Guides · Health
The Air Quality Index, explained without the jargon
6 min read · Updated September 2026
The Air Quality Index turns several different pollutants into one number from 0 to 500. The number you see is whichever pollutant is worst at that moment, so an AQI of 130 on a smoky day and an AQI of 130 on a hot smoggy day are asking you to do different things.
Here is how to read it, and what to do at each level.
The short version
- The AQI reports the single worst pollutant at that moment.
- PM2.5 from smoke affects the heart as well as the lungs.
- A HEPA purifier or a box fan with a MERV 13 filter is the practical indoor fix.
The six categories
- 0 to 50, green, good. No action needed.
- 51 to 100, yellow, moderate. Unusually sensitive people may notice symptoms during long outdoor exertion.
- 101 to 150, orange, unhealthy for sensitive groups. Children, older adults, and people with asthma, COPD, or heart disease should shorten and ease outdoor activity.
- 151 to 200, red, unhealthy. Everyone should reduce prolonged exertion outdoors. Sensitive groups should stay inside.
- 201 to 300, purple, very unhealthy. Stay indoors with filtration; move exercise inside.
- 301 and above, maroon, hazardous. Treat it as an emergency for anyone with a lung or heart condition.
Why PM2.5 is the one to watch
PM2.5 means particles smaller than 2.5 micrometers, roughly a thirtieth the width of a human hair. They are small enough to pass through the lung's defenses into the deep airways and the bloodstream, which is why the health effects are cardiovascular as well as respiratory: emergency room visits for heart attacks and strokes rise on high-particulate days.
Wildfire smoke is mostly PM2.5, which is why smoke drives AQI far higher than typical urban haze. Ozone, the other common driver, is a summertime afternoon problem formed by sunlight acting on traffic and industrial emissions, and it usually peaks between 2 and 7 p.m. while smoke can sit all day and all night.
What actually helps indoors
- Close windows and run air conditioning on recirculate rather than fresh air intake.
- A HEPA purifier sized for the room genuinely works on PM2.5. Look at the clean air delivery rate, not the marketing.
- A box fan taped to a MERV 13 furnace filter, the do-it-yourself approach that spread during recent wildfire seasons, removes a large share of smoke particles for a fraction of the cost. Four filters in a cube works better than one.
- Skip ozone generators and ionizers. They add a pollutant to solve a pollutant.
- Do not vacuum, burn candles, fry food, or smoke indoors while air quality is bad. All of them add particles.
Masks, honestly
Cloth and surgical masks do very little for smoke. A well-fitted N95 or KN95 does, because it is designed for exactly this particle size. Fit matters more than brand: air takes the easiest path, and a gap at the nose bridge defeats the filter.
Planning around it
If you exercise outdoors, check the forecast rather than the current reading, and move your run to the morning on ozone days and skip it entirely on smoke days above roughly 150. Exercise multiplies your exposure because you breathe far more air, and you breathe it through your mouth, bypassing the nose's filtering.