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The “classic” toxic gas compounds found in diesel exhaust include carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO2), and sulfur dioxide (SO2). CO and NOx (i.e., the sum of NO and NO2) are regulated diesel emissions. SO2 emissions from diesel engines, even though not directly regulated, are controlled by diesel fuel standards which legislate increasingly lower sulfur content. In most countries, these gases have also maximum exposure limits established by occupational health authorities.
The above gases are present in diesel exhaust in relatively high quantities, from a few ppm for SO2 to as high as 1000 ppm and higher for NOx. Their health effects vary from acute poisoning, resulting from exposure to high concentrations of gases, to long term urban air pollution effects. While the acute poisoning by CO, NO2, or SO2 are fairly well known and understood, their air pollution effects have not been quantified and are still subject of studies by medical sciences.
The task of quantifying the health effects of air pollution and correlating them with the exposure to particular pollutants is very difficult. Air pollution is seldom caused by a single compound. In so far as pollutants have common sources, fluctuations in their ambient concentrations tend to be correlated with each other. It can, therefore, be difficult to ascertain which pollutant is chiefly responsible for the adverse effects of pollution, or whether they are attributable to the mixture as such. The difficulty is further increased by imprecisions in estimating exposures, influence of other variables (e.g. cigarette smoking), etc. Due to these reasons, the results of various epidemiological studies have not been entirely consistent. Much more work is still needed before the health effects of air pollution and the importance of particular pollutants are better understood.