Chapter 02 · Nature, land & waterWater, Waste & Pollution
Air quality
Definition
The degree to which the air in a location is free of pollutants, assessed by measuring concentrations of key pollutants — PM2.5, PM10, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide — against health-based benchmarks. The WHO global air quality guidelines, updated in September 2021, recommend an annual PM2.5 limit of 5 µg/m³, halving the previous 2005 guideline of 10 µg/m³.
References
WHO AQG history (1987, 1997, 2005, 2021); annual PM2.5 guideline tightened from 10 to 5 µg/m³; non-anthropogenic sources complicating attainment.
99% of world population not meeting WHO guideline levels (2019); key pollutants and health effects.
Overview
What it means
WHO has issued air quality guidelines since 1987 (updated 1997, 2005, 2021) as science-based benchmarks that inform national standards, which remain far looser in most countries. In 2019, 99% of the world's population lived in areas not meeting WHO guideline levels, and modelling indicates that even eliminating anthropogenic emissions would leave some regions above the new PM2.
5 guideline because of natural sources such as dust and wildfire.
How it is used
Governments set legally binding ambient standards informed by WHO guidelines; cities operate monitoring networks and publish real-time air quality indices (such as the US EPA's Air Quality Index) to communicate daily conditions. Air quality management plans pair monitoring with emission controls on vehicles, industry and domestic burning.
Why it matters
Air quality is the measurable interface between air pollution and public health: standards, indices and guidelines translate pollutant science into enforceable limits and daily public guidance. The 2021 WHO tightening increased the estimated global burden of non-compliance, strengthening the policy case for source controls.