PM10 exposure is associated with increased hospitalizations for respiratory syncytial virus bronchiolitis among infants in Lombardy, Italy

Respiratory syncytial virus (RSV) is the primary cause of acute lower respiratory infections in children, bronchiolitis in particular. Airborne particulate matter (PM) may influence the children's immune system and foster the spread of RSV infection. We aimed to verify whether PM10 exposure is associated with hospitalization due to RSV bronchiolitis. We selected hospital discharge records (HRD) with ICD-9-CM code 466.11 of infants < 1 year of age, occurring in the epidemic seasons of two years (2012-2013) in Lombardy, Italy. Cases were assigned daily PM10 and apparent temperature levels of the capital city of their residential province. Different exposure windows were considered: single days preceding hospitalization (lag 0 to 30), their average estimates (lag 0-1 to 0-30), and the four weeks preceding hospitalization (week 1 to 4). Negative binomial regression models adjusted for apparent temperature and season were applied to the daily counts of hospitalizations in each province. Results were expressed as incidence rate ratios (IRR) and 95% confidence intervals (95%CI) per 10 mu g/m(3) increase in PM10 concentration. Random effects meta-analyses of province-specific IRR were performed to obtain regional estimates. 2814 HRD met our inclusion criteria; males represented about 55% of the cases. A 6% increased risk of hospitalization (95%CI: 1.03-1.10) was found at lag 0 and an almost overlapping 7% increase at lag 1. IRR ranged from 1.03 to 1.05 between lags 2 and 11. No increased risk was observed from lag 12. When considering averaged daily lags, risk estimates gradually increased in the two weeks preceding hospitalization from 1.08 (1.04-1.12) at lag 0-1 to 1.15 (1.08-1.23) between lags 0-11 and 0-13. Analyses on weekly lags showed a risk increase of 6% (1.01-1.12) during week 1 and of 7% (1.02-1.13) during week 2. Our study found a clear association between short- and medium-term PM10 exposures and increased risk of hospitalization due to RSV bronchiolitis among infants.