There are two main number systems for outdoor air quality in widespread use, and they exist for different reasons. U.S. EPA NAAQS are enforceable regulatory limits; they balance health protection against attainment cost and technological feasibility. WHO 2021 air quality guidelines are non-binding health-based targets; they describe the air quality at which health risks become small, without weighing what it would cost to get there.
The largest gap is in PM2.5. EPA's annual NAAQS was lowered from 12 to 9 µg/m³ in 2024; WHO's annual guideline is 5 µg/m³. The EPA number is what your county must keep annual averages below. The WHO number is what evidence suggests is needed to push residual cardiovascular and respiratory mortality close to background.
For ozone, NO2, SO2, and CO the picture is similar: EPA limits track several years behind the WHO health-based targets. WHO's 2021 update tightened most thresholds significantly relative to its 2005 predecessor, based on accumulating epidemiological evidence.
The Terrestream device defaults to the EPA scale, the one most U.S. readers know; the only alternative is AQHI+ for Canada (see Canadian AQHI). There is no WHO setting and no stricter mode: everyone gets the same bands. If you prefer to work to WHO guidelines, compare readings with the WHO values yourself; for PM2.5 that means 15 µg/m³ as a 24-hour mean and 5 µg/m³ as an annual mean.
References
- WHO Global Air Quality Guidelines (2021) www.who.int
- EPA - National Ambient Air Quality Standards (table) www.epa.gov
- WHO 2021 air-quality guideline update - summary www.who.int
- EPA - AirNow: AQI Basics www.airnow.gov
