Update of WHO air quality guidelines | SpringerLink

The updated AQG are based on the now extensive body of scientific evidence relating to air pollution and its health consequences. An increasing range of adverse health effects has been linked to air pollution and at ever-lower air pollutant concentrations. This is especially true of airborne particulate matter. New studies use more refined methods and more subtle but sensitive indicators of effects, such as physiological measures (e.g., changes in lung function, inflammation markers). Therefore the updated guidelines are based on these sensitive indicators as well as the most critical population health indicators such as mortality and unscheduled hospitalizations.

Although this information base has gaps and uncertainties, it offers a strong foundation for the recommended guidelines. However, the use of this knowledge for creation of guidelines useful for air quality management is not straightforward.

The experts revising the guidelines agreed that no “threshold” concentration of particulate matter (PM) or ozone had been identified below which there are no adverse health effects. Therefore, no guideline value can be specified that, if achieved, will fully protect human health. However, the previous PM guideline, which comprised a mortality concentration–response function instead of a specific guideline value, was not viewed by the Working Group to be useful guide to policymakers, especially in countries where scientific infrastructure is limited. Moreover, the practical considerations of clarity and applicability of the guidelines for risk communication and management in member states with widely differing levels of local expertise and resources to devote to air quality management led the WHO working group to propose specific guideline values for each pollutant and to identify the residual health risk associated with achieving them.

In setting the PM guideline value, the Working Group considered the range of long-term average PM2.5 concentrations associated with adverse effects on chronic cardiovascular and respiratory disease in epidemiologic studies and set the guideline level—10 μg/m3 as an annual average—at the bottom of this range. The group also took into consideration the results of time-series studies that estimate the effects of short-term exposure to PM2.5 on acute adverse health outcomes conducted in the cities with annual PM2.5 concentrations just above the selected guideline level. Table 1 presents the full set of guideline values from the updated AQG.

Table 1 Updated WHO Air Quality Guideline values

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For ozone, the selected guideline value is still in the range which has been found to increase risk of mortality by some 1–2%. This residual risk was accepted by the group since ozone concentrations at the set guideline level may be due occasionally to natural phenomena, such as intrusion of stratospheric ozone into the troposphere

The Working Group was keenly aware that achieving the AQG might appear impossible in situations where air pollution levels greatly exceed the recommended guideline levels (Fig. 1). Rapid and radical improvement of air quality is rarely possible, and the recommendations have the potential to be easily ignored as not realistic. Therefore, the Working Group recommended a gradual approach to the health risk reduction and improvement of air quality, proposing a set of interim target values in moving towards the strictest guidelines. These targets aim to promote a shift from high air pollutant concentrations, which have acute and serious health consequences, to lower air pollutant concentrations. If these targets are achieved, significant reductions in risks for acute and chronic health effects from air pollution should follow (Table 2). The estimated reduction in health risks associated with the achievement of consecutive interim target levels has been specified in the AQG, allowing the authorities and the public to appreciate the result of air quality improvement results while also providing incentive for further efforts to control air pollution and reduce the risk to health. Progress towards the guideline values should, however, be the ultimate objective of air quality management and health risk reduction in all areas.

Fig. 1

Annual average PM10 concentrations (µg/m3) in selected large cities of the world (WHO 2006a)

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Table 2 WHO air quality guidelines and interim targets for particulate matter: annual mean concentrations

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The AQG have always addressed exposures and health effects of individual pollutants or indicators (such as PM10 mass, an indicator of a complex pollution mixture with multiple sources). However, as understanding of the complexity of the air pollution mixture has improved, the limitations of controlling air pollution and its risk through guidelines for single pollutants have become increasingly apparent. Nitrogen dioxide (NO2), for example, is a product of combustion processes and is generally found in the atmosphere in close association with other primary pollutants, including ultrafine particles. It is itself toxic and is also a precursor of ozone, with which it coexists along with a number of other photochemically generated oxidants. Concentrations of NO2 are often strongly correlated with those of other toxic pollutants. Its concentration is readily measured but needs interpretation as a potential surrogate for a set of sources and the resulting mixture. Achieving guideline concentrations for individual pollutants, such as NO2, may therefore bring public health benefits that exceed those anticipated on the basis of estimates of a single pollutant’s toxicity. Therefore, while the AQG present risk assessments focused on individual pollutants, they also recommend addressing all relevant pollutants in health risk management simultaneously and particularly the four most common, for which guidelines have been reviewed.

Exposure to particulate air pollution from the combustion of solid fuels indoors was estimated to account for 1.6 million deaths in 2000, mainly among women and children in the poorest countries (Smith 2004). The AQG update considered these exposures and the burden of disease that they confer and noted that “Given the recent evidence…it is now reasonable to propose using the same air quality guidelines for both indoor and outdoor exposures.” Although the current guideline values were considered by the WHO Working Group to apply to all non-occupational micro-environments, the update noted “…there is currently a lack of data on indoor air pollution and exposure in vulnerable populations, and such data are not likely to become available [in the near future]…This means that the applicability of [the results of] outdoor air pollution studies to solid fuel use…in developing countries is not …the most critical issue. Rather, the priority is [to develop] guidelines that can be realistically assessed in communities most at risk…and translated into [relevant] standards…” (WHO 2006b, Chapter 9, page 202). Guidelines specifically tailored to indoor combustion of solid fuels are currently being developed by WHO.

The present revision of the AQG provides new guideline values for three of the four pollutants examined. For two of them (particulate matter and ozone), it is possible to derive a quantitative relationship between the concentration of the pollutant, as monitored in ambient air, and specific health outcomes, such as mortality. These relationships are invaluable for health impact assessment and allow insights into the mortality and morbidity burdens from current levels of air pollution as well as the health improvements expected under different air pollution reduction scenarios. The burden-of-disease estimates can also be used to estimate the costs and benefits of interventions that reduce air pollution (AEAT 2006). Approaches to, and the limitations of, health impact assessments are summarized in one of the chapters of the newly published AQG. In particular, selection of the concentration–response functions, reference levels, and assumptions regarding populations for which the impact evaluations are done require special considerations. WHO recently estimated the burden of disease due to exposure to particulate matter above the AQG levels using concentration–response functions as described by Cohen et al. (2004). These estimates indicate that approximately 800,000 premature deaths and more than 6 million years of life lost annually can be attributed to air pollution exposure in large cities of all WHO member states (WHO 2007), with most of the impact occurring in Asia (Fig. 2).

Fig. 2

Estimated impacts of exposure to particulate matter exceeding WHO AQG level in large cities in various regions of the world (see WHO 2007 for definition of the regions)

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