Cars and trucks produce by far the biggest share of fossil-fuel emissions (47 percent by one measure).
The internal combustion engine of petroleum-powered motor vehicles discharges carbon monoxide, lead, nitrogen oxides, aldehydes, and ethylene and other aliphatic hydrocarbons into the atmosphere only a few feet from the breathing zone of the population. Local concentrations of these substances reach appreciable levels, which are highest in urban centres where traffic density is greatest, at major intersections and in so-called 'canyon streets'. Under conditions of poor natural ventilation and strong sunlight, a complex series of reactions takes place between the nitrogen oxides and hydrocarbons leading to the formation of ozone, peroxyacyl nitrates (PAN), and several other substances (usually grouped together as 'photochemical oxidants'). This more extensive type of motor vehicle pollution affects the entire air environment of a community (Los Angeles 'smog'). Emissions of aliphatic hydrocarbons other than ethylene are not considered of importance, except in the causation of photochemical pollution. This may be significant regarding damage to forests and crops.
There are four recognized sources of pollution from the ordinary automobile, namely, the exhaust pipe, the crank case, the carburettor and the fuel tank. Tyre and road dust and asbestos particles from brake linings are not generally included in any discussion of the problem; some pollution from these sources will certainly continue, even if all other emissions can be eliminated. The distribution of pollutants according to sources within the vehicle are: (a) Evaporation losses, tank and carburettor (20% of the hydrocarbons); (b) Crank case blow-by (25% of the hydrocarbons); (c) Exhaust (55% of hydrocarbons and almost all of the lead, carbon monoxide and nitrogen oxides).
Exhaust gases from diesel engines contain negligible amounts of carbon monoxide, no lead and somewhat lower amounts of the lighter hydrocarbons per unit of fuel consumed than are emitted by petrol-powered vehicles. They are nevertheless responsible for much public criticism because they are offensive and malodorous and have a high content of particulate matter. Because diesel vehicles are small in number compared to petrol-powered vehicles, their overall contribution to pollution levels is not great. Nevertheless, because they often discharge pollutants in close proximity to people, they must be regarded seriously, and in cities diesels make a significant contribution to soiling of buildings and materials.
In most parts of the world, the traditional problems associated with industrial production and the consumption of fuels are still of more importance than those due to pollution by motor vehicles. By contrast, in the USA, the motor vehicle is considered the main source of pollution. Some 60% of the total weight of pollutants discharged into the atmosphere of the USA originates from this source. It has been estimated that an uncontrolled vehicle in the USA covering 12,000 miles a year emits the following pollutants: Exhaust: hydrocarbons 135 kg; carbon monoxide 765 kg; nitrogen oxides 40 kg; Crank case: hydrocarbons 60 kg; Evaporation: hydrocarbons 40 kg.
In one year, the average gas-powered car also produces four tonnes of carbon dioxide (CO2) which, as it slowly builds up in the atmosphere, causes global warming. Every litre of gasoline burned up in an auto engine sends over 4 kg of CO2, containing 1 kg of pure carbon, into the atmosphere.
Air pollution from motor vehicles in the developing countries does not yet present a problem of such magnitude as in the highly industrialized countries. The number of cars in use is relatively small and the pollution caused by them is much less than that from industrial complexes. A typical example of such a situation is found in India, where large installations, such as chemical or petrochemical complexes, fertilizer and power plants surround, or are scattered among, most of the large cities. The discharges from these installations are so great that, proportionally, pollution from automobiles is insignificant, except in a few large cities.
However, the effects from automobile exhausts in these cities have some similarity to those in industrial cities in advanced countries because many vehicles have a high weight-to-horsepower ratio and they are often old and poorly maintained. The horsepower of 85% of the cars in India is between 10 and 14, and 60% of all vehicles are more than ten years old. Also maintenance is poor because spare parts are expensive or unavailable and technical competence is low. Consequently pollution is out of all proportion to the number of cars in circulation. Carbon monoxide peaks of 100 ppm have been recorded at street level at important intersections. As the number of vehicles continues to increase, it is expected that oxidant pollution may become a problem in other cities if control measures are not introduced.
When vehicles are first started they take some time for the engine to reach its normal operating temperature. During this period the emissions are substantially higher than when the engine is hot. This is true of petrol engined vehicles without catalysts and those with three-way catalytic converters. Catalytic converters do not become effective until they have reached their normal operating temperature. Vehicles making short journeys or driving in congested traffic in cold weather may not reach their full operating temperature throughout their journey.
In 1998, the California Air Resources Board identified diesel exhaust as a "Toxic Air Contaminant" based on a review of animal and epidemiological studies, which strongly suggest a causal relationship between occupational diesel exhaust exposure and lung cancer. Already in 1989, the International Agency for Research on Cancer (IARC) had concluded that diesel engine exhaust is "probably carcinogenic to humans" (Group 2A), while gasoline engine exhaust was classified as "possibly carcinogenic to humans" (Group 2B).
Data gathered by Environment Canada indicates that overall greenhouse gas emissions in British Columbia increased by 15 percent between 1990 and 1995. The transportation sector was responsible for the majority of that increase, its emissions climbing by 23 percent during the same period.
Long term exposure to exhaust fumes caused the premature death in 1998 of 21,000 people from respiratory and heart disease in the three countries, compared with nearly 10,000 deaths from accidents. In the three countries 300,000 additional cases of bronchitis in children was related to vehicle emissions, as were 15,000 hospital admissions for heart disease, 395,000 adult asthma attacks and 162,000 in children.