Wildfire Smoke AQI vs PM2.5: What Readings Mean & When to Act

Wildfire smoke AQI and PM2.5 readings both measure air pollution, but they answer different questions: AQI tells you the overall health risk in simple terms, while PM2.5 is the raw concentration of the tiny particles that make smoke dangerous. You should act—reduce outdoor activity, close windows, or use an air purifier—when the AQI exceeds 100, or when PM2.5 exceeds 35 µg/m³ over 24 hours, though sensitive groups may need to act at lower levels.

Formula

The Air Quality Index (AQI) is a standardized scale used by the U.S. EPA to communicate health risk. It converts pollutant concentrations—including PM2.5—into a number from 0 to 500. For PM2.5, the breakpoints are:

• AQI 0–50: PM2.5 0.0–12.0 µg/m³ (Good)
• AQI 51–100: PM2.5 12.1–35.4 µg/m³ (Moderate)
• AQI 101–150: PM2.5 35.5–55.4 µg/m³ (Unhealthy for Sensitive Groups)
• AQI 151–200: PM2.5 55.5–150.4 µg/m³ (Unhealthy)
• AQI 201–300: PM2.5 150.5–250.4 µg/m³ (Very Unhealthy)
• AQI 301–500: PM2.5 250.5–500.4 µg/m³ (Hazardous)

The exact formula uses a piecewise linear interpolation: for a given PM2.5 concentration C, find the bracket (C_low, C_high) and corresponding AQI bracket (I_low, I_high), then compute:

AQI = (I_high – I_low) / (C_high – C_low) × (C – C_low) + I_low

Where C is the PM2.5 concentration in µg/m³ (micrograms per cubic meter), and the breakpoints are the official EPA values. The calculator above uses these breakpoints to convert a PM2.5 reading into an AQI estimate, and also categorizes the PM2.5 level directly.

Worked Example

Suppose your outdoor air sensor reads PM2.5 = 55 µg/m³. To find the AQI:

1. Identify the bracket: 55.5 µg/m³ is the lower bound for the 151–200 AQI range, and 150.4 µg/m³ is the upper bound. But 55 is below 55.5, so it falls in the 101–150 bracket (35.5 to 55.4 µg/m³).
2. Apply the formula: AQI = (150 – 101) / (55.4 – 35.5) × (55 – 35.5) + 101 = (49 / 19.9) × 19.5 + 101 ≈ 2.462 × 19.5 + 101 ≈ 48.0 + 101 = 149.
3. So PM2.5 of 55 µg/m³ corresponds to an AQI of about 149, which is “Unhealthy for Sensitive Groups.�

The calculator gives the same result: AQI ≈ 149, category 3 (Unhealthy for Sensitive Groups).

What the AQI Numbers Actually Mean

The AQI is a color-coded index that translates complex pollution data into a simple number. The categories are:

• 0–50 (Green): Air quality is satisfactory, little or no risk.
• 51–100 (Yellow): Acceptable, but a very small number of sensitive people may have mild symptoms.
• 101–150 (Orange): Sensitive groups (children, elderly, people with respiratory or heart conditions) may experience health effects; the general public is less likely to be affected.
• 151–200 (Red): Everyone may begin to experience health effects; sensitive groups may experience more serious effects.
• 201–300 (Purple): Health alert: everyone may experience more serious health effects.
• 301–500 (Maroon): Emergency conditions: the entire population is likely to be affected.

When wildfire smoke blankets an area, PM2.5 is typically the dominant pollutant, so the AQI is often driven almost entirely by PM2.5. That’s why you’ll see the AQI jump to 150+ during heavy smoke events.

PM2.5: The Tiny Particle That Matters

PM2.5 refers to particulate matter with a diameter of 2.5 micrometers or less—about 30 times smaller than a human hair. These particles are dangerous because they are small enough to bypass the body’s natural defenses in the nose and throat, and can penetrate deep into the lungs and even enter the bloodstream. Wildfire smoke is composed largely of PM2.5, along with other gases like carbon monoxide and volatile organic compounds.

Exposure to PM2.5 is linked to a range of health problems, including:

• Asthma attacks and worsened COPD
• Increased risk of heart attacks and strokes
• Respiratory infections
• Low birth weight and preterm birth in pregnant women
• Premature death in people with chronic heart or lung disease

Because of these risks, public health agencies set both short-term (24-hour) and long-term (annual) exposure limits. The U.S. EPA’s current annual standard is 12 µg/m³, and the 24-hour standard is 35 µg/m³. The World Health Organization (WHO) has stricter guidelines: 5 µg/m³ annual and 15 µg/m³ 24-hour.

