Your reports on the health benefits of the reduction of sulphur in fuel since 1990 (South China Morning Post, November 23) have generated much interest and many inquiries to us.

It is clear that some people are puzzled by the reported statement that the intervention (the sulphur restriction regulations) 'was estimated to result in an [average] 20-day increase in life expectancy for women and 40 days for men'. This is thought to be incongruously small when set against our estimate that mortality declined by 2.2 per cent annually, with 600 deaths avoided each year. The following may help clarify the important benefits of this improvement in air quality:

The figures 20 to 40 days are the gains per year for everyone in the whole population and only refer to the beneficial impact of reduced exposures to sulphur pollutants immediately after the intervention. In fact, the calculation is based on the difference in death rates just two years before and two years after the intervention. So, for example, we estimated conservatively that for a 25-year-old male in 1991, with benefits of reduced exposure for 15 years, average life expectancy would be extended by 1.5 years;