PM2.5 Explained: What Fine Particles Are and What Levels Mean at Home
PM2.5 explained: what fine particles are, EPA and WHO limits, the 2024 AQI breakpoints, where indoor PM2.5 comes from, and how to measure and lower it at home.
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PM2.5 is particle pollution with diameters generally 2.5 micrometers and smaller, and EPA says these fine particles pose the greatest risk to health of all particle sizes. EPA's health-based annual standard for outdoor PM2.5 is 9.0 µg/m³, its 24-hour standard is 35 µg/m³, and since May 2024 the AQI calls a 24-hour average up to 9.0 µg/m³ "Good". WHO's 2021 guidelines are stricter: 5 µg/m³ for the year and 15 µg/m³ for a day.
Indoors, the biggest PM2.5 spikes usually come from things you do: cooking, candles, fireplaces, unvented heaters and smoking, according to EPA. A monitor with a laser particle sensor shows those spikes, and an air purifier with a good smoke CADR, a vented range hood and a decent furnace filter bring them down. This guide explains the numbers and what to do about them.
How small is PM2.5?
EPA notes that the average human hair is about 70 micrometers across, which makes it about 30 times larger than the largest fine particle. Particles this small stay airborne for a long time and are drawn deep into the lungs; EPA says some particles smaller than 10 micrometers can get deep into the lungs and some may even get into the bloodstream. PM2.5 includes soot, smoke, and particles that form in the air from gases such as sulfur dioxide and nitrogen oxides from power plants, industry and vehicles.
For the difference between fine and coarse particles, see PM10 vs PM2.5.
The official limits
| Standard | Annual | 24-hour |
|---|---|---|
| EPA primary (health) standard | 9.0 µg/m³, annual mean averaged over 3 years | 35 µg/m³, 98th percentile averaged over 3 years |
| EPA secondary standard | 15.0 µg/m³ | 35 µg/m³ |
| WHO 2021 guideline | 5 µg/m³ | 15 µg/m³ |
| WHO 2005 guideline (replaced) | 10 µg/m³ | 25 µg/m³ |
These are outdoor standards set for long averages, not limits for a single reading from your kitchen. They are still the most useful yardstick for indoor air: if your home's PM2.5 averages above 9 µg/m³ over days, the air inside is worse than the level EPA set for outdoor air.
What the AQI says about a PM2.5 reading
EPA updated the AQI breakpoints for PM2.5 in 2024, effective May 6, 2024. The bands are defined for 24-hour average concentrations.
| AQI category | Index | PM2.5 (µg/m³) |
|---|---|---|
| Good | 0 to 50 | 0.0 to 9.0 |
| Moderate | 51 to 100 | 9.1 to 35.4 |
| Unhealthy for sensitive groups | 101 to 150 | 35.5 to 55.4 |
| Unhealthy | 151 to 200 | 55.5 to 125.4 |
| Very unhealthy | 201 to 300 | 125.5 to 225.4 |
| Hazardous | 301 and up | 225.5 and up |
Before 2024, the Good band ran to 12.0 µg/m³. If your monitor or app still uses the old breakpoints, its colors will look better than the current EPA scale. Our guide to the indoor AQI explains why a minute-by-minute indoor number is not the same as the official index.
Where indoor PM2.5 comes from
EPA lists cooking, combustion (candles, fireplaces, unvented space heaters or kerosene heaters, and cigarette smoking) and some hobbies as indoor sources, plus outdoor particles that migrate inside. In homes without smoking or other strong sources, EPA expects indoor levels to be about the same as or lower than outdoors. That means two kinds of problem:
- Short spikes from activities. Searing, frying, toasting and candles can push a monitor into the hundreds for a few minutes. These are the readings that make people buy purifiers.
- A raised baseline from outdoors. On wildfire smoke days or near heavy traffic, outdoor PM2.5 leaks in through gaps and open windows and keeps indoor levels high for hours.
How to lower PM2.5 at home
- Vent the source. EPA advises venting fuel-burning appliances outdoors and using exhaust fans vented outside when cooking.
- Avoid unvented combustion. EPA advises against unvented stoves, fireplaces and space heaters indoors.
