A strategy sometimes adopted by people who are trying to minimise their risk of adverse health outcomes from smoking is to attempt to smoke only occasionally.1, 2 Many people who have been regular smokers try to reduce their risk of harm by cutting down on the number of cigarettes smoked each day.3 In 2019, about 300,000 Australians smoked at least once a week but not every day. Another 300,000 smoked at least once a month but not every week.4 In 2019, more than half of all current Australian smokers aged 14 years and over reported attempting over the previous year to reduce the amount of tobacco/number of cigarettes they smoked each day—14% unsuccessfully and 39% managing to cut down to at least some extent.4
The logic of minimising or cutting down smoking as a strategy for minimising or reducing harm is supported by a large body of research showing a dose–response relationship between level of consumption and health risks. That is, the association between smoking and many diseases becomes stronger with more cigarettes smoked each day.5, 6 Despite this relationship, evidence from prospective cohort studies does not support low intensity smoking,7-11 or occasional smoking11 as an effective way to avoid risk from tobacco smoking. Neither does it support cutting down as an effective method to substantially reduce overall mortality risk.11-13, 14
As outlined in Section 18.3.1, even very low levels of exposure can cause cancer or create changes in the cardiovascular system: there is no safe level of smoking.5 Different studies over the years have produced varying estimates of the extent of elevation of overall risk and risk of deaths from specific smoking related diseases in light daily smokers compared to never smokers7, 15-17 and even in occasional, non-daily smokers.11 The findings of the largest and most recent studies9-11 and most comprehensive reviews7 are listed below in sections 18.3.2 and 18.3.3. Moving from heavy to moderate and moderate to light levels of smoking does not substantially reduce risk,12, 13 particularly for coronary heart disease and stroke,14,11—see 18.3.4. The main advantage of smokers cutting down the number of cigarettes per day may be as a step that makes it easier to quit completely18, 19 —see 18.3.5.
18.3.1 Biological mechanisms for low-rate smoking posing substantial major health risks
One explanation for lack of substantial risk reduction after reduction in smoking is compensation: cigarette smokers who cut down on number of cigarettes per day (CPD) tend to take more and deeper puffs from each cigarette, and smoke more of it.20, 21 The same phenomenon has been observed with waterpipe smoking.22 In turn, there is a much smaller proportional reduction in intake of nicotine, tar, and other toxins than the amount of cutting down might predict.23 Studies measuring biomarkers of the dangerous substances in tobacco smoke (see Section 18.1.3) find that, compared to quitting completely, smokers who cut down still have relatively high levels of biomarkers of oxidative stress such as glutathione24 and other metabolites such as thiocyanate,25 TNE-2, NNAL, NNN, 2-Fluorene, HPMA, CYMA, and MHB3.26
Another reason why low-rate smoking is not substantially less risky, is that the relationship between exposure to tobacco smoke and damage caused is not linear.5, 6, 27-29
Because there is no safe level of exposure to carcinogens, cancer can develop even with very low levels of exposure,30 including >0–3 cigarettes per day31 and non-daily smoking.32 Duration of smoking has a greater effect on risk than smoking intensity.27 It is more dangerous to smoke a small number of cigarettes for many years than it is to smoke the same number of cigarettes over a shorter period.6, 31, 33-35
With coronary heart disease, the non-linear relationship is even more striking.36 While many of the processes that drive the development of coronary heart disease do result in more risk with greater consumption, there is one notable exception. Aggregation of platelet cells in the blood is elevated at only one cigarette per day and does not worsen substantially with higher cigarette consumption. That is, the maximal effect occurs at a very low dose.37 The so-called sticky blood phenomenon occurs at very low levels of smoking. Platelet aggregation increases the risk of blood clots and heart attacks and stroke at low levels of smoking and even among those merely exposed to second-hand smoke.37(highlighted in USSG 2010,5 Chapter 6, p 358).
