Our study provides further support that current and former smokers experienced greater risk of death at follow-up than nonsmokers in a Chinese rural population. Younger age at smoking initiation and older age at smoking cessation were both associated with increased risk of mortality.
Risk of all-cause mortality was significantly higher in former or current smokers than in nonsmokers. This finding parallels results from The National Longitudinal Mortality Study 20. Another study with a mean (SD) follow-up of 6.6 (1.3) years from the National Institutes of Health–AARP Diet and Health Study21, showed more powerful evidence. There was a dose-dependent association between reported number of CPD (cigarettes per day) at baseline with all-cause mortality as well as with deaths from examined smoking-related outcomes. Even those who reported smoking fewer than 1 CPD (HR, 1.99; 95% CI, 1.76, 2.25) or 1 to 10 CPD (HR, 2.60; 95% CI, 2.45,2.75) had an increased risk of all-cause mortality. Importantly, it extends these findings to show that risk of death associated with smoking remains consistent even for low intensity smoking.
Our study results are striking in that relatively small differences in age at initiation were associated with strong differences in mortality risk 15 years later. A National Health Interview Survey showed that early smoking initiation before age 13 was associated with increased risks for cardiovascular/metabolic (OR:1.67) and pulmonary (OR:1.79) diseases as well as smoking-related cancers (OR:2.1) among current smokers; the risks among former smokers were cardiovascular/metabolic (OR:1.38); pulmonary (OR:1.89); and cancers (OR:1.44). Elevated mortality was also related to early smoking initiation among both current (hazard ratio, 1.18) and former smokers (HR = 1.19) 10. Relative to former smokers, the risk of mortality was lower in individuals who quit smoking at earlier ages. This finding is in agreement with the Sax Institute’s 45 and Up Study 16. Smoking cessation remains beneficial even at age 50. The investigation based on 489,066 participants, aged ≥60 years, from 22 population-based cohorts of the CHANCES Consortium confirmed that for former smokers, excess mortality and risk advancement periods (RAPs) decreased with time since cessation, with RAPs of 3.9 (95% CI, 3.0, 4.7), 2.7 (95% CI,1.8, 3.6), and 0.7 (95% CI,0.2, 1.1) for those who had quit <10, 10 to 19, and ≥20 years ago, respectively22. Similarly, the Zutphen Study found that in 1373 men, stopping cigarette smoking at age 40 increased the life expectancy by 4.6 years, while the number of disease-free life-years was increased by 3.0 years23.
To date, most studies about smoke had been focused on urban or general population 24, only limited studies on smoking and mortality risk have been carried out in Chinese rural areas and in particular none have such a long and large prospective cohort among a male farming population as the current study. Consistently for all these studies, with the increase in pack-years in current smokers, all-cause mortality climbed remarkably. Although the majority of results follow a similar path, our study showed contrary results in former smokers. It is likely that among men who had stopped smoking due to illness, the protective effects of quitting cannot be assessed straightforwardly, even if cessation is substantially protective, because the underlying illness that prompted the smoking cessation may cause a misleadingly elevated risk. The question remains: how does tobacco attribute to all-cause mortality? The " China reported health hazards of smoking" pointed out that tobacco smoke contains 69 known carcinogens, which can cause mutations in key genes, dysregulate normal growth control mechanisms, and eventually lead to the occurrence of cell cancer and malignant tumors 25. In addition, it also damages vascular endothelial function, which can lead to the occurrence of atherosclerosis, narrowing of the arterial vascular cavity and cause a variety of cardiovascular and cerebrovascular diseases.
Our study has two new findings. Firstly, the age of first tobacco use is an important determinant of mortality risk. Age at smoking initiation was strongly associated with mortality in men over 45 years of age. Ever smokers who started smoking earlier were at a progressively higher risk of mortality during follow-up, relative to those who started smoking later. Similarly, risk of mortality was lower when cessation occurred at an earlier age. This finding supports analogous results abroad. A possible reason is that an earlier age of smoking initiation and a later age of smoking cessation, increases duration of exposure. Secondly, our results suggest that nonsmokers who were ever drinkers, had a potent protective factor for mortality risk in this rural Chinese population. While, former smokers who were former drinkers were at risk. We surmise that the increase in mortality risk for those who previously used cigarettes and alcohol among this rural population could be a result of stopping usage due to illness. Because of the cessation patterns in rural areas are different from those in cities, ordinary cessation methods cannot effectively prevent the beginning of smoking and fail to induce residents to quit. For example, indoor smoking bans may not have a substantial impact on middle-aged and elderly people in rural areas, because the majority of them are farmers who worked outsides. Raising the tobacco sales tax may push them to purchase cheaper cigarettes, because the cigarette prices in China vary greatly. Hence if interventions are adopted to the situations and demands for this populations, the protection in rural villages will be more effective, particularly in the context of unbalanced social and economic development 11.
This study has specific strengths and limitations. Firstly, the data were sparse after stratification, resulting in a larger 95% confidence interval and insignificant results. Secondly, in rural China, smoking cessation was motivated mainly by health issues experienced either directly or indirectly. Nearly all participants who have attempted or successfully quit smoking reported experiencing some health issues prior to quitting. And those who successfully quit frequently reported significant health events that prompted a visit to a doctor26. Lastly, only smoking information collected at baseline was available for this analysis, therefore, it is possible that recall bias existed and some participants who were former smokers at baseline may have resumed smoking afterward, leading to an underestimation of benefits related to smoking cessation. Another weakness of the study was the lack of classification and time of death. Data on smoking-attributable causes of death would have been informative in our interpretation of these results.