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:

• 0–50 (Green): Air is clean. Go outside.
• 51–100 (Yellow): Acceptable for most, but sensitive groups should take note.
• 101–150 (Orange): Unhealthy for people with asthma, heart conditions, or lung disease.
• 151–200 (Red): Everyone may start to feel effects. Limit outdoor exercise.
• 201–300 (Purple): Serious risk. Stay indoors if you can.
• 301–500 (Maroon): Hazardous. The kind of air that makes headlines.

The number is calculated against the EPA's legal pollution limits — so an AQI of 100 means the air has hit exactly the safety threshold for that day's dominant pollutant. Every point above 100 means you're breathing air the EPA considers outside acceptable limits.

What the AQI doesn't tell you is equally important. It's measured at official monitoring stations — sometimes miles from your home. It covers only six specific pollutants. And it's a daily average, which can smooth over dangerous spikes during rush hour or when a fire ignites upwind. 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.

Ground-level ozone (O₃) Not the protective ozone layer high above — this is ozone formed when car exhaust and industrial emissions cook in sunlight. It peaks on hot summer afternoons and is one of the main triggers for asthma attacks.

Fine particulate matter (PM2.5) Particles smaller than 2.5 micrometers — about 30 times thinner than a human hair. They travel deep into the lungs and some slip into the bloodstream. Wildfire smoke, diesel exhaust, and coal plants are the biggest sources. PM2.5 is the pollutant most strongly linked to heart attacks and premature death.

Coarse particulate matter (PM10) Larger dust particles from construction sites, unpaved roads, and farming. They irritate the nose and throat but don't penetrate as deeply as PM2.5.

Nitrogen dioxide (NO₂) The signature pollutant of heavy traffic. Children who grow up near busy roads breathe more of it — and research links it directly to the development of asthma.

Sulfur dioxide (SO₂) Comes mainly from coal-burning power plants. It can trigger severe breathing problems in people with respiratory disease, sometimes within minutes of exposure.

Carbon monoxide (CO) An odorless gas from combustion — cars, wildfires, gas appliances. It displaces oxygen in the blood and is most dangerous in enclosed spaces, but outdoor spikes near heavy traffic are more common than most people realize.

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, more calls to pediatric clinics. 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. It's what happens when you live for years near a freeway, downwind of a power plant, or in a city where annual PM2.5 is consistently elevated. 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 a landmark 1993 study — the 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.

The AQI tells you about the air today. It doesn't tell you about the air in the house you grew up in, the school you attended, or the city where you spent your twenties. That cumulative story shapes your long-term health far more than any single bad air day — and it's exactly what PollutionProfile's Historical Exposure Recorder is built to map.

Knowing the number is only half the job. Here's how to actually use it.

Match your behavior to the color: • Green/Yellow: Most people can exercise outside without concern. If you have asthma, keep your inhaler handy on Yellow days. • Orange: Sensitive groups — asthma, heart disease, COPD, elderly, young children — should skip intense outdoor workouts. A walk is fine; a long run isn't. • Red: Everyone should reduce outdoor exertion. Work out inside. Keep windows closed. • Purple/Maroon: Treat it like a weather emergency. Stay in, run an air purifier, avoid opening doors unnecessarily.

Time your outdoor activity smartly: Ozone peaks mid-afternoon on hot days — morning runs are safer in summer. PM2.5 from traffic often spikes during the morning and evening commute. Wildfire smoke can shift by the hour depending on wind.

Track your body, not just the number: Some people notice headaches, fatigue, or chest tightness on moderate AQI days that wouldn't trigger any official alert. Your personal response is data. Logging it over time reveals patterns that generic guidelines never could — and it starts with knowing what you're breathing each day.