The role of temperature on morbidity and mortality in Hong Kong

Abstract

A vast body of epidemiological studies consistently reported that exposure to extreme low and high temperature was both associated with increased risks of morbidity and mortality. However, risk estimates of temperature showed substantial heterogeneity according to geographical regions. The effect heterogeneity may be due to differences in the characteristics of population. However, little is known about the effect heterogeneity of temperature by health conditions. Therefore, a distributed lag non-linear model was incorporated into a nested case-control study design to assess the nonlinear and delayed mortality effects of temperature in the elders with diabetes mellitus, circulatory system diseases, chronic obstructive pulmonary disease, and elders absent of these three diseases, respectively. The cumulative relative risks of temperature were greater in elders with health problems, which suggested that elders with pre-existing diseases were more vulnerable to temperature. Climate change not only increases the global mean temperature but also leads to unstable weather events, including changing temperature variability. Despite extensive investigations on health impacts of mean temperature, evidence on the health effects of temperature variability is sparse, especially for long-term temperature variability (e.g. seasonal). Therefore, a time-varying Cox proportional hazards regression was applied to estimate the hazard ratios of seasonal temperature variability (defined as the standard deviation of summertime and wintertime temperature) for the incidence of total respiratory, pneumonia, and chronic obstructive pulmonary disease, respectively. Wintertime temperature variability was associated with increased risks of incident respiratory diseases. Air pollution is the world’s largest single environmental health risk according to World Health Organization, which caused around 7 million deaths in 2012. Daily temperature and climate may both modify the adverse health effects of air pollution. However, few studies have been conducted to investigate such modification effects. I conducted a time-series study to assess the effect modification of daily temperature on the mortality effects of air pollution by fitting a temperature-stratified generalized additive model. Temperature was classified into low, medium, and high temperature strata, and excess risks of air pollution on mortality were estimated in these three temperature strata, respectively. Excess risks of air pollution were found to be stronger in low temperature strata than in high strata for all-natural mortality. To explore the effect modification of urban climate, urban climatic class derived from the Hong Kong Urban Climatic Analysis Map was used to classify participants from the Hong Kong Chinese Elderly cohort into different climate types according to their residential addresses, and a time-stratified case-crossover study was then conducted to estimate the adverse health effects of air pollution in those corresponding climate types. Urban climate was found to increase the mortality risks of pneumonia associated with air pollution. In summary, pre-existing health problems increase elders’ susceptibility to extreme temperature. Seasonal temperature variability is hazardous to human health. Daily temperature and climate may both intensify the adverse health effects of air pollution.