Washington's worst air quality days

What were Washington's worst days?

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

What caused Washington's worst days?

The air-event catalog has named 23 episodes affecting Washington, 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 · 170.8 µg/m³ here , #1 all-time · from Canada
• Fireworks 2026-07-05 fireworks episode affecting Washington-Arlington-Alexandria, DC-VA-MD-WV Jul 5, 2026 · 54.7 µg/m³ here , #20 all-time
• Fireworks 2025-07-05 fireworks episode affecting Washington-Arlington-Alexandria, DC-VA-MD-WV Jul 5, 2025 · 59 µg/m³ here , #12 all-time
• Wildfire smoke 2023 late-June Canadian wildfire-smoke episode Jun 29, 2023 · 93.1 µg/m³ here , #4 all-time · from Canada
• Wildfire smoke 2023 early-June Canadian wildfire-smoke episode Jun 8, 2023 · 146.8 µg/m³ here , #2 all-time · from Canada
• Fireworks 2021-07-04 Washington-Arlington-Alexandria fireworks episode Jul 4, 2021 · 58.9 µg/m³ here , #13 all-time · from Washington-Arlington-Alexandria, DC-VA-MD-WV

17 more, plus every named episode and its evidence is in the air events.

How does it compare?

The current provisional record is 16% higher than the previous certified record: 170.8 µg/m³ (July 17, 2026) against 146.8 µg/m³ (June 8, 2023).

How unusual is air like this?

Every measured day near Washington, by daily PM2.5; the record stands far out on the right. 159 days at "unhealthy for sensitive groups" or worse, 18 at "unhealthy", 2 at "very unhealthy" (2024 EPA thresholds, across 10,035 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 Washington?

July 17, 2026 (provisional), when daily PM2.5 reached 170.8 µg/m³ (AQI 246), about 7.8 cigarette-equivalents. It ranks first of 10,035 monitored days since 1999.

What is the highest AQI recorded in Washington?

On the worst day, the daily PM2.5 AQI reached 246 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 229 on July 2, 2002.

Is the July 2026 ranking final?

No. Days after May 31, 2026 are preliminary AirNow readings. They rank now so a current event is visible, and they may shift once EPA certifies the data, usually the following spring.

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 Washington get air this bad?

Rarely. The record sits at the far end of 10,035 measured days; Washington 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.