Episodic pollution
Wildfire smoke
Wildfire smoke is overwhelmingly fine particulate matter, PM2.5: particles under 2.5 micrometres, small enough to travel deep into the lung and cross into the bloodstream. A smoke plume also carries carbon monoxide, volatile organic compounds and the precursors that form ozone downwind, but the particles are what drives the index and the health effects.
What makes smoke different from the pollution the Clean Air Act was written for is that it is episodic and enormous. A county can spend four years in the Good and Moderate categories and then record several weeks above the moderate threshold in a single season, from fires hundreds or thousands of miles away.
Who it affects first
Sensitive groups are not a small group
The category the EPA calls “unhealthy for sensitive groups” begins at AQI 101. For particle pollution the sensitive group is people with lung disease, including asthma and COPD, people with heart disease, older adults, children, and pregnant people. In most American counties that is a substantial minority of the population rather than a rare exception.
At AQI 151 and above the effects extend to everyone. Above 200 the EPA describes it as a health alert, and above 300 as emergency conditions.
What actually helps
On a smoke day, the usual advice inverts
Close up
Ventilation is normally the cheapest way to improve indoor air. On a smoke day the outside air is the problem, so close windows and doors and set air conditioning to recirculate.
Filter indoors
A HEPA purifier sized for the room, or a good filter in the central system, genuinely removes smoke particles. This is the one hazard where a purifier is the right tool.
Do not add particles
No candles, no incense, no frying, no vacuuming without a HEPA machine, and no wood burning. Indoor sources stack on top of what has already got in.
Masks, the right kind
A well-fitted N95 or P100 filters fine particles. A cloth or surgical mask does very little against PM2.5, and neither does a dust mask without a seal.
In the monitor data
The counties with the widest year-to-year swings
Chronic pollution is fairly steady from year to year. Episodic smoke is not, so the gap between a county’s calmest and worst year in the EPA record is a decent proxy for how badly smoke has hit it. These are the widest gaps across 2021-2025.
| # | County | Swing | Worst year | Days that year | Calmest year |
|---|---|---|---|---|---|
| 1 | Maricopa County, AZ | 187 | 2021 | 225 | 38 |
| 2 | Tulare County, CA | 91 | 2021 | 134 | 43 |
| 3 | Plumas County, CA | 74 | 2021 | 84 | 10 |
| 4 | Kern County, CA | 63 | 2021 | 128 | 65 |
| 5 | Cameron County, TX | 61 | 2024 | 61 | 0 |
| 6 | Nevada County, CA | 53 | 2021 | 53 | 0 |
| 7 | Boise County, ID | 49 | 2024 | 49 | 0 |
| 8 | Fresno County, CA | 48 | 2021 | 82 | 34 |
| 9 | Klamath County, OR | 47 | 2021 | 50 | 3 |
| 10 | Los Angeles County, CA | 46 | 2024 | 133 | 87 |
| 11 | Placer County, CA | 46 | 2021 | 48 | 2 |
| 12 | Trinity County, CA | 45 | 2021 | 46 | 1 |
| 13 | Larimer County, CO | 45 | 2021 | 50 | 5 |
| 14 | Stanislaus County, CA | 44 | 2021 | 47 | 3 |
| 15 | El Paso County, TX | 44 | 2024 | 63 | 19 |
| 16 | Jefferson County, CO | 42 | 2021 | 55 | 13 |
| 17 | Kings County, CA | 40 | 2021 | 51 | 11 |
| 18 | Riverside County, CA | 38 | 2024 | 147 | 109 |
| 19 | Uintah County, UT | 37 | 2023 | 37 | 0 |
| 20 | Pinal County, AZ | 36 | 2024 | 70 | 34 |
| 21 | San Bernardino County, CA | 36 | 2021 | 146 | 110 |
| 22 | San Diego County, CA | 36 | 2024 | 52 | 16 |
| 23 | Merced County, CA | 35 | 2024 | 35 | 0 |
| 24 | Tehama County, CA | 35 | 2021 | 36 | 1 |
| 25 | Arapahoe County, CO | 35 | 2021 | 37 | 2 |
Derived from EPA Air Quality System annual county summaries. “Swing” is the difference between the county’s highest and lowest count of days above the moderate category across the years it reported. It is a proxy for episodic pollution, not a measurement of smoke: a monitor coming online or going offline moves it too, and the EPA summaries do not attribute days to a cause.
Equipment
What helps on a smoke day
N95 and P100 respirators
Affiliate linksA fitted N95 or P100 filters fine particles. A cloth or surgical mask does very little against PM2.5.
3M
3M Particulate Disposable Respirator 8210, N95, Pack of 20, NIOSH APPROVED
AccuMed
AccuMed BNX N95 Mask Black NIOSH Certified MADE IN USA Particulate Respirator Protective Face Mask (20-Pack, Approval Number TC-84A-9315 / Model H95B) Black
3M
3M 8511 N95 Cool Flow Valved Respirator, 10-Pack
FOXFUN
N95 Masks NIOSH Approved for Adults, 25 Pack Individually Wrapped, Particulate Respirator N95 Face Masks Universal Fit Comfortable
Prices retrieved from Amazon on 2026-08-21 and may since have changed; the price on Amazon at the time of purchase is the one that applies. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. No manufacturer pays for a place in this list.
HEPA air purifiers
Affiliate linksMatch the smoke CADR to the room: roughly two thirds of the floor area in square feet. Removes particles; does nothing for radon or any other gas.
MOOKA
MOOKA Air Purifier for Home Bedroom Large Room 2200 Ft², True HEPA Filter
ECOSELF
Air Purifier for Large Room, Cleans 2,400 ft² in 30 Mins, 3-Stage True HEPA, 360° Airflow, Large Touch Display Control, Real-Time Air Quality (PM2.5) Display, Ultra-Quiet 20dB Sleep Mode, HAP602 White
Levoit
LEVOIT Air Purifier for Home Allergies and Pets Hair Smokers in Bedroom, H13 True HEPA Filter, 24db Filtration System Cleaner Odor Eliminators, Remove 99.97% Dust Smoke Mold Pollen, Core 300, White
MOOKA
MOOKA Air Purifiers for Home Room up to 1076ft², True HEPA Filter, B-D02L
Prices retrieved from Amazon on 2026-08-21 and may since have changed; the price on Amazon at the time of purchase is the one that applies. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. No manufacturer pays for a place in this list.
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