HONG Kong's road tunnels are among the busiest in the world. The traffic, though clean and safe by any Asian measure, is still far from meeting top international vehicle emissions standards. Yet, according to the Environmental Protection Committee, ventilation fans in tunnels are allowed to operate at less than full capacity for three quarters, or more, of the time. Tunnel operators are required to run them only when pollution rises to levels at which a person exposed for one hour is at risk. Even in Hong Kong's worst traffic, it is rare for anyone to be in a tunnel for a whole hour. But repeated exposure is dangerous and tunnel operators should not be saving electricity costs at the expense of public health. The Government is now rightly revising these levels. But still more realistic standards are vital. The most effective safety measure of all - and one which would obviate the need for expensive pollution detection devices to switch fans on automatically -would be to ensure fans ran at full capacity 100 per cent of the time.