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EPA AirNow — when to watch for particulates vs. ozone

When AirNow says AQI 142, it's a one-number summary. The story underneath — wildfire smoke vs. summer ozone vs. industrial NO2 — matters for which actions actually help.

Published 2026-04-25 · Last reviewed 2026-07-03 · methodology

PM2.5 (fine particulate matter)

Sub-2.5-micrometer particles. Penetrate deep into lungs + cross into bloodstream.

Sources: wildfire smoke (dominant in West summer/fall), wood-burning stoves, diesel engines, secondary chemistry from NOx + SO2.

Persistence: can last days at moderate levels during smoke events.

Indoors: HEPA filters effective. MERV-13 furnace filter helps significantly.

Outdoors: N95 masks block ~95% of PM2.5; cloth + surgical do not (gaps).

Ozone (O3)

Ground-level ozone — different from stratospheric (good ozone). Forms from NOx + VOCs in heat + sunlight.

Sources: vehicle exhaust + fossil-fuel chemistry + heat. Peaks 12–6pm in summer.

Persistence: intra-day; clears overnight.

Indoors: HEPA does NOT filter ozone (it's a gas). Activated-carbon filters help.

Outdoors: masks DO NOT help with ozone. Move exercise to early morning.

Reading the AirNow forecast

AirNow's daily forecast names the dominant pollutant.

Western states summer/fall: usually PM2.5 dominant.

East Coast metros June-August: usually ozone dominant in afternoons.

Industrial corridors (Cancer Alley LA, Houston petrochemical, Pittsburgh): year-round mix.

Cold-snap winters (small mountain valleys): inversion layers can spike PM2.5.

Action by pollutant + AQI band

PM2.5 + AQI ≥101: switch to indoor activity, run HEPA, N95 if you must go out.

Ozone + AQI ≥101: shift outdoor activity to early morning, no masks needed.

Both + AQI ≥151: stay indoors, run HEPA + activated-carbon, monitor symptoms.

What zipradar shows

Per-ZIP rolling 30-day AQI summary. Where the dominant pollutant data is available, we surface it. Where it's not, we say so.

Real-time + day-of decisions: airnow.gov is the truth. Our snapshot is for trend reading.