AIR pollution may pose a bigger threat to people's health than smoking. After all, only some people smoke, but no one can escape breathing the air around us. So it is disturbing to learn that a University of Hong Kong study has suggested that the Environmental Protection Department may have picked the wrong targets for its anti-pollution activities. The department has pinpointed taxis and smaller diesel-engined vehicles as prime polluters. It wants their diesel engines replaced by petrol motors, on the correct grounds that the fumes from the latter are easier to control. But the university study suggests that the real culprits are big trucks. They accounted for 48 per cent of nitrogen oxides, and 39 per cent of particulates, in the air, as measured in 1992. These are the two main air pollutants and they may cause breathing problems. Taxis, light buses and small trucks seem to be obvious targets for a clean-up campaign because they pump out black smoke as they move around highly populated city areas. Their exhaust gases look unsightly. The vehicles' emissions should be controlled because the fumes can be trapped in the canyon-like high-rise centres of high population. But if big trucks cause more pollution, they should be the prime concern, particularly as they travel in to city areas, too. This may be another case of appearing to tackle a serious problem while leaving a big cause of it untouched.