San Diego's worst air quality days

What were San Diego's worst days?

What caused San Diego's worst days?

The event catalog has named 5 episodes affecting San Diego, 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 2020-10-04 San Diego-Carlsbad wildfire-smoke episode Oct 5, 2020 · 93 µg/m³ here , #6 all-time · from San Diego-Carlsbad, CA
• Wildfire smoke 2020-09-10 8-area wildfire-smoke episode Sep 14, 2020 · 96.7 µg/m³ here , #5 all-time · from Fresno, CA
• Fireworks 2009-01-01 fireworks episode affecting San Diego-Carlsbad, CA Jan 1, 2009 · 78.3 µg/m³ here , #8 all-time
• Wildfire smoke October 2007 Southern California wildfire-smoke episode Oct 23, 2007 · 126.2 µg/m³ here , #2 all-time · from Southern California
• Wildfire smoke Late-October 2003 Southern California wildfire-smoke episode Oct 27, 2003 · 239.2 µg/m³ here , #1 all-time

every named episode and its evidence is in the event catalog.

How does it compare?

The record is 90% higher than the next-worst day: 239.2 µg/m³ (October 27, 2003) against 126.2 µg/m³ (October 23, 2007).

How unusual is air like this?

Every measured day near San Diego, by daily PM2.5; the record stands far out on the right. 206 days at "unhealthy for sensitive groups" or worse, 25 at "unhealthy", 2 at "very unhealthy" (2024 EPA thresholds, across 10,019 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 San Diego?

October 27, 2003, when daily PM2.5 reached 239.2 µg/m³ (AQI 328), about 10.9 cigarette-equivalents. It ranks first of 10,019 monitored days since 1999.

What is the highest AQI recorded in San Diego?

On the worst day, the daily PM2.5 AQI reached 328 (Hazardous). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 328 on October 27, 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 San Diego get air this bad?

Rarely. The record sits at the far end of 10,019 measured days; San Diego 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.