A FEW of Hong Kong's car exhausts are so poisonous as to be potentially lethal, according to the American inventor of a device used to monitor exhausts in Central. The device, invented by University of Denver chemist Dr Donald Stedman, was demonstrated to the Environmental Protection Department last week as a way of better monitoring pollution from vehicle exhausts. Dr Stedman said at the monitoring site chosen by the department - an entry ramp feeding traffic on to Cotton Tree Drive - that the device had measured some ''absurdly'' high levels of deadly carbon monoxide emissions. The device uses an infrared beam across the road to detect exhaust emissions and grades the levels of carbon monoxide and hydrocarbon emissions from one to greater than 10. In the first two days of testing, , a few cars had registered carbon monoxide readings of between eight and 11. ''That sort of concentration will kill someone in six minutes,'' he said. ''If you are sitting behind it in a tunnel in a traffic jam you are in trouble.'' The biggest danger was to the driver of a car with a carbon monoxide reading greater than eight, especially if rust had eaten holes in the vehicle's chassis. ''There are 500 accidental deaths per year in the US . . . caused by people inhaling the exhausts of their own cars and these are not the suicides,'' Dr Stedman said. ''Typically, you're talking about an old, rusty car stuck in a tunnel in a traffic jam.'' The department said it could not recall anyone dying this way in a traffic jam in one of Hong Kong's tunnels. Its own research two years ago had shown carbon monoxide levels were not a problem within tunnels. ''We have not found any problem in tunnels with carbon monoxide and I think we need to look at what sort of level he is talking about before we can comment on whether we might be concerned about this,'' principal environmental officer Raymond Leung Pak-ming said. The department has yet to analyse Dr Stedman's results. Overall, Dr Stedman said the results showed the majority of vehicles were ''clean''. The remote-sensing device - already in use in Canada, Sweden and the US - is being toured through Asia under a trade programme funded by the United States-Asia Environmental Partnership. The device is a combination of video, remote-sensing and computer technology. A video camera takes pictures of the rear of each car which crosses the infrared beam, and immediately the picture is shown on a screen with emission readings overlaid. At present, the department employs spotters to look for black exhaust emissions. The vehicles they identify are required to attend test centres. and, if found faulty, to be repaired.