How to Read a Sensor or App Reading

When you check an air quality app or a consumer sensor, you’ll often see both an AQI number and a PM2.5 concentration in µg/m³. Here’s how to interpret them:

• AQI = 50 with PM2.5 = 12 µg/m³: Good air, no action needed.
• AQI = 100 with PM2.5 = 35 µg/m³: Moderate, but sensitive people should consider reducing prolonged outdoor exertion.
• AQI = 150 with PM2.5 = 55 µg/m³: Unhealthy for sensitive groups; the general public is still unlikely to be affected, but it’s wise to keep windows closed.
• AQI = 200 with PM2.5 = 150 µg/m³: Unhealthy for everyone; avoid outdoor exertion, use an air purifier indoors.
• AQI = 300+ with PM2.5 = 250+ µg/m³: Very unhealthy to hazardous; stay indoors as much as possible, keep doors and windows sealed, and consider evacuating if you have respiratory conditions.

Note that some low-cost sensors report “PM2.5� in µg/m³ but may also show an “AQI� that is calculated using a different formula (e.g., the NowCast or a simple linear conversion). Always check whether the AQI is the official EPA AQI or a sensor-specific estimate.

When to Act: Practical Thresholds

Your actions should depend on both your personal health and the current reading. Here are general guidelines:

• AQI < 50: No action needed.
• AQI 51–100: Sensitive individuals should limit prolonged outdoor exertion.
• AQI 101–150: Sensitive groups should reduce outdoor activities; everyone else should keep an eye on symptoms.
• AQI 151–200: Everyone should reduce outdoor exertion; sensitive groups should stay indoors as much as possible.
• AQI 201–300: Everyone should avoid outdoor exertion; keep windows closed, run air purifiers, and consider N95 masks if you must go outside.
• AQI > 300: Stay indoors, seal windows and doors, use HEPA purifiers, and avoid all outdoor activity unless absolutely necessary.

For PM2.5 specifically, a 24-hour average above 35 µg/m³ is considered unhealthy for sensitive groups; above 55 µg/m³ is unhealthy for everyone; above 150 µg/m³ is very unhealthy. If you see PM2.5 spikes above 100 µg/m³, treat it as a red flag even if the AQI seems moderate (which can happen with some sensors).

Common Misconceptions and Mistakes

One frequent mistake is assuming the AQI and PM2.5 are the same thing. They are not: AQI is a derived index, while PM2.5 is the raw measurement. Another is using a 1-hour PM2.5 reading to compare against the 24-hour standard—the standards are based on 24-hour averages, so a short spike may not trigger an official alert but can still be harmful.

People also confuse the “NowCast� AQI (used by AirNow) with the standard AQI. The NowCast gives more weight to recent hours, so it can be higher or lower than the standard 24-hour AQI. Always check which index your app is showing.

Finally, many consumer sensors (like those using laser particle counters) may overestimate PM2.5 because they cannot distinguish between smoke particles and other particles like dust or pollen. Calibrate your sensor if possible, or cross-check with official monitoring stations.

How to Protect Yourself During Wildfire Smoke

When smoke is present, the most effective actions are:

• Stay indoors with windows and doors closed.
• Use a HEPA air purifier in the room you occupy most.
• If you must go outside, wear an N95 or P100 respirator (not a cloth or surgical mask).
• Avoid strenuous outdoor activities, including exercise and manual labor.
• Keep indoor air clean by not smoking, burning candles, or using gas stoves.
• If your home has central air, set it to recirculate and use a MERV-13 or higher filter.
• For people with asthma or heart disease, have medications ready and follow your action plan.

Frequently Asked Questions

What is the difference between AQI and PM2.5?
AQI is a color-coded index that combines several pollutants into one number. PM2.5 is a specific pollutant—tiny particles in the air. During wildfire smoke, PM2.5 is usually the main driver of the AQI, but the AQI can also reflect ozone, carbon monoxide, and other pollutants.

Should I trust a $50 air quality sensor?
Consumer sensors can be useful for trend monitoring, but they are less accurate than government monitors. They may overestimate PM2.5 in humid conditions or underestimate in heavy smoke. Use them as a relative guide, not a precise measurement.

What does an AQI of 300 mean for my health?
An AQI of 300 is “Hazardous.� Everyone is at risk of serious health effects. Healthy people should avoid all outdoor exertion, and sensitive groups should stay indoors, use air purifiers, and consider relocating temporarily if possible.

Can I exercise outdoors when AQI is 150?
It’s not recommended. At AQI 150, sensitive groups should avoid outdoor exertion, and even healthy individuals may experience coughing or shortness of breath. If you must exercise, do it indoors with filtered air.

Why does the AQI sometimes show “Good� but I can smell smoke?
Smoke smell can come from gases like volatile organic compounds that are not part of the AQI. Also, local conditions can vary: your immediate area may have lower PM2.5 than a nearby monitor. Check a real-time monitor near you.

How long does PM2.5 stay in the air after a fire?
It depends on weather and fire activity. In stagnant conditions, smoke can linger for days. Rain can help clear it, but PM2.5 can also be resuspended by wind. Monitor readings until they consistently drop below the unhealthy thresholds.

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in aqi | Air Quality Index (AQI) | - | 150 | 0
in pm | PM2.5 concentration | µg/m³ | 55 | 0
out cat | AQI Category | - | if(aqi