PM2.5 indoor levels: what counts as a good number

A good PM2.5 level indoors is under 9.0 µg/m³ averaged over a day, which is the top of the EPA's Good band for fine particles. PM2.5 indoor levels have no standard of their own: the EPA sets those numbers for outdoor air, and they carry indoors because the EPA expects a home with no smoking or other strong particle sources to read at or below the air outside.

You have probably lived the situation without ever seeing a number for it. You sear something at 7pm, the smoke alarm thinks about it, and the kitchen still smells like dinner at bedtime. A reading puts a figure on that half hour and tells you when the room is back to normal.

The rest of this post is where each of those thresholds comes from and what moves the line in a real house. If you would rather watch the readings in your own rooms than read about them, that is what a kit for homeowners does.

Key Takeaways

• Under 9.0 µg/m³ over a day is the top of the EPA's Good band for PM2.5. Moderate runs 9.1 to 35.4 µg/m³, and 35.5 µg/m³ and up is Unhealthy for Sensitive Groups.
• The EPA sets no indoor PM2.5 standard. It does say indoor particles can exceed outdoor particles, and that a home without smoking or other strong sources should read at or below the air outside.
• Those thresholds are 24-hour averages. Frying dinner can put a kitchen over 100 µg/m³ for 20 minutes without the day's average leaving the Good band.
• The comparison that settles things is your indoor reading against the outdoor reading at your own address, not against a number in an article.
• A monitor reports particles. It does not remove any: the range hood, the filter and the purifier do that.

What good PM2.5 indoor levels look like

Four numbers cover most of what you will see on the line:

• Under 9.0 µg/m³ over 24 hours is Good. The EPA's AQI breakpoints for PM2.5 put that band at 0.0 to 9.0 µg/m³, and 9.0 µg/m³ is also the level of the annual outdoor standard the EPA set in 2024.
• 9.1 to 35.4 µg/m³ is Moderate, which the EPA describes as acceptable air with some risk for people who are unusually sensitive to air pollution.
• 35.5 to 55.4 µg/m³ is Unhealthy for Sensitive Groups. The 24-hour outdoor standard sits at 35 µg/m³, right at the edge of that band.
• 55.5 µg/m³ and up is Unhealthy for everyone. A kitchen mid-fry, or a living room with a smoky fireplace, gets there easily.

Under 9.0 µg/m³ for the day is what to aim for. A day that averages past 35.5 µg/m³ is worth doing something about.

Why the EPA has no indoor number

The EPA does not set a standard for indoor particles. Its page on indoor particulate matter says so directly, and adds two lines worth keeping: indoor PM has the potential to exceed outdoor PM and the outdoor standards, and in homes without smoking or other strong particle sources, indoor PM would be expected to be the same as, or lower than, outdoor levels.

That second line is the test. If it reads 12 µg/m³ outside and 11 µg/m³ in your living room, the house is doing what the EPA expects of it and most of those particles came from outdoors. If it reads 12 µg/m³ outside and 40 µg/m³ in the living room, something indoors is making them, and you can go find it.

This is why one number on its own settles less than people expect. 18 µg/m³ in the kitchen means one thing on a clear day and something else on a smoke day when the air outside is at 90 µg/m³. Measured Air puts every reading next to the outdoor air at your own address for that reason, and keeps the history, so this week can be held up against last month. It is the line on the home page: the air outside is measured and the air you breathe isn't.

Spikes and daily averages are two different readings

Every threshold above is a 24-hour average. A monitor reports every few minutes, so the two do not line up, and holding a live reading against a daily standard is the most common way to alarm yourself over nothing.

Searing a steak or frying at high heat can push a kitchen past 100 µg/m³ within minutes. Twenty minutes up there, then an hour coming back down, barely moves the day's average. An afternoon with the windows open on a smoke day moves it a great deal.

So read the spike for what it is, which is where the particles came from and how long the room took to clear, and read the daily average against the 9.0 µg/m³ figure. A kitchen that needs three hours to come back down after dinner is telling you about its ventilation, not about the pan.

Where indoor particles come from

PM2.5 means fine particles: smoke, cooking and dust small enough to get deep into your lungs. The EPA's list of indoor sources is mostly things people do on purpose: candles, fireplaces, unvented space heaters, cigarette smoking, cooking, some hobbies, and outdoor particles drifting in.

In most homes the ranking is shorter than that list:

• Cooking, especially frying, searing and any oil taken past its smoke point.
• Anything with a flame: candles, incense, a fireplace, a gas range with the hood off.
• The air outside, on a smoke day or near a busy road.
• Vacuuming and dusting, which put settled particles back into the air for a while.

Where to put a PM2.5 monitor

Put monitors in the rooms people occupy, at about breathing height, a few feet back from the source. A monitor a foot from the stove reports the pan. One across the kitchen reports what you are breathing.