- Filter the air in the room. A portable purifier's smoke CADR measures how much clean air it delivers for the finest particles in AHAM's test (0.09 to 1 micrometers). AHAM's rule of thumb is a smoke CADR of at least two-thirds of the room's floor area; for wildfire smoke, AHAM suggests a smoke CADR equal to the floor area. See CADR explained and the best air purifiers.
- Upgrade the furnace filter if your system allows it. EPA's guidance on respiratory viruses suggests MERV 13 or higher where the system can handle it. See whether a MERV 13 upgrade is worth it.
- Change filters on schedule. EPA advises changing filters on central systems and air cleaners according to the manufacturer's directions.
Measuring PM2.5
Consumer monitors estimate PM2.5 with a laser that counts light scattered by particles. They are good at showing changes and spikes, but accuracy varies by model. South Coast AQMD's AQ-SPEC program tests sensors beside reference instruments; for example, it reported field R-squared values of 0.85 to 0.93 for the Qingping Air Monitor Lite and 0.69 to 0.73 for the IQAir AirVisual Pro. Our guide to monitor accuracy has the full table, and the best air quality monitors page has our picks.
When PM2.5 matters most
EPA says people with heart or lung disease, children and older adults may be at greater risk from particle pollution. On smoke days, EPA's wildfire guidance recommends creating a clean room with a portable air cleaner; our wildfire smoke purifier picks are sized for that job. A purifier removes particles from the air; it does not treat any condition, and anyone with symptoms should follow their doctor's advice.
Frequently asked questions
What is a good PM2.5 level indoors?
There is no separate indoor standard. EPA's AQI calls a 24-hour average of 0 to 9.0 µg/m³ Good, matching its annual outdoor standard of 9.0 µg/m³, and WHO's 2021 annual guideline is 5 µg/m³.
What PM2.5 level is unhealthy?
On EPA's 2024 AQI scale, a 24-hour average of 35.5 to 55.4 µg/m³ is Unhealthy for Sensitive Groups, and 55.5 µg/m³ and above is Unhealthy for everyone.
Why does my PM2.5 reading spike when I cook?
Cooking is one of the indoor particle sources EPA lists. Searing, frying and toasting release fine particles that a laser sensor picks up within seconds. A range hood vented outside and a purifier running nearby bring the reading back down.
Does an air purifier remove PM2.5?
A purifier with a HEPA-type filter captures fine particles, and its smoke CADR tells you how fast it does that for the smallest sizes AHAM tests (0.09 to 1 micrometers). Size it to the room with AHAM's two-thirds rule.
What is the difference between PM2.5 and PM10?
PM10 includes all inhalable particles up to 10 micrometers, so it contains PM2.5 plus coarser dust and pollen. EPA says the fine particles, PM2.5, pose the greatest risk to health.
Did EPA change the PM2.5 standard?
Yes. In February 2024 EPA lowered the primary annual PM2.5 standard from 12.0 to 9.0 µg/m³ and updated the AQI breakpoints, effective May 6, 2024.
Sources
- Particulate Matter (PM) Basics, US Environmental Protection Agency
- NAAQS Table, US Environmental Protection Agency
- Final Updates to the Air Quality Index (AQI) for Particulate Matter: Fact Sheet and Common Questions, US Environmental Protection Agency
- 40 CFR Appendix G to Part 58: Uniform Air Quality Index (AQI) and Daily Reporting, Legal Information Institute (US Code of Federal Regulations)
- WHO Global Air Quality Guidelines: Particulate Matter (PM2.5 and PM10), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide, World Health Organization (via NCBI Bookshelf)
- Indoor Particulate Matter, US Environmental Protection Agency
- How to Choose the Best Air Cleaner for Your Home, AHAM Verifide
- AQ-SPEC Sensor Evaluations: Summary of PM Sensors, South Coast Air Quality Management District
- Create a Clean Room to Protect Indoor Air Quality During a Wildfire, US Environmental Protection Agency
- Air Cleaners, HVAC Filters, and Coronavirus (COVID-19), US Environmental Protection Agency
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