Follow-up of the air pollution and the human male-to-female ratio analysis in Sao Paulo, Brazil: a times series study.OBJECTIVES:  In order to assess if ambient air pollution in urban areas could be related to alterations in male/female ratio this study objectives to evaluate changes in ambient particulate matter (PM10) concentrations after implementation of pollution control programmes in São Paulo city and the secondary sex ratio (SRR). DESIGN AND METHODS:  A time series study was conducted. São Paulo''s districts were stratified according to the PM10 concentrations levels and were used as a marker of overall air pollution. The male ratio was chosen to represent the secondary sex ratio (SSR=total male birth/total births). The SSR data from each area was analysed according to the time variation and PM10 concentration areas using descriptive statistics. The strength association between annual average of PM10 concentration and SSR was performed through exponential regression, and it was adopted as a statistical significance level of p<0.05. RESULTS:  The exponential regression showed a negative and significant association between PM10 and SSR. SSR varied from 51.4% to 50.7% in São Paulo in the analysed period (2000-2007). Considering the PM10 average concentration in São Paulo city of 44.72 μg/m(3) in the study period, the SSR decline reached almost 4.37%, equivalent to 30 934 less male births. CONCLUSIONS:  Ambient levels of PM10 are negatively associated with changes in the SSR. Therefore, we can speculate that higher levels of particulate pollution could be related to increased rates of female births.