Demographic characteristics of the study subjects
The current study comprised 2,105 subjects, including 885 cases and 1,220 controls. Subjects included 1,354 males and 751 females, with ages between 23 and 90 years of age and average age of 58.94±10.73 years. The case and control groups did not show any significant difference in the distribution of gender, age, ethnicity, and marital status (P>0.05). However, significant intergroup differences were noted in the educational level, occupation, and BMI (P<0.05).
Among the smokers, no significant difference was noted between the case group and the control group in terms of the distribution of gender, ethnicity, and marital status (P>0.05). However, statistically significant intergroup differences were noted in the distribution of education, occupation, and BMI (P<0.05). Among the non-smokers, the case group and control groups did not differ significantly in the distribution of age, ethnicity, and marital status (P>0.05), but showed significant differences in gender, education level, and occupational exposure (P<0.05).
Among the 885 cases included, 551 (62.3%) were of adenocarcinoma, 208 (23.5%) were of squamous cell carcinoma, and 126 (14.2%) were of other pathological types of cancers. Among the 519 smokers, 253 (48.7 %) had adenocarcinoma, 177 (34.1%) had squamous cell carcinoma, and 89 (17.4%) had tumors of other pathological types. Of 366 non-smoking lung cancer patients, 298 (81.4%) had adenocarcinoma, 31 (8.5%) had squamous cell carcinoma, and 37 (10.1%) had lesions of other pathological types (Table 1).
The risk factors of lung cancer
In this study, we explored the potential factors for lung cancer in the three groups: general population, smokers, and non-smokers (Table 2). Of the total population, after adjusting for BMI, education, and occupation, we found that subjects with a family history of lung cancer or medical history of lung disease, drinking alcohol, smoking, passive smoking, or exposure to cooking oil fumes were susceptible to lung cancer. We found that the risk of lung cancer was low for subjects who performed physical exercise and had a regular intake of fruit (more than 3 times per week). Among the smokers, after adjustment for age, BMI, education, and occupation, the risk factors for lung cancer were family history of lung cancer, medical history of lung disease, and exposure to cooking oil fumes, while the protective factors were physical exercise and fruit intake. Similarly, for non-smokers, the risk factors for lung cancer were family history of lung cancer, history of lung diseases, passive smoking, and exposure to cooking oil fumes, after adjustment for gender, education, and occupation, whereas the protective factors were drinking tea, physical exercise, and fruit intake.
Univariate analysis of levels of exposure to atmospheric pollutants in different populations
The annual average of SO2, NO2, and PM10 concentrations for the period between 2005 and 2015 and those of CO, PM2.5, and O3 for the period 2013-2015 are shown in Figure 2. Notably, the distribution of different pollutants differed from one another. Thus, we noted that NO2, PM2.5, and O3 had similar distribution patterns, with the pollutants being highly aggregated in cities along east coast; this may be associated with the dense population and severe traffic pollution. The remaining three common pollutants showed completely different distribution patterns, with CO mainly concentrating in northeast area, SO2 aggregating in the mid–west region, and PM10 mainly affecting the southwest. The discrepancy in the distribution patterns may be attributed to the complex effect of factory contamination and factors such as climatic and geographic conditions.
Univariate analysis was performed to levels of exposure to evaluate the impact of atmospheric pollutants on different populations. The values for concentrations of atmospheric pollutants were classified into four levels based on ±1 standard deviation (Table 3). For the total population, after adjustment for education, occupation, BMI, family history of lung cancer, and medical history of lung disease, alcohol consumption, smoking, passive smoking, exposure to cooking oil fumes, physical exercise, and fruit intake, the results showed that the risk of developing lung cancer with exposure to NO2 concentrations of 19–29 (μg/m3) was 1.356 times greater than at concentrations of less than 19 (μg/m3) (95% CI: 1.028–1.788). As compared to exposure to PM10 concentrations of less than 51 (μg/m3), exposure to concentrations of 51–57 (μg/m3) increased the risk of developing lung cancer by 2.450 times greater (95% CI: 1.728–3.474), while exposure to concentration of 57–64 (μg/m3)and increased the risk by 1.637 times (95% CI: 1.178–2.276).
