But campaigners for cleaner city air face a big problem in using the new anti-pollution technology - there is no law that compels the guilty drivers to clean up their emissions.
Yesterday's RAC demonstration in central London, attended by MPs and Robert Atkins, the Environment Minister, was of a temporary unit. It shoots a beam of infra- red light through the exhaust plume of a car. If a detector finds the pollution level to be too high the car is identified on a videotape - all in the space of one-third of a second. Later this week, Southwark Borough Council will set up a permanent monitoring station in the busy Old Kent Road in south London using similar equipment.
Southwark's environmental health officers would like to send letters to the worst polluters asking them to clean up their exhaust emissions. But the Driver and Vehicle Licensing Centre has not yet decided whether to identify the names and addresses of drivers from the photographed number plates.
There is now a check for carbon monoxide in the compulsory, annual MOT test for older cars but passing it is only a legal requirement at the time of the test. Police also have powers to stop cars causing gross, visible pollution such as clouds of soot but many of the most harmful emissions are invisible.
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