Short-term effects of air pollution and temperature on daily morbidity in Chiang Mai Thailand

Air pollution is associated with mortality and morbidity worldwide. Hot and cold temperature is also related to increased deaths and possibly hospital visits and admissions in many settings. Climate change is anticipated to pose increasing risks of deaths and illnesses associated with air pollution and temperature variations, particularly in developing world. To date, research studies about health effects of air pollution and temperature have been conducted in developed countries with cool climate more than in developing countries with subtropical or tropical climate. Furthermore, studies to identify susceptible populations are still limited. This study aims to investigate heath effects of air pollution and temperature and to identify people who are more susceptible to air pollution and temperature in a developing, tropical country, Thailand.

A regression analysis of retrospective time series data was employed to assess the shortterm effects of air pollution and temperature on daily out-patient visits and hospital admissions in Chiang Mai, Thailand, from October 2002 to September 2006. Generalised negative binomial regression was used to model the relationships between the exposure and health outcomes, controlling for seasonal patterns and other possible potential confounders. Lag effects up to 4 days for air pollution, and up to 13 days for temperature were considered. Effect modification by age, sex, occupation, season, and previous out-patient visits before admissions were also examined.

There were positive, but not significant, effects of air pollution for some pollutants (particularly for S02), with notably larger effect sizes compared to previous studies in Western countries. There was evidence of hot temperature effects (though wide confidence intervals), with an increase in diabetic visits of 26.3% (95% Cl, 7.1% to 49.0%), and in circulatory visits of 19.2% (95% Cl, 7.0% to 32.8%) for each 1°C increase in temperature above 29°C. There was a rise of both the visits (3.7% increase, 95% Cl, 1.5% to 5.9%) and admissions (5.8% increase, 95% Cl, 2.3% to 9.3%) due to intestinal infectious disease for each 1°C increase across the whole temperature range. Despite no statistically significant differences between subgroups, air pollution effects were stronger in the elderly, females and manual workers, whereas temperature effects were stronger in the elderly, male and unemployed people.

This study suggests that while there was little evidence of air pollution effects, there was significant evidence of high temperature effects on daily morbidity in Chiang Mai. The elderly seemed to be more vulnerable to the daily changes of both air pollution and temperature.