18.3.2 Risks associated with low-rate daily smoking
A 2018 meta-analysis of 141 cohort studies summarised in 55 scientific papers reported that, on average, smoking only one cigarette per day carries a risk of developing coronary heart disease and stroke much greater than expected: around half that for people who smoke 20 per day—that is, 10/20th rather than the 1/20th that might be expected.7 This strongly suggests that there is no safe level of smoking for cardiovascular disease. The authors estimated that among a group of 100 middle-aged people, a 20-a-day cigarette habit would cause an extra seven heart attacks or strokes. But if those people drastically cut back to one a day, smoking would still cause three additional heart attacks.7
Figure 126.96.36.199 Risk of heart disease among those that smoke 1 cigarette per day compared to those that smoke 20 cigarettes per day
Source: based on Hackshaw et al,
Subsequent to the 2018 meta-analysis, a prospective study of 188,167 cardiovascular disease (CVD)- and cancer-free individuals aged >/= 45 years from the NSW general population joining the 45 and Up Study from 2006 to 2009 revealed similar findings.9 Using questionnaires linked to hospitalisation and death data up to the end of 2015, the researchers calculated hazard ratios (HRs) for hospitalisation with or without mortality from CVD among current and past versus never smokers, including according to intensity and recency of smoking. They adjusted for age, sex, urban/rural residence, alcohol consumption, income and education and estimated population-attributable fractions. During a mean 7.2 years follow-up fatal and non-fatal major CVD events occurred, including 4548 (crude rate 3.2/1000 person-years) acute myocardial infarction (AMI; or heart attack), 3991 (2.8) cerebrovascular disease, 3874 (2.7) heart failure and 2311 (1.6) peripheral arterial disease (PAD) events. CVD risks were elevated at almost all levels of current smoking intensity examined and increased with smoking intensity. While risks of CVD mortality were largest for those who smoked >/= 25 cigarettes/day (HR=4.90 (3.79–6.34); never-smoker referent group), they were still considerable for those who smoked 4–6 cigarettes per day (1.92 (1.11–3.32)).
A very large US study published in 2020 similarly found surprisingly high levels of risk for those smoking only 1 or 2 or 3–5 cigarettes per day.10 The aim of this study was to examine the association of light cigarette smoking with all-cause and cause-specific mortality among adults in the United States. Data were from 13 waves of the National Health Interview Survey (1997 to 2009) that were linked to the National Death Index records through December 31, 2011. A total of 329,035 participants aged 18 years and over were included. Deaths were from all cause, cancer, CVD and respiratory disease and were confirmed by death certification. During a median follow-up of 8.2 years, 34,862 participants died, of which 8,415 were from cancer, 9,031 from CVD, and 2,040 from respiratory disease. Compared with never-smokers, participants who smoked 1–2 or 3–5 cigarettes per day were at higher risk of all-cause mortality after adjustment for demographic variables, lifestyle factors and physician-diagnosis of chronic disease. The risks were similar for the two levels of smoking—1–2 cigarettes per day (HR = 1.94, 1.73–2.16) and 3–5 cigarettes per day (HR = 1.99, 1.83–2.17). The associations were strongest for respiratory disease-specific mortality, followed by cancer-specific mortality and CVD-specific mortality. For example, among those smoking 1–2 cigarettes per day, risk of death was 9.75 times greater (HR 95%CI 6.15–15.46), 2.28 (1.84–2.84) times greater, and 1.93 (1.58–2.36) times greater, respectively, for these three cause-specific mortalities. This study indicates that light cigarette smoking increases risk of all-cause and cause-specific mortality in US adults.
An analysis of data from the US Current Population Survey published in 2020 similarly found that mortality rates were only slightly lower among those that smoke 3–10 cigarettes per day than among those who smoked 11–20 and may not have been lower at all among daily smokers who smoked >1–2 cigarettes.11
Taken together this comprehensive meta-analysis and three subsequent very large Australian and US cohort studies suggest that people wishing not to increase their risk of angina, heart attack or stroke need to cut out all smoking rather than continuing to smoke even a few cigarettes a day.
18.3.3 Risks associated with non-daily smoking
In the United States since the 2010s, several very large cohort studies linked to national morbidity and mortality data bases have increasingly allowed researchers to study the effects of infrequent smoking over long periods of time.
