ix As motorized travel in developing countries is concentrated in a few large cities, urban freight and passenger demand will increase at a rate well above the average growth in transport. Projections of emission levels from passenger transport for a sample of large cities in developing countries indicate a doubling or more of emissions by the year 2000, assuming modal shares remain unchanged from a 1980 base level. With the anticipated increase in automobile ownership and the heavy reliance on buses (often poorly-maintained) for public transportation, pollutant emissions from motor vehicles could well be higher by a wide margin. Extrapolating current trends to the year 2000, motor vehicle air pollution in many large cities in the developing world will be much worse than the projected levels for major cities in industrialized countries. The risk of exposure to dangerous levels of air pollution will be high given the dense concentrations of urban population and the life-styles in many developing countries.

How vulnerable cities in developing countries are to the adverse impacts of motor vehicle air pollution depends on a number of factors other than city size, population, and level of motorization. Altitude is an important factor; fuel combustion efficiency of motor vehicles declines with altitude resulting in increased pollutant emissions, particularly of carbon monoxide. Temperate zone cities located in valleys and basins surrounded by mountains commonly experience thermal inversions, which compress pollutants within a reduced ceiling height. Other meteorological factors include direction and speed of prevailing winds, amount of sunlight, precipitation, and humidity. Abundant sunshine is a critical factor in the formation of photochemical ozone and smog as in Mexico City.

The type of built-up environment also has a significant influence -- the canyon effect of tall buildings can cause elevated roadside levels of CO and NOx. On the other hand green spaces, forests and woodlands as well as urban precincts with prohibited or restricted motor vehicle use can help to mitigate the effects of air pollution. Concentration of stationary sources of pollution such as refineries, thermal power plants, and chemical industries within urban boundaries multiplies the level of air pollution and exposes a much larger population to health hazards. Motor vehicle emissions can aggravate the already elevated CO and SOx levels from domestic heating and cooking with coal, lignite, firewood, charcoal and animal dung, as in Seoul, Ankara, Warsaw, and numerous cities in South Asia and Sub-Saharan Africa. Diesel exhaust emissions also add materially to the existing particulate load from natural dust and mineral particles and ash emitted by industry (e.g., cement, steel, and coal-fired thermal power plants).

There are no straightforward generalized approaches to determine the contribution of land transport and particularly motor vehicles to pollutant emissions and ambient pollution concentrations. A case by case assessment is required based on detailed emissions inventories and air quality monitoring programs. But it can be stated with certainty that:

- the contribution of transport to air pollution in developing countries will increase substantially with the projected growth in motorization levels; and