Exposure to PM2.5 is also a risk factor for lung cancer. For exposure to concentrations of 20–27 (μg/m3), the OR was 33.658 (95% CI: 15.450–73.325), and for exposure to concentrations of 27–35 (μg/m3), the OR was 5.059 (95% CI: 2.390–10.712). Furthermore, exposure to O3 is also a risk factor for lung cancer. For exposure to O3 concentrations of 47–65 (μg/m3), the OR was 125.056 (95% CI: 29.902–523.012); 17.746 (95% CI: 4.322–72.862), for O3 concentrations of 65–83 (μg/m3); and 50.896 (95% CI: 11.069–234.032) for concentrations of more than 83 (μg/m3) .
Next, we compared the results for smokers, after adjusting for age, education, occupation, BMI, family history of lung cancer, and medical history of lung disease, exposure to cooking oil fumes, physical exercise, and fruit intake. Further, for non-smokers, the results were compared after adjusting for gender, education, occupation, family history of lung cancer, medical history of lung disease, passive smoking, exposure to cooking oil fumes, drinking tea, physical exercise, and fruit intake. The analysis indicated that for both groups, exposure to PM10 and PM2.5 was a risk factors for lung cancer. Moreover, exposure to PM2.5 had a greater impact on non-smokers than on smokers, as shown by the following results for both groups: OR was 61.431 (95% CI: 18.041–209.181) and 24.545 (95% CI: 8.12–74.265) (in non-smokers and smokers, respectively, for exposure to PM2.5 concentrations of 20–27 (μg/m3)) and OR was 11.814 (95% CI: 3.622–38.540) and 2.630 (95% CI: 0.910–7.595) (for non-smokers and smokers, respectively, for exposure to PM2.5 concentrations of 27–35 (μg/m3). Since none of the participants were exposed to O3 concentrations of less than 47 (μg/m3)in the control group, the effect of O3 exposure on the smokers could not be assessed. Among the non-smokers, exposure to O3 was identified as a risk factor for lung cancer. However, SO2 and lung cancer did not show any associated, both in the general population and in the group of smokers or non-smokers. Similarly, none of the participants in the control group were exposed to a low concentration of CO, and therefore, the association of CO with lung cancer could not be evaluated (Table 3).
We also analyzed the relationship between air pollution and lung cancer stratified by passive smoking and exposure to cooking oil fumes, and the results were consistent with those of smoking. The detailed results are shown in Additional Table 1 to 3 (for passive smoking) and Additional Table 4 to 6 (for cooking oil fume exposure).
Multivariate analysis of levels of exposure to atmospheric pollutants in different populations
All the variables that were found to have a significant impact on the development of lung cancer in the previous analysis were further subjected to multi-factor unconditional logistic regression analysis using the backward stepwise method. We used P<0.05 as the inclusion criterion and P>0.10 as the exclusion criterion. The results of the analyses indicated that the following factors increased the risk of lung cancer among the general population: smoking;exposure to cooking oil fumes; passive smoking;medicalhistory of lung disease; family history of lung cancer;and exposure to O3, PM10, and PM2.5. On the other hand, fruit intake and physical exercise were found to be protective factors against risk of lung cancer (Table 4). For smokers, medical history of lung disease; family history of lung cancer; and exposure to PM10, and PM2.5 were factors that increased the risk of lung cancer, whereas fruit intake and physical exercise were factors that reduce the risk of lung cancer (Table 5). Similarly, among the non-smokers, cooking oil fumes; medical history of lung disease; family history of lung cancer; exposure to PM10, PM2.5, and O3 may increase the risk of lung cancer, while fruit intake, physical exercise, and drinking tea were found to protect against the risk of lung cancer (Table 6).
The results of the analysis in different populations are shown in Additional Table 7-9 for passive smoking and in Additional Table 10-12 for exposure to cooking oil fumes.