Wildfire Smoke AQI: What the Numbers Mean

Wildfire smoke AQI numbers tell you how clean or polluted the air is right now, and the key threshold for staying indoors is an AQI above 150, which is considered unhealthy for everyone. The scale runs from 0 (clean air) to 500 (hazardous), and each category comes with specific health warnings. This guide explains what each range means, how to interpret the numbers on your phone or weather app, and exactly when you should keep windows closed and limit outdoor activity.

Formula

The calculator above uses the standard EPA AQI category thresholds to give you plain-language health advice. The formula is a set of if-then conditions based on the AQI value you enter (t) and whether you consider yourself sensitive (h). The AQI itself is not a simple formula; it is derived from concentrations of pollutants like PM2.5, ozone, and carbon monoxide using a piecewise linear breakpoint table. For this calculator, the output is the official health message for that AQI range.

Worked Example

Suppose the current AQI is 175 and you are not in a sensitive group. The calculator checks the first condition: t is greater than 150 and less than or equal to 200, so it returns “Unhealthy: everyone should reduce prolonged outdoor exertion.” This matches the EPA category for AQI 151–200, which is labeled “Unhealthy.” If you had entered 175 and selected “Sensitive,” the same AQI would still return the “Unhealthy” message because the threshold for sensitive groups ends at 150.

What the AQI Categories Actually Mean

The Air Quality Index is divided into six color-coded categories, each with a specific health message. The scale is standardized by the U.S. Environmental Protection Agency (EPA) and used by many countries, though some use different breakpoints. Here is what each range means for your daily activities:

• 0–50 (Green): Good. Air quality is satisfactory, and air pollution poses little or no risk. You can go outside and exercise normally.
• 51–100 (Yellow): Moderate. Air quality is acceptable, but there may be a risk for people who are unusually sensitive to air pollution, such as those with asthma or heart conditions. If you are sensitive, consider reducing prolonged outdoor exertion.
• 101–150 (Orange): Unhealthy for Sensitive Groups. Children, older adults, and people with lung disease (like asthma) or heart disease are at greater risk. Members of sensitive groups should limit prolonged outdoor exertion; everyone else is less likely to be affected.
• 151–200 (Red): Unhealthy. Everyone may begin to experience health effects. Sensitive groups may experience more serious effects. Reduce outdoor activity, and if you have symptoms, stay indoors.
• 201–300 (Purple): Very Unhealthy. Health alert: the risk of health effects is increased for everyone. Avoid outdoor exertion, and keep windows closed.
• 301–500 (Maroon): Hazardous. Health warning of emergency conditions. Everyone is likely to be affected. Stay indoors, keep windows closed, use air purifiers if available, and avoid all outdoor exertion.

How to Check Your Local AQI

Most weather apps and websites display the AQI for your area. In the U.S., you can check AirNow.gov, which reports the official AQI from monitoring stations. Many smartphone weather apps also show a separate air quality section, often with a color-coded dot. If you live near a wildfire, the AQI can change rapidly, so check it multiple times a day. You can also use the visual range: if you cannot see a landmark that is normally visible a few miles away, the air is likely unhealthy.

When to Stay Indoors: The 150 Threshold

The most important number to remember is 150. When the AQI exceeds 150, the air is unhealthy for everyone, not just sensitive groups. At that point, you should stay indoors as much as possible, keep windows and doors closed, and avoid vigorous outdoor activities like running or biking. If you must go outside, wear an N95 or P100 respirator that is properly fitted; cloth masks and surgical masks do not filter out fine particles from smoke. For sensitive individuals, the threshold is lower: if you are in a sensitive group, consider staying indoors when the AQI is above 100.

What the Numbers Don’t Tell You

The AQI is a snapshot of the current or forecasted air quality, but it does not tell you the specific pollutant causing the problem. During wildfires, the main pollutant is PM2.5—tiny particles smaller than 2.5 micrometers that can penetrate deep into the lungs and bloodstream. The AQI also does not tell you how long the poor air will last, and it may not reflect microclimates. For example, a monitoring station a few miles away may read 120 while your neighborhood is much smokier due to wind patterns. If you smell smoke or see ash falling, the air is likely worse than the reported number.

Common Misconceptions About AQI and Smoke

• “It’s okay because I don’t have asthma.” Even healthy people can experience eye irritation, coughing, and shortness of breath at AQI above 150. Long-term exposure to wildfire smoke can have lasting health effects.
• “I can just open a window for a few minutes.” During a smoke event, even brief openings allow fine particles to enter. Keep windows and doors sealed, and use an air purifier with a HEPA filter.
• “A face mask will protect me.” Only N95 or P100 respirators (or higher) are effective against smoke particles. Bandanas, scarves, and cloth masks do not provide adequate protection.
• “The AQI is the same everywhere in my city.” AQI can vary significantly by location due to wind, terrain, and proximity to the fire. Check your specific area, not just the city-wide number.

How to Protect Yourself Indoors

When the AQI is high, your home can become a safe haven if you take the right steps. First, close all windows and doors, and seal gaps with towels or weather stripping. Use a portable air cleaner with a HEPA filter in the room you spend the most time in; make sure it is sized for the room. Do not use a swamp cooler or a whole-house fan that draws in outside air. Also, avoid activities that create indoor pollution, such as burning candles, smoking, vacuuming (which stirs up dust), or frying food. If you have an HVAC system, set it to recirculate mode and use a filter rated MERV-13 or higher. If you have asthma or a heart condition, have your medication and an action plan ready.

Frequently Asked Questions

What AQI is safe to exercise outside?

For most healthy people, exercising outdoors is generally safe when the AQI is below 100. If you are in a sensitive group, consider exercising indoors when the AQI is above 50. When the AQI exceeds 150, everyone should avoid strenuous outdoor activity, as the particles can cause immediate respiratory stress.

Why does the AQI sometimes say “Good” but I can smell smoke?

This can happen when the monitoring station is not near you, or when the smoke is at ground level but the station is measuring a different pollutant. The AQI is based on measurements from specific locations, and smoke can be patchy. If you smell smoke, trust your senses and reduce your exposure even if the AQI is low.

How long does wildfire smoke stay in the air?

It depends on the fire’s size, wind, and weather. A small fire may clear in a day, while large wildfires can produce smoke that lingers for weeks. The AQI forecast can help you plan, but be prepared for sudden changes.

Can I use a regular air conditioner during a smoke event?

Yes, but set it to recirculate mode so it does not pull in outside air. Make sure the filter is clean and ideally rated MERV-13 or higher. If you have a window unit, close the outdoor air damper.

What does PM2.5 mean in the AQI?

PM2.5 refers to particulate matter that is 2.5 micrometers or smaller in diameter. These tiny particles are the main health threat in wildfire smoke because they can bypass your body’s defenses and get deep into your lungs. The AQI for PM2.5 is calculated from the concentration of these particles in the air.

Is it safe to go to work or school when the AQI is 200?

If you must go out, try to limit your time outside and avoid physical exertion. Many schools and workplaces have policies for high AQI days, such as moving activities indoors. If you have a chronic condition, consult your healthcare provider for personalized advice.

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in t | Current AQI | AQI | 175 | 0
in h | Sensitivity | Normal=0, Sensitive=1 | 0 | 0
out advice | Health advice | - | if(h==0, if(t