Season and humidity dependence of the effects of air pollution on COPD hospitalizations in Hong Kong

Associations between ambient pollution and respiratory morbidity including chronic obstructive pulmonary disease (COPD) have been confirmed. Weather factors, such as temperature, season and relative humidity (RH), may modify the effects of air pollution. This time series study was conducted to examine whether the effects of air pollution on emergency COPD hospital admissions in Hong Kong varied across seasons and RH levels, and explore the possible joint modification of season and RH on the effects of pollution. Data of daily air pollution concentrations mean temperature and RH, and COPD hospital admissions from 1998 to 2007 were collected. Generalized additive Poisson models with interaction terms were used to estimate the effects of pollution across seasons and RH levels. We observed an increase in the detrimental effects of air pollution in the cool season and on low humidity days. On the cool and dry days, a 10 μg m−3 increment of lag03 exposure was associated with an increase in emergency COPD admissions by 1.76% (95%CI: 1.19–2.34%), 3.43% (95%CI: 2.80–4.07%), and 1.99% (95%CI: 0.90–3.09%) for nitrogen dioxide (NO2), ozone (O3), and sulfur dioxide (SO2), respectively, all of which were statistically significantly higher than those on the other days. No consistent modification of weather factors was found for the effects of particles with an aerodynamic diameter less than 10 μm (PM10). The results suggested that season and RH jointly modified the effects of gaseous pollutants, resulting in increased emergency COPD hospitalizations on the cool and dry days.