Allentown's worst air quality days

What were Allentown's worst days?

Days marked provisional are preliminary AirNow readings after March 31, 2026, the latest date EPA has certified for Allentown; they may shift once finalized.

What caused Allentown's worst days?

The event catalog has named 8 episodes affecting Allentown, 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 16, 2026 · 132 µg/m³ here , #2 all-time · from Canada
• Wildfire smoke 2023 late-June Canadian wildfire-smoke episode Jun 29, 2023 · 79.9 µg/m³ here , #6 all-time · from Canada
• Wildfire smoke 2023 early-June Canadian wildfire-smoke episode Jun 7, 2023 · 258.9 µg/m³ here , #1 all-time · from Canada
• Wildfire smoke 2021-07-20 11-area wildfire-smoke episode Jul 20, 2021 · 60.2 µg/m³ here , #15 all-time · from Albany-Schenectady-Troy, NY
• Winter inversion 2010-12-31 through 2011-01-01 winter-inversion episode affecting Bridgeport-Stamford-Norwalk, CT, Springfield, MA, Allentown-Bethlehem-Easton, PA-NJ Jan 1, 2011 · 53.2 µg/m³ here , #25 all-time
• Wildfire smoke July 2004 eastern U.S. Alaskan wildfire-smoke episode Jul 22, 2004 · 54.5 µg/m³ here , #24 all-time · from Alaska

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

How does it compare?

The record is 96% higher than the next-worst day: 258.9 µg/m³ (June 7, 2023) against 132 µg/m³ (July 16, 2026).

How unusual is air like this?

Every measured day near Allentown, by daily PM2.5; the record stands far out on the right. 149 days at "unhealthy for sensitive groups" or worse, 21 at "unhealthy", 2 at "very unhealthy" (2024 EPA thresholds, across 9,846 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 Allentown?

June 7, 2023, when daily PM2.5 reached 258.9 µg/m³ (AQI 368), about 11.8 cigarette-equivalents. It ranks first of 9,846 monitored days since 1999.

What is the highest AQI recorded in Allentown?

On the worst day, the daily PM2.5 AQI reached 368 (Hazardous). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 368 on June 7, 2023.

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 Allentown get air this bad?

Rarely. The record sits at the far end of 9,846 measured days; Allentown averages about one "unhealthy" day every 2 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.