One large study of 39,555 participants in the US Million Heart studies found no difference in blood pressure and cholesterol levels between ‘social’ smokers and current smokers: rates of hypertension and elevated cholesterol were significantly higher among both groups compared to never smoker.38
Another large US cohort study published in 2018 tracked death and disease up to 2011 among 70,913 adults who were interviewed in the US National Health Interview Surveys in 1991, 1992 and 1995.39 The long-term non-daily smokers smoked a median of 15 days (i.e. typical of weekend-only smokers) and 50 cigarettes a month. Daily smokers in the cohort by contrast smoked an average of 600 cigarettes per month. As expected, daily smokers had the highest mortality risks (HR=2.50, 95% CI=2.35, 2.66) and shorter survival (10 years less) than never smokers. However, mortality risks were also surprisingly high among those who only smoked occasionally.
Compared with never smokers, lifelong non-daily smokers who had never smoked daily had a 72% higher mortality risk (HR=1.72 95% CI=1.36, 2.18) including for cancer, cardiovascular disease, and respiratory disease mortalities. Higher mortality risks compared to those of never smokers were observed among lifelong non-daily smokers who reported smoking:
- 11–30 cigarettes per month (HR=1.34, 95% CI=0.81, 2.20);
- 31–60 cigarettes per month (HR=2.02, 95% CI=1.17, 3.29); and
- >60 cigarettes per month (HR=1.74, 95% CI=1.12, 2.72).
Median life-expectancy was about 5 years shorter for lifelong non-daily smokers than for never smokers. The researchers concluded that although the mortality risks of non-daily smokers are lower than those of daily smokers, they are still substantial.39
A study even larger than the National Health Interview study published by the same researchers in 2020 looked at records for more than 500,000 American ever-smokers completing a Tobacco Use Supplement administered as part of the US Census Bureau’s Current Population Survey between 1992–93 and 2010–11.11 Records were linked to the US National Death Index as part of a National Lifetime Mortality Study.
As with the earlier study, the median number of cigarettes smoked per month was 600—10–20 cigarettes per day—for daily cigarette smokers and 40 cigarettes per month for lifelong non-daily smokers.
Once again, current daily smokers had higher all-cause mortality risks than never smokers (HR= 2.32; 95% CI, 2.25–2.38). And once again, lifelong non-daily smokers also had surprisingly higher levels of risk than never smokers (HR= 1.82; 95% CI, 1.65–2.01). Lifelong non-daily smokers had higher mortality risks than never smokers, even among those smoking as few as 6 to 10 cigarettes per month.
18.3.4 Effects of moving from heavier to lighter rates of smoking
A 2020 meta-analysis has analysed the findings from all well-designed studies that have examined the effects of moving from heavy to moderate or light levels of smoking.14 In this analysis, the researchers defined the following smoking categories:
- heavy smokers smoked ≥15–20 cigarettes per day (CPD)
- moderate smokers smoked 10–19 CPD, and
- light smokers smoked <10 CPD.
They looked at the results of 19 studies including four case-control and 15 cohort studies and examined the effects on lung cancer, other smoking related cancers, stroke, coronary heart disease (defined as myocardial infarction and ischemic heart disease combined) and cardiovascular disease (defined as stroke and coronary heart disease.
Compared to continuing heavy smokers, they found decreased lung cancer risk:
- for those who reduced CPD by more than 50% (HR= 0.72, 95%CI: 0.52, 0.91),
- among those who changed from heavy to moderate smoking (HR=0.66, 95%CI: 0.46, 0.85),
- among those who changed to heavy to light (HR=0.60, 95%CI: 0.49, 0.72).
They also found lower risk of cardiovascular disease (CVD)
- for those who reduced from heavy to light smoking (HR= 0.78, 95%CI: 0.67, 0.89).
However, they did not find significant reductions in risk of deaths among those who reduced even by as much as 50% either for
- coronary heart disease (HR=1.09, 95%CI:0.86, 1.32) or
- cardiovascular disease (HR=1.05, 95%CI:0.89, 1.21).
