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Wildfire smoke air purifier calculator
What will the air in your room be like with a purifier running? Enter today's outdoor AQI, your room and your purifier's CADR. You'll see the indoor AQI, how fast it gets there, and the CADR that would bring it back to Good.
CADR is rated at top speed. Running it on low for sleep? Enter a smaller number.
- Indoor PM2.5
- Filtration from purifiers
- 90% of the way there after
- CADR for a Good indoor AQI
Affiliate link. Estimates for a well-mixed room at steady state. See the method below.
How the estimate works
Indoors, smoke levels settle where particles coming in balance particles being removed. Outdoor air brings smoke in by leaking through the building. Your purifier takes it out, and some settles onto surfaces. Write each as air changes per hour (ACH) and the indoor level is a simple ratio:
indoor ÷ outdoor = leakage ACH ÷ (leakage ACH + 0.2 + purifier ACH)Purifier ACH is total CADR × 60 ÷ room volume, the same as in the ACH calculator. The 0.2 per hour is a typical rate at which smoke-sized particles settle on indoor surfaces. The ratio applies to PM2.5 concentration. The calculator converts outdoor AQI to PM2.5, applies the ratio, and converts back using the EPA's 2024 AQI breakpoints. Because the AQI scale isn't linear, cutting PM2.5 by 80% doesn't cut the AQI by 80%.
Jeff Kaufman worked through a small 34 CFM purifier in a 10 × 10 ft bedroom with an 8 ft ceiling. That's 800 ft³, so the purifier adds 34 × 60 ÷ 800 = 2.55 ACH. With 0.5 ACH leaking in, indoor PM2.5 settles at 15% of outdoor, so outdoor AQI 200 becomes about 70 indoors. (His version leaves out surface deposition and gets 16%.)
Indoor AQI by purifier strength (typical home, windows shut)
| Outdoor AQI | No purifier | 2 ACH | 5 ACH | 8 ACH |
|---|---|---|---|---|
| 100 | 81 | 36 | 17 | 11 |
| 150 | 111 | 53 | 27 | 17 |
| 200 | 175 | 77 | 55 | 40 |
| 300 | 236 | 116 | 71 | 58 |
| 400 | 271 | 139 | 79 | 63 |
Assumes 0.5 ACH of outdoor air and all leaking particles getting through the walls. Real homes filter some smoke at cracks, so readings are often a little lower. A recent cigarette, candle or cooking session will push them higher.
Running it overnight: a purifier as a white noise machine
A white noise machine is mostly a fan in a box. An air purifier is a fan in a box that pulls air through a filter. Run one in the bedroom all night and you get the masking sound and cleaner air, which matters most on smoke days. Jeff Kaufman makes the case that for most people who use a noise machine, a small purifier is simply better.
The trade-off is noise versus airflow. Purifiers are usually quieter per CFM when they're bigger and running slowly, so a larger unit on low often beats a small unit on high for both sound and cleaning.
| Level | Sounds like | Purifier setting |
|---|---|---|
| 30 dB | Whisper | Sleep mode on most purifiers |
| 40 dB | Quiet library | Low speed |
| 45 dB | Quiet room with a fan running | Typical small purifier on high |
| 50–55 dB | Light rain, quiet office | Mid-size purifier on high |
| 60 dB | Normal conversation | Large purifier on turbo |
These are typical values. Check the dB rating at each speed for your model. All are well below the 70 dB 24-hour average the EPA identified in 1974 as protective against hearing loss.
Wondering what running one all night costs? Use the air purifier electricity cost calculator. For an efficient purifier it's usually a few dollars a year.
Questions
Do air purifiers help with wildfire smoke?
Yes. Smoke is mostly fine particles (PM2.5), which HEPA and other high-efficiency filters capture well. With windows shut, a purifier delivering 5 air changes per hour typically keeps a room below a tenth of the outdoor level. Check the smoke CADR, not the pollen or dust rating.
Should I open the windows while running an air purifier?
Not when the outdoor air is smoky. An open window can let in several air changes per hour of outdoor air, which overwhelms most home purifiers. Set the leakage option to "window open" to see the effect. Shut windows first, then add filtration.
How many air purifiers do I need for smoke?
Size for the room you will spend most time in, usually the bedroom. The calculator shows the total CADR needed to get that room back to a Good AQI. If one purifier cannot reach it, two smaller units add their CADRs together.
Is it OK to run an air purifier all night?
Yes. Purifiers are designed for continuous use, and an efficient one costs a few dollars a year to run overnight. Many people find the steady fan noise works like a white noise machine. Avoid ozone generators and "ionizers" that emit ozone; the EPA advises against them.
Does my HVAC system count?
It can. A central system running its fan continuously through a MERV 13 or better filter adds filtration to every room it serves. The calculator leaves it out, so treat the result as a cautious estimate if your system runs.
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