At a glance
What matters most
- 01
The current global guideline edition was published in 2021.
- 02
The guidelines cover PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide.
- 03
WHO guideline values are health-based recommendations, not national legal limits.
- 04
Short-term and long-term values use different averaging periods.
WHO 2021 guideline levels
| Pollutant | Averaging period | Guideline level |
|---|---|---|
| PM2.5 | Annual | 5 µg/m³ |
| PM2.5 | 24-hour | 15 µg/m³ |
| PM10 | Annual | 15 µg/m³ |
| PM10 | 24-hour | 45 µg/m³ |
| Ozone | Daily maximum 8-hour mean | 100 µg/m³ |
| Ozone | Peak season | 60 µg/m³ |
| Nitrogen dioxide | Annual | 10 µg/m³ |
| Nitrogen dioxide | 24-hour | 25 µg/m³ |
| Sulfur dioxide | 24-hour | 40 µg/m³ |
| Carbon monoxide | 24-hour | 4 mg/m³ |
Guideline values versus legal standards
WHO guideline levels are designed as evidence-informed public-health references. Countries may use them when developing laws and policies, but national standards can differ because of legal frameworks, monitoring systems, feasibility and policy decisions.
A concentration can comply with a national standard and still be above a WHO guideline. The correct comparison depends on the pollutant, unit and averaging period.
Why averaging periods matter
Annual means describe long-term exposure, while 24-hour and 8-hour values address shorter pollution episodes. Comparing a live hourly reading directly with an annual or 24-hour guideline is not a like-for-like compliance assessment.
Frequently asked questions
Are WHO guideline values legally binding?
No. They are evidence-informed health recommendations. Governments decide whether and how to incorporate them into enforceable standards.
Can I compare a live PM2.5 reading with the annual guideline?
Not directly. The annual guideline applies to a full-year mean, while a live value represents a much shorter period.
Why are Indian limits different from WHO guidelines?
WHO publishes health-based recommendations, while India’s NAAQS are national regulatory standards established through India’s legal and policy process.
Primary sources
This reference was prepared from the following primary public-health and regulatory sources: