Background Previous studies have found associations between meteorological variables and asthma hospitalisations but the nature of these associations has varied and few studies have been done in subtropical areas or evaluated effect modification by age.

Objectives This study aimed to evaluate associations between asthma hospitalisations and meteorological factors and to assess effect modification of these associations by age and season in Hong Kong.

Methods Poisson generalised additive models combined with distributed lag nonlinear models and piecewise linear models were used to model associations between daily asthma hospitalisations from 2004 to 2011 and meteorological factors and air pollutants, adjusting for day of week, seasonality and trend. Subgroup analyses by age and season were performed.

Results In the hot season, hospitalisations were lowest at 27°C, rose to a peak at 30°C, then plateaued between 30°C and 32°C. The cumulative relative risk for lags 0–3 days (RRlag0–3) for 30°C vs 27°C was 1.19 (95% CI 1.06 to 1.34). In the cold season, temperature was negatively associated with asthma hospitalisations. The cumulative RRlag0–3 for 12°C vs 25°C was 1.33 (95% CI 1.13 to 1.58). Adult admissions were most sensitive to temperatures in both seasons while admissions among children under 5 were least associated. Higher humidity and ozone levels in the hot season, and low humidity in the cold season were also associated with more asthma admissions.

Conclusions People with asthma should avoid exposure to adverse conditions by limiting outdoor activities during periods of extreme temperatures, combinations of high humidity and high temperature, and low humidity and low temperature, and high ozone levels.