Joint effects of diesel engine emissions and smoking and risk of lung cancer, pooled studies
| Smoking status | Diesel exposure | Controls | Cases | OR* 95% CI |
| Never-smokers | Never | 182 | 22 | 1.00 (ref.) |
| Non-substantial | 57 | 4 | 0.65 (0.2 to 2.0) | |
| Substantial | 13 | 5 | 3.02 (0.9 to 9.9) | |
| Ever smokers | Never | 769 | 1006 | 9.23 (5.7 to 14.8) |
| Non-substantial | 247 | 317 | 11.34 (6.8 to 18.8) | |
| Substantial | 62 | 137 | 17.08 (9.7 to 30.2) | |
| Never–low smokers | Never | 412 | 118 | 1.00 (ref.) |
| Non-substantial | 142 | 23 | 0.77 (0.5 to 1.3) | |
| Substantial | 25 | 17 | 2.29 (1.1 to 4.6) | |
| Medium–heavy smokers | Never | 539 | 910 | 5.68 (4.4 to 7.3) |
| Non-substantial | 162 | 298 | 7.97 (5.8 to 11.0) | |
| Substantial | 50 | 125 | 9.84 (6.4 to 15.1) |
↵* Adjusted for age, ethnicity, schooling level, socioeconomic status, respondent status and occupational carcinogens.