Insights into the new WHO <italic>Global Air Quality Guidelines</italic>

Abstract

<p indent=0mm>Exposure to air pollution has been linked to a massive global disease burden. To protect public health, the World Health Organization (WHO) released new global air quality guidelines (AQGs) on September 22, 2021, based on integrated evidence from > 500 research articles. This document reported recommended AQGs and corresponding interim targets (ITs) for six ambient air pollutants, namely inhalable particles with diameter < 10 micrometers (PM₁₀), fine inhalable particles with diameter < 2.5 micrometers (PM_2.5), ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and carbon monoxide (CO). An AQG represents the theoretical concentration providing minimal risk attributable to single-pollutant exposure, and the corresponding ITs are concentrations associated with various levels of tolerable risk based on quantitative health impact assessment. ITs can act as key references for air quality regulation. WHO last published AQGs in 2006. Compared to the previous guidelines, the new AQGs further limit some air pollutants due to increased confidence regarding their health effects at low levels of exposure. Specifically, the annual limits on PM_2.5, PM₁₀, and NO₂ have been lowered from <sc>10,</sc> <sc>20,</sc> and <sc>40</sc> to <sc>5,</sc> <sc>15,</sc> and <sc>10 μg/m³,</sc> respectively. The 24-hour limits on PM_2.5 and PM₁₀ were lowered from 25 and <sc>50 μg/m³</sc> to <sc>15</sc> and <sc>45 μg/m³,</sc> respectively. The new guidelines offer additional limits on peak-season O₃ <sc>(< 60 μg/m³)</sc> and 24-hour NO₂ (< <sc>25 μg/m³)</sc> for controlling their health impacts. The revised AQGs are applicable to both outdoor and indoor environments but not occupational environments. AQGs also provide qualitative information on the health effects of component-specific particles, including black carbon or elemental carbon, ultrafine particles, and particles originating from sand and dust storms. In this guideline update, WHO recommends strict limits on a few pollutants (e.g., PM_2.5), which should be appropriately interpreted when planning air quality policies in China. Since 2013, China’s clean air actions have rapidly reduced PM_2.5 pollution, supporting a new national ambient air quality standard that is more rigorous than the current level <sc>(35 μg/m³,</sc> equivalent to the WHO first-stage IT). The AQG revision is based on a synthesis of accumulated evidence about the health effects of air pollution. Such evidence has grown rapidly over the past <sc>15 years,</sc> but remains limited for characterizing the complex factors underlying the disease burden of mixed air pollution. Development of the new AQGs depended strongly on observations of the health effects of low-concentration exposures obtained from high-income countries (e.g., Canada and United States). Therefore, application of the AQGs in highly- or moderately-polluted countries should be examined further through local studies. Additionally, the AQGs are applicable to a single exposure type, but not multi-pollutant mixtures, to which people are exposed in the real world. Limits, including those described above, should be considered by governments developing local air quality standards. Development or revision of national ambient air quality standards should be highly reliant on environmental health studies that focused on the local population, with reference to the new WHO AQGs.