More advanced models can capture the spatial variation of air pollution levels within cities. Photo: © TonyV3112 /

WHO guidelines are based on science – policy should follow

Detailed models and data from more parts of the world are behind the lower acceptable levels for particles, ozone and nitrogen dioxide in the new WHO air quality guidelines.

The new air quality guidelines from the World Health Organization (WHO)were published in September and the acceptable levels have been lowered for almost all pollutants (figure). This means that we now have the evidence that air pollution is harmful for health at much lower levels than previously believed.

The previous WHO air quality guidelines (AQGs) were published more than 15 years ago. Since they were issued, air pollution research has rapidly developed. Air pollution is now recognised as the single biggest environmental threat to human health. Although air quality in past decades has improved gradually in high-income countries, there are still many areas that do not reach the levels recommended in the previous WHO AQGs. Air quality has generally deteriorated in most low- and middle-income countries, in step with large-scale urbanisation and economic development that rely on the burning of fossil fuels. Disparities in air pollution exposure are, therefore, increasing worldwide.

Since the 2005 air quality guidelines were established, thousands of new studies have continued to document the adverse health effects of air pollution. During this time, enormous advances have also occurred in exposure assessment. In particular, the use of satellite remote-sensing instruments in combination with advanced chemical transport models and ground-based measurements has substantially improved the understanding of pollution levels at a much higher spatial scale. For example, the WHO AQG from 2005 for annual mean PM2.5 was largely based on results from two American studies where exposure to PM2.5 was assessed from just a few monitoring sites per city. Today’s more advanced models can capture the spatial variation of air pollution levels within cities, which gives us a better understanding of the impact of living in the more polluted parts of the city, compared with cleaner areas. This has been useful not only for population studies of health effects but also to estimate the worldwide health impact of air pollution

These new methods of exposure assessment have facilitated studies of nationwide populations, not just in cities but also in rural areas where air pollution monitoring is sparse or even absent. Studies conducted in low- and middle-income countries where concentrations are high are of great importance; however, equally important are studies in very clean areas, as they can answer important questions on the effects of low-level exposures and the evaluation of thresholds. These studies provide critical information on the benefits that might be expected if air pollution levels were reduced worldwide. In view of these many advances, revision of the old WHO AQGs was both timely and necessary and followed a rigorous predefined process outlined in the WHO handbook for guideline development. A long-term AQG level is defined as the lowest exposure level to an air pollutant above which there is confidence that there is an increase in adverse health effects. The gap between the WHO air quality guidelines and the levels adopted in national regulations reflects the policy-making process. Whereas the WHO guidelines are evidence-informed, health-oriented recommendations, the process of developing legally binding regulations is driven by national policymakers and the willingness to set environmental standards. This process involves different actors and may be influenced by a range of economic drivers and powerful lobby organisations, but the new WHO AQGs are what we should strive for if we value health.

Ebba Malmqvist


In this issue