Indoor air quality monitors: what PM2.5, CO2 and VOC mean

What a typical monitor measures

PM2.5: fine particles

The EPA defines PM2.5 as fine inhalable particles with diameters generally 2.5 micrometers and smaller. An average human hair is about 70 micrometers across, about 30 times larger than the biggest fine particle. The EPA notes that some particles under 10 micrometers can get deep into the lungs and some may enter the bloodstream. Indoors, the usual sources are cooking, candles, fireplaces, smoke drifting in from outside and dust that gets stirred up.

A monitor reports PM2.5 in micrograms per cubic meter. A spike while you fry food is normal; a steady reading that stays high after the source ends suggests you need ventilation or filtration. During wildfire smoke, check outdoor conditions on AirNow, which provides the Air Quality Index and a Fire and Smoke Map. See wildfire smoke and indoor air.

CO2: a clue about ventilation

Carbon dioxide in a home mostly comes from breathing, so it rises when people share a closed room. The Minnesota Department of Health says outdoor air holds about 400 parts per million (ppm) or somewhat more in busy areas, and that ventilation rates are typically set to keep CO2 below 1,000 ppm, a level acceptable to most people. NIST adds a limit: indoor CO2 reflects contaminants linked to occupants, not other pollutants such as emissions from building materials.

• Close to the outdoor level: the room is well ventilated.
• Above about 1,000 ppm: the room is stuffy for the number of people. Open a window, run a fan or the ventilation system.
• A bedroom with the door and windows closed often climbs overnight.

CO2 alone is not a pollution score. A room can have low CO2 and high PM2.5 after cooking, or the opposite after a meeting.

VOCs: indicative only

Volatile organic compounds are gases emitted by some solids and liquids. The EPA says concentrations of many VOCs are consistently higher indoors, up to ten times higher, than outdoors. Typical sources are paints, varnishes, wax, cleaning products, cosmetics, hobby supplies and dry-cleaned clothing. The EPA advice is to increase ventilation when using products that emit VOCs and to identify and, if possible, remove the source.

A consumer monitor reports a single TVOC number. It cannot tell you which compound is present or whether it is harmful. Treat a spike as a prompt to ventilate and find the source, not as a safety verdict. Formaldehyde, which the EPA calls one of the few indoor air pollutants that can be readily measured, needs a sensor designed for it.

Humidity and temperature

Most monitors include humidity. The EPA recommends keeping indoor relative humidity below 60% and ideally between 30% and 50%. Readings above that range can point to a dehumidifier, a bath fan or a leak. See smart humidity control.

How accurate are consumer monitors?

• PM2.5 sensors estimate particle counts with a small laser. They are useful for trends and for seeing a spike, less so for exact numbers, and their readings can drift over time.
• CO2 sensors come in two kinds. NDIR sensors measure CO2 directly. Some low-cost monitors estimate “CO2 equivalent” from a VOC sensor, which can jump with cooking or cleaning products. For ventilation decisions, look for a monitor that states it uses an NDIR CO2 sensor.
• TVOC readings are unitless or approximate and vary between brands. Compare a monitor with itself over time, not with another monitor.

Place the monitor at about breathing height, away from the kitchen stove, vents, windows and humidifiers. Give it a day or two to settle after you plug it in.

What a monitor does not measure

• Carbon monoxide. Needs a UL-listed CO alarm on each level and near sleeping areas. See carbon monoxide detector placement.
• Radon, mold spores and pollen. Special tests or devices.
• Health. A monitor shows conditions in the room. It cannot say whether anyone is affected. Talk to a doctor about symptoms.

Using readings: simple automations

To size a purifier for the room, use the air purifier room size calculator.