Objective We want to determine: ¿Which of the two factors is more harmful? ¿What is the magnitude of its effect? and ¿What happens when someone is exposed to both factors at once?. Methods An ecological study that evaluates the epidemiological association (relative risk) between air pollution and smoking with lung functions as assessed by functional spirometry in 489 adults. We compare two environments with different pollution levels 30 and 60 g/m³ of particulate matter (PM10) and groups of smokers against nonsmokers. Results Lung function is impaired (FEV1 <80 %) in smokers at higher rates than the non-smokers, with an excess risk of 52 % (RR. 1.52 CI 95 % 1.11 -2.07). People exposed to higher levels of pollution have a higher proportion of pulmonary dysfunction than those exposed to less polluted environments with an excess risk of 64 % (RR. 1,64 CI 95% 1.19-2.25). When subjects are exposed to both factors, that is smokers who also live in contaminated environments, the excess of risk reaches 129 % (RR 2.29 CI 95 % 1.45-3.61). Conclusions People who breathe in polluted environments have impaired lung function in a similar magnitude or greater than smokers. Therefore, we could say that breathing contaminated air is equivalent to smoking, but sadly it occurs against the affected party's will.