THE sun's early rays are fighting their way through the blue exhaust haze over Phra Kanong junction by the time traffic sergeant Phooyai Boonmun has wrapped up against the creeping death all around him. Gas mask for the fumes. Goggles, helmet and gloves for the suspended carbon particles. Scarf and thick clothing for the dust; earplugs for the noise and a plastic sheet in case the monsoon rains start running with black ooze. ''Even with all this protection, my wife still fears for my health. I watch my colleagues growing ill each day from the foul air and wonder when my time will come,'' said Phooyai. The 1,750 policemen who direct traffic in congested Bangkok are at the frontline of the world's most dangerous air pollution, exposed like laboratory rats to a critical test of human tolerance. Japanese scientists gave Bangkok the worst rating of any major city after testing air quality at 30 intersections earlier this month. They concluded most of it was unfit for breathing. In another study, researchers from the University of Hawaii and Bangkok's Mahidol University found 18 types of cancer-causing agents circulating in the black pall of dust and fumes that permanently hangs over the city of 8.5 million people. These included benzine, known to cause leukaemia in some types of petrochemical processes, toulene vapours - which can damage the kidneys, liver and nerve system - and pentane, a respiratory irritant. Members of the US team were so horrified by the findings that they refused to return to the city for fear of contracting lung cancer themselves. ''The Japanese study confirmed that Bangkok has a more severe risk of carbon monoxide poisoning than any other city in the world. It's not a situation we like, but perhaps this will force something to be done about it,'' said Bangkok public health director Chaiyan Kampanatsaenyakorn. A 1989 report by the Population Action Institute in Washington had already given Bangkok the lowest rating on a quality of life index, which measured air, water and noise pollution: it was two points behind other cities. Police doctors monitoring the deteriorating health of their officers reported late last month that air pollution was twice the safety limit set by the World Health Organisation. Yet health effects of the pollution remain largely undocumented because of the difficulty of establishing causal links. While carbon monoxide is believed to weaken heart muscles by depriving the body of oxygen, the evidence is inconclusive. Lead from fuel is thought to add to brain retardation, especially in children, and chemical additives have been proven to damage bone marrow and cause various cancers, but only after sustained exposure. Doctors have tentatively linked contaminated dust to the deaths of at least 1,400 people a year and say as many as 1.7 million Bangkokians suffer from bacteriological diseases. Blood tests of 100,000 people have shown evidence of large lead intake, while400 deaths have been traced directly to the toxic metal. It is not certain, though, that all originated from car omissions. ''The largest number of people going to have treatment at hospital clinics in the last few years has been for respiratory illnesses. If this trend continues for the long term, I am afraid about cancers because of the build-up of air carcinogens from the exhausts of cars,'' said Professor Thephanom Muangman, dean of environmental studies at Mahidol University, who led the Thai-Hawaiian team. Most of the pollution is blamed on the city's traffic gridlock, with 500 new cars being added daily to the 2.4 million motor vehicles and one million motorcyclists already on the roads. About 85 million tonnes of carbon dioxide is spewed out in Thailand each year, with 80 per cent coming from cars. Power generation, forest burning and fossil fuels contribute the rest. The Government is moving some of the 20,000 factories out of Bangkok to reduce the output of industrial pollutants, but this is not expected to improve air quality greatly. Lead-free petrol has been available for two years, and catalytic converters some compulsory in cars from last month. Both, however, have a reputation for being expensive and only late-model vehicles will immediately benefit. There are regulations on exhaust emissions, but none is strictly enforced. Many of the two-stroke motorcycles that cram every available inch of road space - reputedly the largest number on any city's roads - remove their mufflers altogether in a blast ofdefiance against the authorities. With little parkland to absorb the fumes and dust, most end up trapped in narrow streets that were never intended for large numbers of vehicles. Car movement averages only 4 kilometres per hour in peak traffic, exposing millions of commuters to lethal gases for up to five hours a day. Police have published a list of 22 intersections where pedestrians and passengers on non-air-conditioned public transport may literally be taking their life in their hands. At the worst road junction, dust particles are 37 per cent greater than accepted Thai standards, and the citywide average is 23 per cent above. But the permitted pollution safety levels are themselves highly suspect. ''Part of the problem is the standards that Thailand uses to measure air pollution, which sometimes allow as much as double the levels overseas,'' said Professor Thephanom. ''Here, we set the threshold for lead at 10 microgrammes per 10 cubic metres of air. In America, if it's above 1.5mg, it's considered to be abnormal, but in our streets it varies from 0.3mg to 3mg - and Thailand says this is normal.'' Mahidol University has warned that bus drivers inhaling carbon monoxide through open windows for up to 12 hours at a time are a prime cause of road accidents, which rise sharply in the afternoon hours. Drivers were found to be suffering from mental disorders and even losing consciousness at the wheels after constantly breathing in the fumes. ''Of course it's difficult to stay awake, so most of the drivers take pills [amphetamines]: then you don't notice anything much after a while,'' said one driver with the government bus service. ''Some drivers have to take time off for breathing problems.But the traffic jams are a worse problem for us than the pollution.'' The government department responsible for measuring air pollution has no physicians on its staff and has limited authority to enforce regulations, even when they do exist. With few public information campaigns, awareness of the potential menace from exhaust emissions remains low. ''Toxic gas is not visible and therefore it is difficult to make people realise its hazards,'' said Major-General Somsak Pathumrak, commander of the police traffic division. Traffic police who spend their entire working hours at smog-shrouded intersections, have become a gauge for measuring the worsening effects of the pollution. Twenty per cent are suffering from heart ailments blamed on respiratory problems, almost a quarter have hearing defects and one in 10 has contracted pneumonia. ''None of us can breathe properly anymore, our eyes burn, we can't sleep at night and we get ill-tempered. We would all like to quit, but where does that leave Bangkok?'' said Sergeant Phooyai. ''The police know better than anyone what is happening here.I just hope the rest of this city wakes up while there is still time.''