Air Quality Index and Your Health: What the Numbers Mean

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What the AQI Scale Measures

The Air Quality Index (AQI) is a 0-500 scale published by the EPA that converts pollutant concentrations into a single number. Higher is worse. The scale is not linear — the categories represent health thresholds, not equal increments.

AQI reports the worst-performing pollutant for the day. If PM2.5 is at AQI 80 and ozone is at AQI 110, the reported AQI is 110 (ozone, the worse of the two).

The Six AQI Categories

|---|---|---|---|

| 151-200 | Unhealthy | Red | Everyone |

Most major US cities spend the majority of days in the 0-100 range. During wildfire season or heat events, AQI can spike to 200+ even in areas that normally score well.

The Two Main Pollutants That Drive AQI

**PM2.5 (Fine Particulate Matter)**: Particles smaller than 2.5 microns. Primary driver of AQI during wildfire smoke events and in areas with heavy vehicle traffic. PM2.5 penetrates the lungs and enters the bloodstream. This is what air purifiers address best. The 24-hour average PM2.5 concentration that corresponds to AQI 100 is 35.4 micrograms per cubic meter.

**Ozone (ground-level)**: A gas formed by chemical reactions between vehicle emissions and sunlight. Peaks on hot, sunny afternoons. Ozone cannot be filtered by a standard HEPA + carbon air purifier — it requires specialized activated carbon media or a dedicated ozone scrubber. If the day's AQI is driven by ozone rather than PM2.5, your air purifier addresses less of the risk.

When to Check AQI and How to Act

**For sensitive groups (asthma, lung disease, children under 14, adults over 65):**

AQI 51-100: Consider limiting prolonged outdoor exertion
AQI 101-150: Reduce outdoor activity; run your air purifier indoors
AQI 151+: Stay indoors; seal windows, run purifier continuously

**For healthy adults:**

AQI 101-150: Sensitive groups should limit outdoor activity; healthy adults can usually tolerate moderate exertion
AQI 151-200: Limit all strenuous outdoor activity
AQI 201+: Avoid outdoor activity; indoor air management is essential

Where to Check AQI

The EPA's AirNow.gov is the official source. It reports both current AQI and forecasts by ZIP code. The IQAir AirVisual app aggregates EPA and sensor data and shows hyperlocal readings from nearby outdoor monitors.

For real-time neighborhood-level data, PurpleAir.com uses a network of community-owned sensors. Note that PurpleAir raw readings run higher than EPA-calibrated values — PurpleAir applies a correction factor (EPA AQS conversion) that you can enable in the settings.

AQI and Your Air Purifier

The action rule: above AQI 100 with PM2.5 as the driver, run your air purifier on medium or high continuously. A CADR-matched purifier can reduce indoor PM2.5 to Good levels (AQI < 50) even when outdoor air is Unhealthy (AQI 150-200), as long as windows and doors are kept closed.

At AQI 200+, an air purifier alone may not fully compensate if the building has significant air infiltration. Adding weatherstripping, using HVAC with MERV 13 filters, and sealing gaps around windows and doors significantly improves outcomes.

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