Methods
Guideline development was initiated and overseen by a WHO steering group. AQG recommendations and other guidance were informed by systematic reviews and determined by the consensus decisions of a guideline development group (GDG) comprised of subject matter experts in the area of air quality and health. Evidence evaluation was guided by the Grading of Recommendations Assessment, Development and Evaluation approach.6 AQG levels were defined on the basis of moderate or high certainty evidence supporting a link between a pollutant and a particular health outcome over a long-term (annual mean or highest six-month average) or short-term (24-hour) period. Thus, long-term AQGs took precedence over short-term AQGs when both were considered. Additional methodological details can be found in the complete guideline text.
The AQG and interim target recommendations are summarized inTable 1 . Previously established WHO AQGs that are not addressed in this update remain valid. Good practice statements for the management of certain types of health risk-associated PM – black carbon/elemental carbon (BC/EC), ultrafine particles (UFP), and sand and dust storms (SDS) – are listed in Table 2 . Further discussion of the rationale and complete citations for the recommendations can be found in the guideline.