Urban air pollution:

a new look at an old problem

WHO estimates that air pollution causes more than 2 million premature deaths each year (WHO 2006). In 1987, WHO published Air Quality Guidelines for Europe. In 2005, following important new research from low- and middle-income countries, it released new Air Quality Guidelines for four common pollutants (PM, O3, NO2, and SO2) that are intended to inform policymakers on appropriate targets related to air-quality management (Table 1; WHO 2006).

Major international events have often helped draw attention to air-quality issues and attract international research efforts. For example, in the run-up to the 2008 Beijing Olympics an international collaborative research project, Campaigns of Air Quality Research in Beijing and Surrounding Regions (CAREBEIJING), was implemented. The project was designed to study the regional air pollution processes that affect air quality in Beijing and to formulate air-pollution control strategies for the Olympics. When considered on an average annual basis, the air quality in Beijing has certainly improved although the recent smog suggests that there is some distance to travel.

Ultimately, however, the old adage “You can’t manage what you can’t measure” applies here. Successful implementation and enforcement of air-quality measures is dependent on access to scientific information. The megacities assessment clearly shows the disparity in scientific knowledge across large cities. Cities that have had access to high-quality information – Los Angeles, for example – have benefited. Could newer cities learn from the experience of older cities to avoid teething troubles?

Experience shows that this can indeed be the case. A prime example is shown in Figure 2, which shows how ozone and PM levels changed through time in three similarly sized megacities: Los Angeles, Mexico City and Beijing. During the time period depicted in the figure, each city had a population within ± 40% of the average of the three cities. In Los Angeles, ozone concentrations peaked in the early 1970s and have been decreasing over the past four decades. We can see that the ozone levels in Mexico City never reached the peaks of Los Angeles and levels fell faster. Today, the ozone and PM concentrations in Los Angeles and Mexico City are nearly the same. Mexico City avoided severe air pollution by translating the lessons of Los Angeles to implement emission controls.

Beijing, however, seems to be on a different track. PM concentrations remain higher than the two other cities but are decreasing. In contrast, ozone concentrations, which have been low, are rising. As mentioned earlier, Beijing has taken numerous measures during the past few years to control air pollution. Assessing how these measures are working will not only help Beijing to fine-tune its policies but will also hold lessons for more recently urbanising centres.  

The WMO/IGAC Impacts of Megacities on Air Pollution and Climate assessment sought to evaluate the current state of knowledge and identify gaps. It reveals that the quality and quantity of available information is highly variable, with air pollution in some cities being much better characterised than that in others. The gaps underscore the need for continued international efforts to measure urban air quality and use this information to improve air quality while simultaneously mitigating climate change.