What Your AQI Score Actually Means for Your Health

On the morning of September 9, 2020, residents of San Francisco woke up to an orange sky. It looked like Mars. The sun was a dim red disc. And the Air Quality Index across the Bay Area was pushing 200 — a level the EPA labels "Very Unhealthy."

Most people had never paid attention to the AQI before that day. By noon, everyone knew what it meant.

The AQI is a single number — 0 to 500 — that translates the messy chemistry of outdoor air into something you can actually act on. Think of it like a speedometer for pollution. Every point above 100 means you're breathing air the EPA considers outside acceptable limits for that day's dominant pollutant.

PollutionProfile pulls your local AQI directly to your address every day — so the number you see is yours, not a regional average.

The AQI doesn't measure all pollution — just six pollutants scientists and regulators have identified as the most widespread health threats. On any given day, whichever one is highest sets the score.

But one matters far more than the others: PM2.5. Fine particulate matter — particles smaller than 2.5 micrometers — is about 30 times thinner than a human hair. That's not just a trivia fact. At that scale, particles bypass the filters in your nose, travel deep into your lungs, and some slip into the bloodstream directly. They can reach your heart and your brain. No other AQI pollutant does this.

The other five — ground-level ozone, PM10, nitrogen dioxide, sulfur dioxide, and carbon monoxide — each have their own sources and effects. But when it comes to long-term health damage, PM2.5 is the one that dominates the research.

Here's something most AQI explainers skip: the number on your phone is designed to protect you today. It says almost nothing about what years of breathing your local air is doing to your body. Those are two very different problems.

Short-term exposure is what makes the news. A red-day AQI triggers more asthma attacks, more ER visits for chest pain. The harm is real — but it's often reversible. Lung function dips and then recovers when the air clears.

Long-term exposure is quieter and harder to see. The damage accumulates: lungs age faster, arteries harden earlier, cognitive decline accelerates in older adults, life expectancy shortens — measurably, across entire populations.

The evidence landed in the 1993 Harvard Six Cities Study, which tracked over 8,000 adults for 16 years. Cities with higher pollution had higher death rates, not from acute spikes but as a persistent background effect. That finding has been replicated across dozens of countries.

Knowing the number is only half the job. The other half is matching your behavior to the color — and recognizing that it means different things depending on whether you're in a sensitive group.

Ozone peaks mid-afternoon on hot days — morning runs are safer in summer. PM2.5 from traffic often spikes during commute hours. Wildfire smoke can shift by the hour. The same AQI number means different things depending on what pollutant is driving it.

The single highest-impact habit: check AQI every morning before outdoor plans. It takes ten seconds. It guides everything else.

The number on your phone is almost certainly the US EPA AQI — but it's not the only standard in use, and it's notably more lenient than what the World Health Organization recommends. In 2024, the EPA tightened its annual PM2.5 standard from 12 µg/m³ to 9 µg/m³. The WHO's annual guideline is 5 µg/m³ — almost half the EPA limit.

This means a city that shows "Good" on AirNow every day of the year can still have an annual PM2.5 average that's nearly twice the WHO recommended maximum. "Good" is relative to the standard being used.

And different apps use different sensors entirely. AirNow reports from roughly 1,000 official EPA reference monitors across the entire US. PurpleAir's network has over 30,000 low-cost community sensors. They often show higher PM2.5 readings because they're physically closer to local sources.