Dayton's worst air quality days

What were Dayton's worst days?

Days marked provisional are preliminary AirNow readings after April 30, 2026, the latest date EPA has certified for Dayton; they may shift once finalized.

What caused Dayton's worst days?

The event catalog has named 7 episodes affecting Dayton, each resolved to a cause only where retained evidence supports it: EPA monitor qualifiers, NWS alerts, agency statements, and reporting. The ranking above stays cause-neutral; this is where the causes live.

• Wildfire smoke July 15–20, 2026 Great Lakes and eastern U.S. wildfire-smoke episode Jul 17, 2026 · 99.3 µg/m³ here , #2 all-time · from Canada
• Fireworks 2025-07-04 fireworks episode affecting Cincinnati, OH-KY-IN, Indianapolis-Carmel-Anderson, IN, Dayton, OH, Louisville/Jefferson County, KY-IN Jul 5, 2025 · 47 µg/m³ here , #18 all-time
• Wildfire smoke 2023 mid-July Canadian wildfire-smoke episode Jul 17, 2023 · 48.2 µg/m³ here , #14 all-time · from Canada
• Wildfire smoke 2023 late-June Canadian wildfire-smoke episode Jun 28, 2023 · 128.6 µg/m³ here , #1 all-time · from Canada
• Wildfire smoke 2023 early-June Canadian wildfire-smoke episode Jun 7, 2023 · 54.5 µg/m³ here , #7 all-time · from Canada
• Wildfire smoke September 2005 central and eastern U.S. western-fire smoke episode Sep 13, 2005 · 49.1 µg/m³ here , #11 all-time · from Idaho and Montana

1 more, plus every named episode and its evidence is in the event catalog.

How does it compare?

The record is 30% higher than the next-worst day: 128.6 µg/m³ (June 28, 2023) against 99.3 µg/m³ (July 17, 2026).

How unusual is air like this?

Every measured day near Dayton, by daily PM2.5; the record stands far out on the right. 93 days at "unhealthy for sensitive groups" or worse, 5 at "unhealthy", 1 at "very unhealthy" (2024 EPA thresholds, across 6,917 measured days).

Year by year (1999–2026)

Worst single day each year, by the selected measure.

Why did reported numbers differ?

During a major event, AirNow, weather apps, news reports, and neighborhood monitors can all show different figures at the same moment. All of them can be right:

• Concentration vs AQI. µg/m³ is the measurement; AQI is a 0 to 500 index derived from it. "330 µg/m³" and "AQI 500" can describe the same air.
• Hourly vs NowCast vs daily. An hourly peak, the NowCast rolling estimate apps display, and the full-day average answer different questions; this page ranks full days.
• One monitor vs a composite. A single sensor can read far above or below a citywide figure. The measure toggle above shows both framings.
• Live vs certified. Real-time feeds get corrected when EPA certifies data months later, which is why recent days here carry a provisional label.

What about indoors?

Citywide monitors can tell you what happened outside. They cannot tell you what happened inside a particular home, school, or facility: filtration, ventilation, and building operation decide that. During and after days like the ones above, outdoor particles become an indoor load question.

Check current conditions and the next clean-air window → Estimate recent filter load where you are →

Common questions

What was the worst air-quality day in Dayton?

June 28, 2023, when daily PM2.5 reached 128.6 µg/m³ (AQI 204), about 5.8 cigarette-equivalents. It ranks first of 6,917 monitored days since 1999.

What is the highest AQI recorded in Dayton?

On the worst day, the daily PM2.5 AQI reached 204 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 217 on June 24, 2003.

Were these days caused by wildfire smoke?

This page ranks measured PM2.5 without assigning a cause: a high day can be smoke, fireworks, dust, an inversion, or industry. A separate wildfire-smoke event ranking, using a published attribution method with its version disclosed, is the next layer of this catalog.

How often does Dayton get air this bad?

Rarely. The record sits at the far end of 6,917 measured days; Dayton averages about one "unhealthy" day every 6 years.

Why did different apps show different numbers that day?

Concentration (µg/m³) and AQI are different scales; hourly, NowCast, and daily values answer different questions; a single monitor differs from a citywide composite; and live feeds get revised when EPA certifies data. This page uses the certified daily record, with the provisional tail labeled.