Cigarette equivalent of wildfire smoke

A rough rule of thumb from Berkeley Earth: breathing about 22 µg/m³ of PM2.5 for a day ≈ one cigarette. Enter today's PM2.5 (or use your location) to see the equivalent.

Rule of thumb, not a medical measurement. Based on Berkeley Earth's PM2.5–cigarette equivalence (Pope et al. mortality data). Not medical advice.

What this calculates

One line of arithmetic: cigarettes = PM2.5 ÷ 22, where PM2.5 is in µg/m³ and the result is per 24 hours of exposure. If you enter an AQI instead of a concentration, the tool first converts it back to µg/m³ through the 2024 EPA PM2.5 breakpoints — the inverse of the calculation on every city page — and then divides.

So an AQI of 150 (about 55.4 µg/m³) works out to roughly 2.5 cigarettes a day, and a genuinely severe smoke day at 250 µg/m³ is about 11.

Where the number comes from

The 22 µg/m³ figure is a rule of thumb published by Berkeley Earth, derived by comparing the particulate dose from ambient PM2.5 against the dose from smoking. It exists to make an abstract number legible — "AQI 150" means little to most people, "two and a half cigarettes" means a lot. We use their constant unchanged rather than deriving our own.

What this does not tell you

It is a dose comparison for one pollutant, not an equivalence of harm. Cigarette smoke carries nicotine, carbon monoxide and a set of carcinogens that wildfire smoke does not deliver in the same proportions, and it is inhaled hot and concentrated rather than diluted in ambient air. Nobody should read "11 cigarettes" as "identical to smoking 11 cigarettes".

The constant is approximate and its author says so. It is a rule of thumb with one significant figure. Treating the output as precise to a decimal place reads more into it than the input supports.

It assumes a full 24 hours at that concentration. Real exposure is not flat: you sleep indoors, the plume thins overnight, you may run a purifier. If you spent the day in filtered air, your actual dose is a fraction of the number shown.

It says nothing about who you are. The same dose is not the same risk for a healthy adult and for someone with COPD.

A worked example

A Canadian plume settles over the Upper Midwest and your city page reads AQI 168. Converting through the 2024 table, that is about 85 µg/m³ of PM2.5. Divided by 22: roughly 3.9 cigarettes for a full day outdoors at that level.

Now apply the caveat that matters. You are outside for two hours and spend the rest of the day in a house running a box-fan filter that holds indoor PM2.5 near 12 µg/m³. Your real day is roughly two hours at 85 and twenty-two at 12 — a dose-weighted average near 18 µg/m³, or about 0.8 cigarettes. Same city, same day, one fifth the exposure. The gap between those two numbers is the entire argument for filtering indoor air.