I. WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide – 2021

The 2021 WHO air quality guidelines encompass a comprehensive framework applicable to both outdoor and indoor environments, covering various settings where people spend their time¹⁰⁷. While these guidelines extend to numerous contexts, they do not address occupational settings due to unique exposure characteristics and risk reduction policies, along with potential differences in population susceptibility within the adult workforce¹⁰⁸.

The primary objective of these updated global guidelines is to provide evidence-based quantitative recommendations for air quality management, offering long- or short-term concentration values for key air pollutants. Exceeding the air quality guideline levels is linked to significant public health risks. Although not legally binding standards, these guidelines equip World Health Organisation member states with an evidence-driven tool to shape legislation and policies, with the ultimate aim of reducing air pollutant levels and alleviating the substantial global health burden from air pollution exposure¹⁰⁹.

Table 2: Recommended AQG levels and interim targets (Extracted from WHO IAQ Guidelines 2010)

Pollutant	Averaging time	Interim target				AQG level
		1	2	3	4
PM2.5, µg/m3	Annual	35	25	15	10	5
	24-hour[1]	75	50	37.5	25	15
PM10, µg/m3	Annual	70	50	30	20	15
	24-hour[2]	150	100	75	50	45
O3, µg/m3	Peak season[3]	100	70	–	–	60
	8-hour[4]	160	120	–	–	100
NO2, µg/m3	Annual	40	30	20	–	10
	24-hour[5]	120	50	–	–	25
SO2, µg/m3	24-hour[6]	125	50	–	–	40
CO, mg/m3	24-hour[7]	7	–	–	–	4

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[1] 99th percentile (i.e. 3–4 exceedance days per year).

[2] 99th percentile (i.e. 3–4 exceedance days per year).

[3] Average of daily maximum 8-hour mean O3 concentration in the six consecutive months with the highest six-month running-average O3 concentration.

[4] 99th percentile (i.e. 3–4 exceedance days per year).

[5] 99th percentile (i.e. 3–4 exceedance days per year).

[6] 99th percentile (i.e. 3–4 exceedance days per year).

[7] 99th percentile (i.e. 3–4 exceedance days per year).

Table 2.1. Air quality guidelines for nitrogen dioxide, sulfur dioxide and carbon monoxide (short averaging times) that were not re-evaluated and remain valid (WHO, 2021)

Pollutant	Averaging time	Air quality guidelines that remain valid
NO2, µg/m3 1-hour 200	1-hour	200
SO2, µg/m3 10-minute 500	10-minute	500
CO, mg/m3	8-hour	10
	1-hour	35
	15-minute	100

References

107 The World Health Organisation, “WHO Global Air Quality Guidelines,” the World Health Organisation, 2021. Available online: https://apps.who.int/iris/bitstream/handle/10665/345334/9789240034433-eng.pdf?sequence=1&isAllowed=y

108 The World Health Organisation, “WHO Global Air Quality Guidelines,” the World Health Organisation, 2021. Available online: https://apps.who.int/iris/bitstream/handle/10665/345334/9789240034433-eng.pdf?sequence=1&isAllowed=y

109 The World Health Organisation, “WHO Global Air Quality Guidelines,” the World Health Organisation, 2021. Available online: https://apps.who.int/iris/bitstream/handle/10665/345334/9789240034433-eng.pdf?sequence=1&isAllowed=y