As if Hong Kong's air pollution problem was not worrying enough, research from the United States suggests it may be affecting men's chances of fathering a child. In a study published last month in the journal Environmental Health Perspectives, Rebecca Sokol and colleagues from the Keck School of Medicine at the University of Southern California, Los Angeles, found a link between levels of ozone pollution and lower sperm counts among a population of long-term sperm donors. But it is unlikely a similar study could be replicated in Hong Kong, as the city has a scarcity of sperm donors, the number falling from 92 in 2002 to just 44 last year. Fertility levels have been falling around the globe in recent decades and many have pointed the finger at pollution. But until recently, few studies have drawn a strong link between the two. Dr Sokol and her team analysed 5,134 samples from 48 repeat sperm donors in Los Angeles and cross-referenced them with air-pollution levels in the donors' neighbourhoods on days leading up to the time of donation. It takes about 92 days for a sperm to come into being, mature and move into a position ready for deployment. The team correlated the data with information on levels of ozone, nitrogen dioxide and carbon monoxide at periods from 0-9 days, 10-14 days and 70-90 days before collection between January 1996 and December 1998. 'Various chemicals have been implicated as reproductive toxicants,' Dr Sokol writes. 'A number of these chemicals categorised as air pollutants are present in the blood, urine and semen of exposed men and may affect sperm quality. 'We found that an inverse relationship exists between ambient ozone levels and sperm concentration.' Ground-level ozone is formed when sunlight triggers a chemical reaction between volatile organic compounds and oxides of nitrogen. Anthony Hedley, from the University of Hong Kong's department of community medicine, said the results added to the volume of evidence about how detrimental pollution can be to human health. 'It would not be surprising that the inhaling of air pollutants by males can have a damaging toxic effect on the germ cells of the gonads,' Professor Hedley said, noting that all Dr Sokol's donors were non-smokers. 'These are very well described features of sperm pattern in smokers, and the combined effects of air pollution in urban environments are very much like smoking. I've described it as a medical emergency.' At present, there is little information about whether Hong Kong's rapidly worsening air pollution problem is contributing to a drop in fertility in the city. There has not been enough research conducted over a long enough period to justify ironclad conclusions, scientists say. 'We have not conducted any territory-wide survey on the sperm counts of Hong Kong men, hence cannot draw a general picture on the situation as a whole,' said a spokeswoman for the Hong Kong Family Planning Association, which manages the city's sperm bank. 'Our sample group is very small, so small that we believe any generalisation on semen quality is not indicative.' She was unable to explain the rapid decline in the number of donors over the past few years, except to say that it was an individual's choice.