AQI and Your Health: What Each Level Means

Last updated April 29, 2026 · 8 min read

An air-quality reading is only useful if you know what to do with it. This guide walks through what each AQI level actually does to the body, who is most vulnerable, and a concrete action plan you can apply at every level.

Who is "sensitive"?

The phrase "sensitive groups" gets repeated in every AQI app and rarely defined. The U.S. EPA's working definition is broad and worth memorizing because it likely includes someone in your household:

• People with asthma, COPD, or other chronic lung disease
• People with heart disease or recent cardiac events
• People with diabetes (cardiovascular complications make air pollution riskier)
• People with active respiratory infections — yes, this includes a bad cold
• Children, especially under 14 (developing lungs and higher breathing rates per kg of body weight)
• Pregnant people (PM2.5 exposure is associated with reduced birthweight and preterm birth in observational studies)
• Adults aged 65 and older
• Anyone who is exercising heavily outdoors at the moment, regardless of underlying health (heavy breathing means more dose)

If you fall into any of these groups, your AQI threshold for action is roughly one category lower than the general public's. AQI 80–100 (upper Moderate) is when you start paying attention; AQI 100+ (orange) is when you actually adjust behavior.

The action plan, level by level

AQI 0–50 (Good, green)

No restrictions. The air is doing what air is supposed to do. This is the right time for high-intensity outdoor exercise, marathon training, kids playing outside as long as they want.

AQI 51–100 (Moderate, yellow)

For the general public: still no meaningful restriction. For unusually sensitive individuals — people with severe asthma, advanced COPD, or recent cardiac events — symptoms can begin in the upper half of this range, especially during prolonged or intense outdoor exertion. If you've noticed your symptoms tend to start around AQI 75–100, that's a reliable personal threshold and worth respecting even when the EPA category is still "Moderate".

AQI 101–150 (Unhealthy for Sensitive Groups, orange)

This is the action threshold for sensitive groups. Concretely:

• Asthma and COPD: keep rescue medication on hand. Reduce prolonged or intense outdoor exertion. Move workouts indoors.
• Children: shorten outdoor recess and outdoor sports. Substitute indoor games when feasible. School closures generally don't kick in until red, but cross-country and soccer practices often move indoors at orange.
• Older adults and people with heart disease: limit extended outdoor exertion. A walk is fine; a vigorous hike is not.
• Everyone else: business as usual, but a heavy workout outdoors will likely feel a bit harder than usual. That's a reliable cue that the air is the limiter.

AQI 151–200 (Unhealthy, red)

Now the general public starts to feel it: throat scratchiness, watery eyes, mild cough, more pronounced shortness of breath during exertion. The EPA recommends:

• Sensitive groups: avoid all outdoor exertion. Stay indoors with windows closed. Run an air purifier if you have one. If you must go outside, a properly fitted N95 or KN95 reduces PM2.5 dose substantially (it doesn't help against ozone).
• General public: limit prolonged or intense outdoor exertion. Reschedule workouts indoors. Driving with windows up and recirculate-on reduces in-cabin PM2.5 dramatically.
• Schools: many districts trigger their air-quality protocols at red — outdoor PE moves indoors, recess shortens, athletic events may be postponed.

AQI 201–300 (Very Unhealthy, purple)

Health alert level. Everyone is at increased risk for effects, and sensitive groups are at meaningful risk for serious effects. Stay indoors with the windows closed and HVAC set to recirculate. Run a HEPA air purifier in the room you spend the most time in (sized for the room: a unit's CADR rating in cubic feet per minute should be at least two-thirds of the room's volume). If your home doesn't seal well, "create a clean room": pick one interior room, close the door, weatherstrip if you can, and run the purifier there. Cancel outdoor exercise; even brief outdoor exertion can be enough to trigger asthma exacerbation at this level. See Indoor Air Quality for the practical setup.

AQI 301+ (Hazardous, maroon)

Emergency conditions. Everyone is more likely to be affected; sensitive groups are at serious risk. Stay indoors. Wear an N95 if you must briefly leave the house. Local agencies often issue "stay indoors" advisories at this level. School closures, race cancellations, and air-quality-triggered telework policies are common. If you are caring for someone with asthma or heart disease, watch for symptoms — chest tightness, persistent cough, breathing harder than usual at rest — and have a clinician's number handy.

Acute vs. chronic exposure

Health effects come in two flavors. Acute effects show up within hours: airway irritation, asthma exacerbation, headache, fatigue, even emergency-room visits during severe smoke events. Chronic effects show up over years: reduced lung function, increased risk of cardiovascular disease, increased risk of lung cancer, and (for very high long-term exposures) reduced life expectancy. The WHO estimates ambient air pollution contributes to around 4.2 million premature deaths globally per year, primarily through PM2.5-driven cardiovascular and respiratory disease.

Because chronic exposure is the much bigger health story but produces no acute signal, it's easy to undercount the value of paying attention to AQI on Moderate days. Living in a city with an annual average PM2.5 of 12 µg/m³ — a number that rarely produces an AQI above 100 — still raises long-term risk relative to a city averaging 6 µg/m³.

For immunocompromised readers, the AQI threshold for action shifts down further when there's an active respiratory-illness wave: the inflammatory baseline from PM2.5 stacks with infection susceptibility. Pandemic Watch tracks current outbreak surveillance and is worth reading alongside AirNow on Moderate or Unhealthy-for-Sensitive-Groups days.

Special situations

Pregnancy

Observational studies link maternal PM2.5 exposure with reduced birthweight, preterm birth, and gestational hypertension. The EPA includes pregnant people in sensitive groups. Practical guidance: treat orange (101+) as your action threshold; minimize time outdoors during smoke events; air-purifier the room you sleep in.

Children at school

Children breathe more air per kilogram of body weight than adults and are typically more active, so their dose is higher. Schools in California, Oregon, and Washington — states with frequent wildfire smoke — generally use AirNow data to drive their outdoor-activity decisions: green/yellow normal, orange shortened outdoor PE, red and above indoor only. If your district doesn't have a written policy, a polite email to the principal asking about it tends to produce one.

People with cardiovascular disease

Air pollution's cardiovascular effects often surprise people: PM2.5 exposure raises the risk of heart attack and stroke within days of a spike, mediated through inflammation and changes in heart-rate variability. The American Heart Association formally recognized PM2.5 as a cardiovascular risk factor in 2010. If you've had a recent cardiac event, AQI 100+ is your action threshold even if you have no respiratory symptoms.

What you can't outrun: chronic baseline

Most of the health hit from air pollution comes not from the few red-AQI smoke days each year but from the year-round Moderate days you probably ignore. The strongest single thing most people can do is reduce their indoor baseline — see Indoor Air Quality — because you spend roughly 90% of your time indoors, and indoor PM2.5 typically tracks outdoor PM2.5 closely without active filtration.

Primary sources: EPA AirNow — AQI Basics · EPA — PM Health Effects · WHO Ambient Air Quality Fact Sheet · CDC — Air Quality