• The kitchen, or the room next to it, a few feet back from the range. The daily peak is almost always here.
• The bedroom. It is eight hours of exposure, and it is the easiest room in the house to judge a purifier in. CO₂ tells a separate story there: see CO₂ levels in a bedroom.
• The room where people spend the evening.

Keep monitors out of window drafts and off heat vents, and keep them out of the bathroom, where shower steam can show up on a particle reading.

What lowers PM2.5 indoor levels

Work down this list, and read the monitor after each change instead of assuming:

1. Run the range hood the whole time you cook, and leave it going for 10 to 15 minutes after. The EPA's advice on indoor particles starts with venting cooking exhaust outside, which is also the thing to check: plenty of hoods recirculate into the kitchen instead.
2. Drop the flames you can do without. Candles, incense and a fireplace with a weak draft are each worth one evening of readings to see what they cost.
3. Upgrade the HVAC filter. For smoke the EPA recommends a MERV 13 filter, or as high a rating as your system fan and filter slot can take, fitted snugly so air cannot slip past the edges, with the fan set to On rather than Auto.
4. Add a HEPA room purifier sized to the room, in the room you use most, left running. This is the fix with the clearest before-and-after on the line.
5. On a clear day, open the windows. On a smoke day, keep them shut and let the filter do the work. The outdoor reading is what tells you which day it is.

Every one of those trades something. A range hood is loud. A MERV 13 filter asks more of the blower and needs changing on schedule. Shutting the windows against smoke sends the CO₂ up, because the two fixes pull in opposite directions. A reading is how you know which trade you are making.

What a PM2.5 reading does not tell you

A monitor reports particles. It does not remove one of them. Nothing in the room changes until the hood runs, the new filter goes in, or the purifier comes on, and the reading's job is to show which of those did anything.

PM2.5 also says nothing about the rest of the air. A living room at 4 µg/m³ can still be sitting at 1,600 ppm of CO₂ with the door shut, VOCs off-gassing from a new couch, or a radon level that needs months of readings before it means anything.

A reading is not a diagnosis either. The bands above come from the EPA and describe air quality categories. None of them says what a given number will do to a particular person.

Frequently asked questions

What is a good PM2.5 level indoors?

Under 9.0 µg/m³ as a daily average. That is the top of the EPA's Good band for PM2.5 and the level of the annual outdoor standard. Short spikes above it while you cook are ordinary, and what matters is how fast the room comes back down.

Is 12 µg/m³ of PM2.5 bad indoors?

It falls in the EPA's Moderate band, 9.1 to 35.4 µg/m³, which the EPA describes as acceptable air with some risk for people who are unusually sensitive to air pollution. Whether it is worth acting on depends on the air outside: at 20 µg/m³ outdoors your house is filtering well, and at 3 µg/m³ outdoors something inside is making particles.

How high does PM2.5 get when you cook?

Past 100 µg/m³ within minutes, if you are frying or searing. A daily average absorbs a spike that short, so the number to watch is how long the room takes to get back to where it started. That is a question about the range hood.

Where should I put a PM2.5 monitor?

In the rooms people occupy, at about breathing height, a few feet back from the source. The kitchen or the room beside it catches the daily peak, and the bedroom covers the eight hours you cannot watch. Keep monitors out of window drafts and out of the bathroom.

Does an air purifier lower indoor PM2.5 levels?

A HEPA room purifier sized to the room and left running does bring the particle reading in that room down, and it is the easiest fix to check: note the line before you switch it on, then again an hour later. It does nothing for CO₂, which passes straight through the filter.

Do I need a monitor in every room?

No. Start with the kitchen, the bedroom and the room where people spend the evening. A kit is sized to the rooms you want covered, plus an outdoor reference, and monitors arrive preconfigured, so setup is power and a Wi-Fi password.

The short version

Good PM2.5 indoor levels come down to three numbers: under 9.0 µg/m³ as a daily average, 9.1 to 35.4 µg/m³ for the EPA's Moderate band, and 35.5 µg/m³ as the point where the EPA starts naming sensitive groups. A fourth number matters as much as any of them, and that is whatever it reads outside your own address at the same moment.

The cheap test comes first. Run the range hood the entire time you cook, watch the kitchen line, and time how long the room takes to get back to where it started. If the peak drops and the recovery shortens, the hood was the answer. If it doesn't, the hood is probably recirculating, and the next two things to try are a MERV 13 filter and a HEPA room purifier in the room you use most.

Guessing settles none of it, because a kitchen at 8 µg/m³ and a kitchen at 45 µg/m³ smell about the same an hour after dinner. Order a kit sized to your rooms, put a monitor where you cook and one where you sleep, and stop guessing whether the air in your house is fine.