Rapid urbanization, motorization and economic growth are contributors to an increasing air pollution problem in most large developing urban centers. Comparative risk assessment and health studies, carried out in a number of cities (e.g., Bangkok, Cairo, Mexico City, Quito, Santiago, cities of Central and Eastern Europe), have indicated that typically fine suspended particulates (PM10 and smaller) and exposure to lead (Pb) cause the greatest damage to human health. Other pollutants of concern are sulfur dioxide (SO2) (to the extent it contributes to fine particulates and long-range environmental damage); ozone (O3) (mainly in warmer, sunny locations with unfavorable topographic conditions), volatile organic compounds (VOC) (some of which are known carcinogens); nitrogen oxides (NOx) (as contributors to ozone formation) and carbon monoxide (CO) (associated with global warming).
Main Sources of Pollution
Anthropogenic air pollution originates from (i) large stationary sources (industries, power plants and municipal incinerators); (ii) small stationary sources (households and small commercial boilers); and (iii) mobile sources (traffic) (Figure 1). Many of these sources are closely related to the production and consumption of energy, especially fossil fuels. Besides power plants and industries, domestic fossil fuel use, especially heavy fuel oil, biomass, and brown coal, is a significant source of ambient particulates and sulfur dioxide, especially in temperate regions (e.g., in China, Eastern Europe). Traffic is a large contributor to both particulate and sulfur emissions in cities with frequent traffic congestion of large and poorly maintained vehicle fleets using high-sulfur diesel fuel (e.g. in Asia). In cities where leaded gasoline is still used, traffic may contribute 80-90 percent of atmospheric lead concentrations, while poorly controlled emissions from lead smelters could be also significant factors. The role of natural and anthropogenic sources are equally important in the formation of ground-level ozone: natural sources, such as biogenic emissions from plants and trees, and traffic emissions are the largest sources of atmospheric VOC; while natural, mobile and stationary combustion sources are all significant contributors to NOx concentrations. Motor vehicles are typically responsible for the majority of CO emissions.
The impact of emissions on human exposures depends on the location and dispersion of pollution: large stationary sources, often located farther away from most densely populated city centers, disperse into higher layers of the atmosphere, while households and traffic emit near ground levels in highly populated areas. As a result, mobile and small stationary sources contribute more to ambient urban pollutant concentrations, and resulting health effects, than their share in total emission loads indicates.
Alternatives to Reduce the Harmful Impacts of Pollution
Measures to mitigate the negative effects of pollution may focus on (i) separating pollution sources and receptors; (ii) reducing the polluting activity; (iii) reducing its pollution characteristics; and (iv) controlling emissions with filtering devices. Not all of these alternatives are available for all pollutants. Changing the location of pollution source may be an effective strategy for universally mixed pollutants with only localized health effects (such as particulates). Urban planning, zoning, and other land use regulations can influence urban air quality through micro-level decisions. However, these measures are not effective for persistent pollutants (such as heavy metals) and pollutants with significant regional and global impacts (such as SO2 and CO2). Opportunities for applying alternatives of emission reduction also vary across pollution sources:
- The impact of emissions from large stationary sources can be reduced by (i) choosing a location away from populated areas; (ii) using clean fuels such as gas, low-sulfur or low-ash coal; (iii) applying cleaner technologies such as fluidized bed combustion or low-NOx burners; (iv) improving maintenance and housekeeping; and (v) installing proper end-of-pipe control technologies such as electrostatic precipitators and baghouses.
- The impacts of traffic-related emissions may be mitigated by (i) diverting traffic away from heavily populated areas (for example, building a ring road around cities; restricting downtown traffic); (ii) converting high-use vehicles to cleaner fuels (for example, buses to natural gas); (iii) improving vehicle maintenance; (iv) increasing the share of less polluting traffic modes; (v) using more fuel efficient vehicles; and (vi) installing catalytic control devices. However, supply-side traffic management measures aimed at reducing congestion (for example, by improving road infrastructure) rarely lead to significant overall emission reduction due to their effects on increased traffic flows.
The emissions from households and other small stationary sources can be reduced most effectively by converting them to cleaner fuels.