Short-term exposure to fine particulate matter and pneumonia-related hospitalizations: a systematic review and meta-analysis

Previous epidemiological reports have emphasized the impact of ambient particulate matter (PM) on pneumonia-related hospitalizations. However, these reports vary across different study populations. We aimed to evaluate the impact of short-term exposure to PM on hospital admissions due to pneumonia. A systematic literature review was performed to identify studies quantifying the impact of PM <= 2.5 mu g m(-3) (PM2.5) and PM <= 10 mu g m(-3) (PM10) on pneumonia-related hospitalizations. A meta-analysis was performed by using pooled analyses of each pollutant with a random effects model. Subgroup analyses were performed according to various lag times and age groups, along with meta-regression analyses. A total of ten studies were eligible for analysis. In the overall population, a 10 mu g m(-3) increase in the daily PM2.5 was associated with an increased risk ratio (RR) of hospitalization for pneumonia (pooled RR 1.007, 95% confidence interval [CI]: 1.000-1.014 per 10 mu g m(-3) increase), but the PM10 was not (pooled RR 1.008, 95% CI: 0.998-1.018 per 10 mu g m(-3) increase). A lag effect was not observed in the subgroup analysis of different lag times. In the subgroup analysis of elderly individuals (>= 65 years), both the PM2.5 and PM10 resulted in increased RRs for pneumonia-related hospitalizations, with pooled RRs of 1.026 (95% CI: 1.006-1.047 per 10 mu g m(-3) increase) and 1.016 (95% CI: 1.013-1.019 per 10 mu g m(-3) increase), respectively. By contrast, studies that included young patients revealed a nonsignificant relationship between exposure to high levels of PM2.5 and pneumonia-related hospitalizations (RR 1.003, 95% CI: 0.999-1.008 per 10 mu g m(-3) increase). In the meta-regression analysis, results from recent study periods indicated that the effect of PM2.5 on pneumonia-related hospitalizations was less than that of earlier studies (P= 0.009). Our results suggest that PM2.5 and PM10 may affect elderly individuals in terms of pneumonia-related hospitalizations, which may vary over time.