In this study, we estimated PM2.5 data using a satellite-driven PM2.5 exposure model  which provided daily population-weighted average PM2.5 concentrations for all districts of Lima, from 2010 to 2016. We then examined PM2.5 short-term exposure in relation to all cause and cause-specific mortality in Lima, one of the most polluted cities in Latin America . We observed positive associations of daily PM2.5 exposure with all-cause mortality, with an increase of 1.8% per IQR (6.19 µg/m3) increase in PM2.5 concentration, driven largely by those over 65 years of age. Our results are congruent with previous findings of statistically associations of PM2.5 with all-cause [1, 3, 23]. In a systematic review of seven time series studies of all-cause deaths, the researchers found an increase of 1.4% per 10 µg/m3 increment in PM2.5 . In another systematic review of 37 studies, the researchers reported a 0.9% increase in all-cause mortality per 10 µg/m3 increase in PM2.5 .
For circulatory mortality, our results show an increase of 3.7% per IQR increase in PM2.5 for both mortality for all ages, and 3.8% those over 65 years. We also found increased risk with CVD, a large subset of circulatory disease. Newell, in a systematic review of 91 published studies, found a 0.47% increase in cardiovascular mortality per 10 µg/m3 increase in PM2.5 . Likewise, Zhao et al observed an increased CVD mortality risk (0.68%) associated with a 10 µg/m3 increase in PM2.5 .
For respiratory mortality, we found an association only for the elderly group, which was borderline significant. Our results showed a slight increased risk of 3% per IQR increase of PM2.5. One systematic review reported positive effects, with a 10 µg/m3 increment in PM2.5 being associated with 1.51% (95% IC 1.01 to 2.01) in all cause respiratory mortality .
Our study showed that PM2.5 exposure has a significant impact on the health of people in Lima, in particular among elderly people. The population structure is changing ; the number of individuals over 65 years is increasing. PM2.5 exposure along with a pre-existing condition, such as cardiometabolic disorders  or diabetes , has been shown to increase the risk of CVD deaths. Elderly people are more likely to have more of these conditions and be more vulnerable to the effects of air pollution.
Our study has several strengths. Firstly, we used a predictive model that estimated daily PM2.5 concentrations for 39 districts of Lima and for the entire study period, which replaced the relative scarce data from ground monitoring. Second, the allocation of PM2.5 concentrations to each death according to the district of residence allowed us to reduce possible exposure measurement by using district-specific PM2.5 estimates than using city-wide average data, thereby avoiding bias in our RR estimates. Third, we used the data of people who had habitual residence in Lima and died there. A limitation observed in the study is our inability to estimate associations with some disease-specific mortality such as, ischemic heart disease, stroke, or specific respiratory diseases (eg COPD) [28, 29]. Another limitation is our lack of data regarding the occupation, which will have led to some mismeasurement of estimated air pollution exposure for person who worked outside of their district.
Our study provides evidence regarding the effects of PM2.5 on cardio-respiratory deaths in moderate concentrations. The local recommendations for daily PM2.5 established by the Peruvian Ministry of Environment, are currently 25 µg/m3, compared to the recommendations given by WHO, which are 10 µg/m3. We believe that these recommendations for air quality in Peru are likely to be too high, and probably needs to be reviewed.