Wildfire Smoke Calculator
See what wildfire smoke is actually doing to your lungs. Enter today’s AQI and how long you’re outside, and this converts it to PM2.5 and to a cigarette equivalent — a scale most people have intuition for. Estimates only, not medical advice.
Example: with AQI 155 · 4 hrs outdoors · windows closed · over 3 days → Cigarette equivalent (per day): 1.8 cigarettes.
- Outdoor PM2.561.2 µg/m³
- EPA air quality categoryUnhealthy
- Total for the smoke episode5.4 cigarettes
Computed by the calculator below using its default values. Change any input to see your own numbers.
How you compare
🌬️ Compare air purifiers & HVAC filters
Check it outWhat this number is, and what it isn’t
Wildfire smoke is mostly fine particulate matter — PM2.5 — small enough to reach deep into the lungs and cross into the bloodstream. The Air Quality Index you see on a weather app is a scaled version of that same measurement, so it can be converted back to a concentration in micrograms per cubic metre, and a concentration multiplied by time is a dose. That is all this calculator does: it converts your AQI to PM2.5, weights it by how long you are outside versus inside, and expresses the result on a scale people have intuition for.
The cigarette comparison comes from Berkeley Earth, which observed that breathing air at 22 µg/m³ for a day carries roughly the same long-term mortality risk as one cigarette. It is a risk analogy, not a chemical one. Cigarette smoke and wildfire smoke contain different compounds, and a cigarette delivers its dose in minutes while smoke exposure is spread across a day. The number is useful for scale, not for diagnosis.
| EPA category | AQI | PM2.5 (µg/m³) | Cigarettes/day if outdoors |
|---|---|---|---|
| Good | 0 – 50 | 0.0 – 9.0 | up to 0.4 |
| Moderate | 51 – 100 | 9.1 – 35.4 | 0.4 – 1.6 |
| Unhealthy for sensitive groups | 101 – 150 | 35.5 – 55.4 | 1.6 – 2.5 |
| Unhealthy | 151 – 200 | 55.5 – 125.4 | 2.5 – 5.7 |
| Very unhealthy | 201 – 300 | 125.5 – 225.4 | 5.7 – 10.2 |
| Hazardous | 301 – 500 | 225.5 – 325.4 | 10.2 – 14.8 |
How it’s calculated
AQI converts to PM2.5 with the EPA’s piecewise-linear formula, run in reverse: C = (Chi − Clo) ÷ (Ihi − Ilo) × (I − Ilo) + Clo, using the 24-hour PM2.5 breakpoints EPA revised in May 2024 (50 = 9.0 µg/m³, 100 = 35.4, 150 = 55.4, 200 = 125.4, 300 = 225.4, 500 = 325.4).
Your day is then split in two. Time outdoors is exposed to the full outdoor concentration; the remaining hours are exposed to an indoor fraction of it. EPA puts that fraction at about 55–60% with windows and doors closed and no air cleaner running (we use 57.5%, the midpoint), roughly 100% with windows open, and about half again as much with a MERV 13 filter running continuously. The two shares are averaged across 24 hours to give a daily equivalent concentration, which is divided by 22 µg/m³ — Berkeley Earth’s one-cigarette figure — to get the cigarette equivalent.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional. This tool converts published air-quality figures into a dose comparison; it does not interpret symptoms or assess anyone’s individual health risk.
Sources: EPA Air Quality Index (PM2.5 breakpoints, revised May 2024); EPA, Strategies to Reduce Exposure Indoors; Berkeley Earth, Air Pollution and Cigarette Equivalence.
Common mistakes
- Comparing today’s AQI to numbers from before May 2024 — EPA tightened the PM2.5 breakpoints, so the same air reads higher on the index now.
- Assuming indoors means clean. With windows shut and no filtration, EPA puts indoor PM2.5 at roughly 55–60% of outdoor — better, but not zero.
- Reading the cigarette figure as chemistry. It equates long-term mortality risk, not the contents of the smoke.
- Using an instantaneous sensor reading as if it were the AQI, which is defined on a 24-hour average.
Frequently asked questions
How many cigarettes is wildfire smoke equivalent to?
Berkeley Earth’s rule of thumb is that breathing air at 22 µg/m³ of PM2.5 for 24 hours carries roughly the same long-term mortality risk as smoking one cigarette. At an AQI of 155 — about 61 µg/m³ — a full day outdoors works out to roughly 2.8 cigarettes. The comparison is about risk, not chemistry: wildfire smoke and tobacco smoke are not the same mixture.
Does staying indoors actually help?
Yes, but indoors is not a clean room. EPA reports that with windows and doors closed and no air cleaner running, indoor PM2.5 generally settles around 55–60% of the outdoor level, though it ranges from about 30% to 100% depending on the home. Upgrading to a MERV 13 filter and running the HVAC fan continuously cuts indoor particles by roughly half again.
What AQI counts as unhealthy?
On the EPA index, 101–150 is unhealthy for sensitive groups, 151–200 is unhealthy for everyone, 201–300 is very unhealthy, and 301 and above is hazardous. EPA revised the PM2.5 breakpoints in May 2024, so the same microgram reading now maps to a higher AQI than it did before — older AQI comparisons understate today’s numbers.
Why does my weather app show a different AQI than my sensor?
The AQI is defined on a 24-hour PM2.5 average, while apps typically report a NowCast estimate weighted toward recent hours, and low-cost sensors report an instantaneous reading. During a fast-moving smoke plume these can differ a lot. Enter whichever number you have — switch the reading type to match.