Table 2.

References to thresholds published by the EPA and WHO for PM_2.5, NO₂, and 8-hour max O_3.

Pollutant	EPA threshold	EPA reference	WHO threshold	WHO reference
PM2.5	12 µg/m3	2020 NAAQS primary annual mean, averaged over 3 years standard (https://www.govinfo.gov/content/pkg/FR-2020-12-18/pdf/2020-27125.pdf)	5 µg/m3	2020 long-term/annual AQG (https://www.who.int/publications/i/item/9789240034228)
NO2	53 ppb	2018 NAAQS primary 1-year annual mean standard (https://www.federalregister.gov/documents/2018/04/18/2018-07741/review-of-the-primary-national-ambient-air-quality-standards-for-oxides-of-nitrogen)	5 ppb	2020 long-term/annual AQG of 10 µg/m3, which corresponds to approximately 5.07 ppb. Same reference as above.
O3	70 ppb	2020 NAAQS primary annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years (https://www.govinfo.gov/content/pkg/FR-2015-10-26/pdf/2015-26594.pdf)	30 ppb	2020 long-term, peak season AQG of 60 µg/m3 (which corresponds to approx. 30 ppb). Same reference as above.

Abbreviations: AQG, Air Quality Guidelines; EPA, Environmental Protection Agency; NAAQS, National Ambient Air Quality Standards; WHO, World Health Organization.