Activists tend to remember landmark victories in the fight against air pollution. But most people would be surprised to hear that it is 25 years since the government took a policy decision which, according to researchers, made a difference to air quality overnight. That was to impose a cap on the sulphur content of industrial fuel used in factories. The year was 1990. Ever since, sadly, other pollution sources have undermined air quality. But a similar landmark victory may not be far off, according to a think tank that specialises in environmental issues. Simon Ng Ka-wing, chief research officer at Civic Exchange, says this is likely when it becomes mandatory for ocean-going vessels to use low-sulphur marine diesel fuel when berthing in Hong Kong, under a proposed law that activists hope will take effect as early as July. Although ship emissions account for only 18 per cent of sulphur dioxide in the city's air, they penetrate residential areas easily. As with the factories in 1990, Ng expects the policy to have a dramatic effect because sulphur emissions will be cut by almost 80 per cent immediately. This is the kind of boost to its environmental credentials the city needs, given the importance of good air quality in attracting the talented people and tourists so important to its economy. The latest data shows that measures such as phasing out dirty diesel vehicles and buses that fail even outdated European emission standards are having a welcome impact. But average concentrations of pollutants remain too high, especially for ozone. The ultimate goal of tackling marine diesel emissions in Hong Kong is a recognised clean-fuel emissions control area (ECA) for the Pearl River Delta, like those in North America, the Baltic Sea and the North Sea. That would depend on participation of Guangdong ports and support from Beijing in the International Maritime Organisation. Meanwhile, if a mandatory cap were to have the predicted dramatic effect, it should prompt the government to redouble efforts to upgrade local bus and truck fleets to combat roadside air pollution.