Particulate matter: PM2.5 and PM10
PM2.5 means particles of 2.5 micrometres or smaller, roughly 30 times finer than a human hair. The WHO 2021 global air quality guidelines set PM2.5 at 5 µg/m³ annual mean and 15 µg/m³ 24-hour mean. The US EPA strengthened the annual PM2.5 National Ambient Air Quality Standards to 9.0 µg/m³ in 2024 and kept the 24-hour standard at 35 µg/m³. These are outdoor limits, but they are the reference numbers most indoor monitors compare against.
PM10 is the coarser fraction: particles of 10 micrometres or smaller. WHO 2021 sets PM10 at 15 µg/m³ annual and 45 µg/m³ 24-hour. Indoor sources include cooking, candles, and resuspension from floors. A vacuum with a HEPA filter, per the US DOE definition, captures at least 99.97% of particles of 0.3 micrometres.
The WHO estimates that 99% of the world's population breathes air exceeding its guideline levels. That figure is about outdoor air, but it explains why indoor filtration and ventilation get so much attention. Indoor PM2.5 often tracks outdoor PM2.5 plus indoor sources.
A short worked example: a kitchen frying fish can push PM2.5 well above the WHO 24-hour guideline of 15 µg/m³ for the duration of cooking. Running a vented range hood and opening a window brings the level down faster than a purifier alone.
Gases: NO2 and CO2
Nitrogen dioxide (NO2) is a gas produced by combustion. The WHO 2021 guideline sets NO2 at 10 µg/m³ annual and 25 µg/m³ 24-hour. Indoor sources are gas stoves, gas ovens, and unvented space heaters. A vented range hood is the standard control.
CO2 is not a combustion pollutant indoors; it is mostly exhaled breath. Outdoor air holds about 420 ppm of CO2. Many ventilation guidelines treat about 1000 ppm indoors as the upper comfort band. A non-dispersive infrared (NDIR) CO2 sensor reports ppm directly, unlike estimated readings from some low-cost monitors.
CO2 at 1000 ppm is not a health limit; it is a ventilation indicator. ASHRAE Standard 62.1 and Standard 62.2 define ventilation rates for acceptable indoor air quality. ASHRAE Standard 241 (2023) adds equivalent clean airflow for infectious aerosols.
A bedroom with a closed door and two occupants can exceed 1000 ppm overnight. That is a signal to increase ventilation, not necessarily to buy a purifier. CO2 is the easiest indoor pollutant to measure and the easiest to fix with fresh air.
VOCs, radon, and ozone
Volatile organic compounds (VOCs) are a broad family of carbon-based gases. Common indoor sources are paints, adhesives, cleaning products, and furniture. There is no single WHO limit for total VOCs; formaldehyde has its own WHO indoor guideline. The US EPA publishes indoor air quality guidance on VOCs but no single numeric limit.
Radon is a radioactive gas from soil and rock. The US EPA radon action level is 4 pCi/L, about 148 Bq/m³. The WHO reference level is 100 Bq/m³. Short-term radon tests run 2 to 90 days; long-term tests run more than 90 days. Radon is the second leading cause of lung cancer in the US after smoking, per EPA.
Ozone is a lung irritant. Outdoors it forms from sunlight and traffic emissions. Indoors, some air cleaners generate it on purpose. The California Air Resources Board (CARB) limits ozone emissions from indoor air cleaning devices sold in California to 0.050 ppm. That limit is a real regulatory number, not a marketing claim.
The EPA Air Quality Index (AQI) runs from 0 to 500. Bands are 0–50 Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, and 151–200 Unhealthy. The AQI is an outdoor index, but it is the fastest way to decide whether to ventilate today.
How the limits compare
WHO 2021 guidelines are health-based and often stricter than national standards. The US EPA NAAQS are legally enforceable outdoor limits. Indoor limits are mostly guidelines, not law, except for specific devices like ozone generators under CARB rules.
A useful habit is to compare any indoor reading to the WHO 2021 24-hour guideline for that pollutant. For PM2.5 that is 15 µg/m³; for PM10 it is 45 µg/m³; for NO2 it is 25 µg/m³. If your indoor reading is above the 24-hour guideline, investigate the source.
Radon is the exception: it has no safe threshold, so the action level is a policy choice. The US EPA chose 4 pCi/L; the WHO chose 100 Bq/m³. Both are based on risk models, not on a sharp health cliff.
The EPA AQI bands give a quick outdoor context. When the AQI is above 100, closing windows and running a HEPA purifier sized by the AHAM two-thirds rule is reasonable. When it is below 50, ventilation is usually the better move.
Key figures at a glance
| WHO 2021 PM10 guideline | 15 µg/m³ annual; 45 µg/m³ 24-hour | Global Air Quality Guidelines, World Health Organization, 2021. |
|---|---|---|
| WHO 2021 NO2 guideline | 10 µg/m³ annual; 25 µg/m³ 24-hour | Global Air Quality Guidelines, World Health Organization, 2021. |
| CO2 reference bands | ≈420 ppm outdoor; ≈1000 ppm indoor comfort upper band | Outdoor baseline and common ventilation guideline band. |
| CARB ozone limit | 0.050 ppm | California Air Resources Board limit for indoor air cleaning devices sold in California. |
How to run a short-term radon test
Choose the lowest lived-in level
Place the radon test kit in the lowest level of the home that is regularly used. A basement or ground-floor room is typical. Avoid bathrooms, kitchens, and laundry rooms because of moisture and drafts.
Close windows and doors
Keep windows and doors closed as much as possible during the test. Short-term radon tests run 2 to 90 days. The US EPA recommends a short-term test first, then a follow-up if the result is near the action level.
Place the kit correctly
Put the kit at least 20 inches above the floor. Keep it away from exterior walls, drafts, and direct sunlight. Follow the kit's instructions for exact placement; the US EPA publishes a citizen's guide to radon.
Send the kit to a lab
After the exposure period, seal the kit and mail it to the lab. The lab reports radon concentration in pCi/L or Bq/m³. The US EPA action level is 4 pCi/L; the WHO reference level is 100 Bq/m³.
Decide on follow-up
If the short-term result is at or above 4 pCi/L, run a long-term test (more than 90 days) or a second short-term test. If the long-term result stays at or above the action level, consider radon mitigation. A qualified mitigator can install a vent pipe and fan system.
Frequently asked questions
What is the difference between PM2.5 and PM10?
PM2.5 means particles of 2.5 micrometres or smaller. PM10 means particles of 10 micrometres or smaller. PM10 includes PM2.5 plus coarser particles. WHO 2021 sets PM2.5 at 5 µg/m³ annual and PM10 at 15 µg/m³ annual.
Is there a safe level of radon?
No threshold is known below which radon is risk-free. The US EPA action level is 4 pCi/L, about 148 Bq/m³. The WHO reference level is 100 Bq/m³. Both are policy choices based on risk models.
Do VOCs have a single limit like PM2.5?
No. VOCs are a family of gases, not one pollutant. Formaldehyde has its own WHO indoor guideline. The US EPA publishes indoor air quality guidance on VOCs but no single numeric limit. Source control and ventilation are the main tools.
What does the EPA AQI measure?
The EPA Air Quality Index (AQI) runs from 0 to 500. It combines several outdoor pollutants into one number. Bands are 0–50 Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, and 151–200 Unhealthy. It is an outdoor index, not an indoor one.
Can an air purifier remove NO2?
Most HEPA purifiers do not remove NO2 because it is a gas, not a particle. The US DOE HEPA definition covers particle capture. Activated carbon can adsorb some NO2, but a vented range hood is the standard control for gas cooking.