Legislation on the permissible composition of exhaust is being continuously tightened. The figure shows the periodic change in the limits of motor vehicle emissions. Apart from a progressive decrease of the permissible carbon monoxide emissions since 1977, it has been required by law that the sums of HC and NOX are maintained.
The new legislation applicable for Europe since 1988, no longer uses the vehicle mass as a basis for evaluation, instead the engine displacement. From that point of view, the numerical values can only be compared with each other to a limited extent.
In the context of the indicated regulations, questions arise as to what direct effect the mentioned exhaust components have on human beings and at what concentrations serious consequences on health have to be taken into account.
Carbon monoxide is a severe blood poison. Its affinity to hemoglobin is about 300 times larger than oxygen. Half an hour exposure to air with 0.3 Vol-% CO can lead to fatal internal asphyxiation. Due to the additive effect, a long exposure to air with a low CO-concentration can be even more dangerous than a short one in a high concentration. Tests have shown that exposure to a CO-concentration of 30 ppm for a couple of hours, can already lead to respiratory difficulties.
Unburned hydrocarbons have adverse effects on health only at high concentrations. However, partially burned hydrocarbons are undesirable due to their annoying smoke and irritation to the eyes and breathing.
Oxides of nitrogen, similar to carbon monoxide, are severe blood poisons. A concentration of 700 ppm leads to fatal lung oedema. The MAK-value (maximum concentration at workplace) amounts to 25 ppm.
In addition, compounds of lead in engine emission resulting from the use leaded fuel are poisonous and are transferred into the human body mainly through inhaled air and also through the stomach and intestine.
Particulate emissions are also poisonous for human beings, most of all due to substances present in the particles which have been found to be carcinogenic.
In contrast to the direct effects on human health, the influence of indirect effects upon human living space is increasingly gaining in significance in discussions related to pollution. In this case one should mention the responsibility of vehicle emissions in forest depletion which has not entirely been investigated. Moreover, the atmospheric temperature increases as a result of the increasing concentrations of carbon dioxide in the air due the high consumption of fossil fuels (Greenhouse effect). Vehicles contribute considerably to this effect.
Repeated attempts have been made in the past in order to use renewable fuels instead of fossil fuels to power vehicles. Fuels made out of a bio-mass are theoretically CO2 neutral and their application in combustion engines technically does not present a problem. On the other hand, there are a number of disadvantages. Almost half of the energy generated is re-used for production and processing. The surface area required in order to produce energy worth mentioning is considerable. The production is inevitably connected with the emission of new pollutants (fertilizers, pesticides). At the moment biological fuels are generally uneconomical compared to crude oil. In the 70s and 80s Brazil is the only country that took essential steps to promote alternative fuels (methanol derived from sugarcane), but has today given up.