use. Nicotine and other chemicals in these emissions are absorbed through the oral or nasal mucous membranes of the user and enter the bloodstream, circulating throughout the body. Oral smokeless tobacco products are mixed with saliva through chewing or sucking. Saliva contains compounds that can modify the pH of the solution as well as start the process of digestion, where large carbon-based (organic) components are broken down into smaller, more water-soluble compounds. The emission from oral smokeless tobacco may therefore be modified before entering the body’s cells and bloodstream, however, these modifications remain poorly characterised due to a lack of research.

Nicotine is the main addictive chemical present in smokeless tobacco products.⁶⁷ Nicotine is found in moist snuff and chewing tobacco at similar levels to conventional cigarettes¹⁰¹ but concentrations across products and brands differ widely. A study of 31 popular brands of smokeless tobacco in the US showed that total nicotine levels in unused products ranged from 8.77 to 18.16 mg/g of the product, and free nicotine ranged from 0.48 to 6.99 mg/g of the product.¹⁰² A study of nicotine in smokeless tobacco emissions, using artificial saliva, found that nicotine release was dependent on the form and cut of the smokeless tobacco products, with a slower release observed for snus and loose-leaf, compared to chopped and loose moist snuff smokeless tobacco.¹⁰³

Nicotine is absorbed into the body more slowly from oral snuff and chewing tobacco than from cigarettes, but it reaches similar peak levels in the blood.¹⁰¹ ^,¹⁰⁴ Unlike nicotine from cigarettes that quickly drops in concentration, nicotine levels in the blood plateau after the use of these smokeless tobacco products.¹⁰¹ The rate of nicotine delivery across mucous membranes, which affects its addictive properties, can be increased by a higher (less acidic) pH level in smokeless tobacco (see Section 12.4.3.1). The pH levels of smokeless tobacco products vary widely, from pH 5 to 8.¹⁰⁵ For products that mix with saliva, pH may be affected by the buffering chemicals present in the saliva, which could increase the pH of tobacco emissions with a low pH.¹⁰⁴ However, there is evidence that manipulation of the pH of moist snuff by manufacturers is the primary means by which the rate of nicotine delivery is controlled.¹⁰⁴

Most of the research on smokeless tobacco chemicals has detected them in the unused product, rather than its emissions.⁶⁷ Chemicals found in various types of smokeless tobacco products include toxic and carcinogenic chemicals such as N-nitrosamines, aldehydes, benzofluoranthenes, lactones, benzo[a]pyrene, aldehydes, arsenic, cadmium, polonium and uranium.⁶⁷ ^,¹⁰⁵ Additives, which often remain a trade secret, may include menthol, methyl or ethyl salicylate, β-citronellol, 1,8-cineole or benzyl benzoate. There is a wide variety in the amount of N-nitrosamines found in different brands of smokeless tobacco.⁶⁷ Some Swedish Snus manufacturers use processes that minimise the amount of N-nitrosamines in their products.⁶⁷

In the absence of comprehensive studies of the chemicals emitted from smokeless tobacco products during their use, biomarkers provide evidence that these chemicals enter the body. Biomarker studies have shown that users of smokeless tobacco products are exposed to nicotine,¹⁰⁶ carcinogenic N-nitrosamines such as NNN and NNAL (see Section 12.4.3.7),¹⁰⁶ ^,¹⁰⁷ polycyclic aromatic hydrocarbons and volatile organic compounds.¹⁰⁸ People who only use smokeless tobacco have higher concentrations of “total nicotine equivalent” biomarker and tobacco-specific N-nitrosamines than exclusive cigarette smokers. Conversely, they had lower biomarker levels for polycyclic aromatic hydrocarbons and volatile organic compounds than cigarette smokers.¹⁰⁸

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