Motor vehicle traffic is an important source of particulate pollution in cities of the developing world, where rapid growth, coupled with a lack of effective transport and land use planning, may result in harmful levels of fine particles (PM2.5) in the air. However, a lack of air monitoring data hinders health impact assessments and the development of transportation and land use policies that could reduce health burdens due to outdoor air pollution. To address this important need, a study of traffic-related PM2.5 was carried out in the city of Nairobi, Kenya, a model city for sub-Saharan Africa, in July 2009. Sampling was carried out using portable filter-based air samplers carried in backpacks by technicians on weekdays over two weeks at several sites in and around Nairobi ranging from high-traffic roadways to rural background. Mean daytime concentrations of PM2.5 ranged from 10.7 at the rural background site to 98.1 μg/m3 on a sidewalk in the central business district. Horizontal dispersion measurements demonstrated a decrease in PM2.5 concentration from 128.7 to 18.7 μg/m3 over 100 meters downwind of a major intersection in Nairobi. A vertical dispersion experiment revealed a decrease from 119.5 μg/m3 at street level to 42.8 μg/m3 on a third-floor rooftop in the central business district. Though not directly comparable to air quality guidelines, which are based on 24-hour or annual averages, the urban concentrations we observed raise concern with regard to public health and related policy. Taken together with survey data on commuting patterns within Nairobi, these results suggest that many Nairobi residents are exposed on a regular basis to elevated concentrations of fine particle air pollution, with potentially serious long-term implications for health.

Few studies examine urban air pollution in sub-Saharan Africa (SSA), yet urbanization rates there are among the highest in the world. In this study, we measured 8-hr average occupational exposure levels of fine particulate matter (PM2.5), black carbon (BC), ultra violet active-particulate matter (UV-PM), and trace elements for individuals who worked along roadways in Nairobi, specifically bus drivers, garage workers, street vendors, and women who worked inside informal settlements. We found BC and re-suspended dust were important contributors to PM2.5 levels for all study populations, particularly among bus drivers, while PM2.5 exposure levels for garage workers, street vendors, and informal settlement residents were not statistically different from each other. We also found a strong signal for biomass emissions and trash burning, which is common in Nairobi’s low-income areas and open-air garages. These results suggest that the large portion of urban residents in SSA who walk along roadways would benefit from air quality regulations targeting roadway emissions from diesel vehicles, dust, and trash burning. This is the first study to measure occupational exposure to urban air pollution in SSA and results imply that roadway emissions are a serious public health concern.

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We conducted an urban "metabolic" study of inputs and outputs of food, water, energy, and pollutants from Los Angeles County, USA. This region has been the subject of recent debate about the nature of population density and distribution as it relates to urban form and associated environmental impacts. We found that with the exception of food imports and wastewater outputs, inputs of resources and outputs of pollutants generally declined on a per capita basis from 1990 to 2000. Reductions likely reflected a combination of changes in public policy, improvements in technology and public infrastructure, and impacts of increases in population density. However, in comparison to other municipalities and urban regions, resource consumption per capita was still relatively high in some categories, particularly inputs of water and transportation energy. Per capita greenhouse gas emissions were lower in Los Angeles County than for the average of the USA as a whole but higher than previous analyses of urban areas internationally, largely due to comparatively high emissions from the transportation sector. Wastewater discharge accounted for less than 40% of water inputs excluding precipitation; however the partitioning of water outputs through other potential losses, specifically evapotranspiration, runoff, and groundwater recharge, remains highly uncertain. We suggest that more detailed information about water losses will greatly improve future mass and energy accounting for analyses of urban environmental sustainability in this semi-arid region.