PM2.5 concentration levels, corresponding AQI, and what your monitor reading means

What PM2.5 is and why it matters indoors

PM2.5 stands for particulate matter 2.5 micrometers and smaller. These are small particles: dust, smoke, wildfire ash, pollen pieces, bacteria, and other floating material. PM2.5 is measured in micrograms per cubic meter of air.

PM2.5 is used to assess air quality because these small particles penetrate deep into the lungs and damage the alveoli where gas exchange happens. Larger particles like pollen stay in the nose and throat. Smaller particles like viruses are captured by other measurements.

An air quality monitor shows real-time PM2.5 concentration indoors. This reading changes as windows open or close, as pollution sources become active (cooking, smoking), or as the purifier runs.

Where indoor PM2.5 comes from

• Outdoor infiltration: wildfire smoke, urban pollution, pollen, dust
• Indoor sources: cooking, smoking, combustion appliances, vacuuming, candles
• Resuspension: dust settling on floors and being disturbed by movement

Good air: 0 to 12 micrograms

PM2.5 below 12 micrograms per cubic meter is clean, healthy air. This is typical in homes that are closed to outdoor pollution and have no active indoor sources.

In a closed home on a low-pollution day (no wildfires, low outdoor AQI), indoor PM2.5 usually stays below 12 micrograms. You do not need to run a purifier.

Moderate air: 12 to 35 micrograms

At 12 to 35 micrograms, air is transitioning from clean to degraded. People without respiratory sensitivity notice little, but the concentration is climbing.

This level happens when outdoor pollution is moderate (AQI 51 to 100), or when an indoor source is active (cooking, smoking). If the monitor shows 20 micrograms and climbing, it is time to close windows and consider running a purifier.

Unhealthy for Sensitive Groups: 35 to 55 micrograms

At 35 to 55 micrograms, air quality is visibly degraded. If you look across a room, you might see a slight haze. Most people without respiratory issues feel fine, but people with asthma, allergies, or heart disease start to feel symptoms.

This is when you should run a purifier on medium speed. If you can smell pollution (cooking smoke, wildfire smoke), run on high. Keep windows and doors closed to prevent outdoor pollution from entering.

Unhealthy: 55 to 150 micrograms

At 55 to 150 micrograms, indoor haze is obvious. The air looks gray or yellow-tinted depending on the pollution source (wildfire smoke is orange-yellow).

At this level, everyone should stay indoors if possible, and run the purifier on high continuously. Children, older adults, and anyone with respiratory or heart disease are at high risk.

Very Unhealthy and Hazardous: above 150 micrograms

Above 150 micrograms, air is extremely polluted. This happens during severe wildfires, in industrial zones, or in rare pollution events in major cities.

Keep windows and doors closed. Run the purifier on high 24/7. If you must go outside, wear an N95 or P100 mask. Minimize exertion because exercise increases breathing rate and particle exposure.

Indoor vs. outdoor PM2.5 readings

Outdoor PM2.5 is measured by air quality monitoring networks and published as AQI data. Indoor PM2.5 is what your home monitor shows, which is usually lower than outdoor levels if windows are closed.

In a typical home with closed windows, indoor PM2.5 reaches 50 to 70 percent of outdoor PM2.5 within a few hours due to infiltration through doors, air vents, and window frames.

If windows are open, indoor PM2.5 quickly matches outdoor levels. This is why closing windows when outdoor AQI is unhealthy is the first line of defense.