Other factors have impact on local rates of cancer
We refer to the story on a Friends of the Earth report (South China Morning Post, November 11), which claims that pollution in the Pearl River Delta has been responsible for sending 'Hong Kong's rates of cancer and respiratory illnesses soaring'. This is very misleading.
The figures for cancers and hospital admissions quoted in the article are crude rates which cannot be used to make causal inferences about environmental risks and the health of the population.
Since 1985 the population of Hong Kong has increased; the age structure has changed with a large increase in the middle-aged and elderly, and the capacity of the healthcare system has expanded enormously.
The age of a community is a major determinant of its experience of cancer and chronic degenerative disease. Furthermore, there are many lifestyle factors which determine the incidence rates of most cancers and cardio-respiratory diseases. The report acknowledges this but does not name them; they include tobacco smoking and nutrition. We have demonstrated very important effects of air pollution on the health of both children and adults, including hospital admissions and mortality.
There is no question that pollution exerts a powerful short-term effect on morbidity and mortality in those with pre-existing heart and lung diseases and it probably does so on a long-term basis in otherwise healthy non-smokers. Unfortunately there are no local data on long-term effects of air pollution on cancer and other diseases and we are obliged to extrapolate from overseas studies. There should be better resources available to mount long-term studies in Hong Kong. However, the proportional contribution of pollution to health problems overall will predictably be very much smaller than lifestyle risks such as smoking.
Age-specific mortality rates from all causes in Hong Kong, at all ages, have fallen between 1978 and 1998. Crude death rates for lung cancer have risen but age standardised incidence and death rates have fallen and overall the age standardised rates for malignant diseases - and certainly for those which may be influenced by pollution - have fallen (Department of Health 1988-89), indicating a declining risk for the population, given its age. The modest but important reduction in smoking in the 1980s and 1990s, from about 23 to 15 per cent, will have made a major contribution, especially to a reduction in male risks of cancers, cardio-respiratory problems and premature deaths.
On the other hand, the factors influencing hospital admissions are many and complex. They include patient expectations, doctors' referral practices and clinical decision making, and both the facilities and capacity of the healthcare system. All of these cannot be deduced by simply quoting numbers.
In short, there is no evidence to support the speculation that ambient air pollution has contributed to a deterioration in population health in Hong Kong on the scale quoted in the Friends of the Earth report.
We do believe that there is an urgent and badly neglected need to reduce air pollution as an important ubiquitous health hazard, but the science must be valid and not misrepresented.
Air Pollution Research Group
University of Hong Kong and Chinese University of Hong Kong