They found no significant reduction in risk for stroke among those who reduced from heavy to moderate (HR=0.98, 95%CI:0.85, 1.12). Compared with continuing heavy smokers, they also did not find any significant difference in all-cause mortality (HR=1.02, 95%CI:0.91, 1.13), all-cancer risks (HR=0.82, 95%CI:0.62, 1.10), and smoking-/tobacco-related cancer risk (HR=0.89, 95%CI:0.64, 1.13) among those who reduced smoking by more than 50%.
The researchers concluded that substantial smoking reduction may decrease lung cancer risk but results on cardiovascular disease (coronary heart disease (including myocardial infarction and ischemic heart disease) and stroke combined) risk were mixed and the relationships between smoking reduction and other endpoints examined were not significant. They conclude that while smoking reduction may decrease risks of lung cancer, the relationships between smoking reduction and other endpoints, including all-cause mortality and cardiovascular disease, remain unclear. Although smoking reduction may decrease lung cancer risks by about one-quarter to one-third, the absolute magnitude of lung cancer risk remains high. Among smokers, complete cessation remains the most effective approach for cancer and CVD prevention.
As discussed in 18.3.3 above, the notably large Current Population Survey cohort survey did find some reduction in risk for former daily smokers who became non-daily smokers.11
Relative to never smokers, mortality risks for a smoking related cancer were
- 2.16 times (95%CI, 1.64–2.86) higher among lifetime non-daily smokers,
- 3.65 times (95%CI, 3.11–4.27) higher among non-daily previous daily smokers and
- 4.88 times (95%CI, 4.57–5.22) higher among continuing daily smokers.11
Non-daily smokers who previously smoked every day had lower overall mortality risks than daily smokers, with similar risks after 10 or more years of non-daily smoking as lifelong non-daily smokers (HR=1.73; 95% CI, 1.56–1.92; never smoker reference group). However, this risk was still much higher than for daily smokers who quit completely. Among former daily smokers who had quit for ten years or more, the risk comparative to non-smokers had reduced to HR=1.18 (95% 1.15–1.22).
Figure 188.8.131.52 Risk of dying early compared to someone who has never smoked
Source: Inoue-Choi et al 2020 11
The authors conclude that although reducing smoking from daily to non-daily was associated with decreased mortality risk, cessation was associated with far greater benefit.
18.3.5 Cutting down as a step towards quitting
The primary benefit of cutting down seems to be its role as a step toward quitting.16, 18, 40-42 However there is still considerable debate as to whether cutting down is a less effective strategy to quit than going ‘cold turkey’.43 A 2019 review exploring the usefulness of cutting down determined that smokers who reduce the number of daily cigarettes smoked are more likely to attempt and actually achieve smoking cessation, particularly when combined with nicotine replacement therapy.19, 44 Smokers not yet ready to quit have also proved willing to use anti-smoking drug varenicline to cut down. This similarly has been associated with increased confidence in ability to quit at a later date.45
The existing epidemiological evidence on cutting down providing limited health benefit (as summarized above) is generally in the context of no additional source of nicotine, such as NRT or e-cigarettes. Obtaining nicotine from an alternative source may reduce some of the compensatory smoking behaviour that would otherwise limit the benefit from cutting down. Studies of smokers cutting down while using NRT have found that while blood nicotine levels generally remain stable or slightly higher, carbon monoxide readings decreased.46-48 This has led some to advocate concomitant use of NRT while smoking as a harm reduction strategy.49 Evidence suggests that use of NRT to ‘cut down to quit’ is effective and cost effective compared to no quit attempt,18 but data on the health risks/benefits of long-term dual use of NRT and other alternative nicotine delivery products and cigarettes are lacking.
The fact remains that even low-intensity smoking—even occasional non-daily smoking—is associated with substantially elevated risks of death among ever-smokers. Complete cessation of tobacco smoking is the only way of reducing these risks.
Relevant news and research
For recent news items and research on this topic, click here. ( Last updated January 2021)
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8. Inoue-Choi M, Liao L, Reyes-Guzman C, Hartge P, Caporaso N, et al. Association of long-term, low-intensity smoking with all-cause and cause-specific mortality in the National Institutes of Health-AARP Diet and Health study. JAMA Internal Medicine, 2017; 177(1):87-95. Available from: http://jamanetwork.com/journals/jamainternalmedicine/article-abstract/2588812
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