San Jose's worst air quality days

What were San Jose's worst days?

What caused San Jose's worst days?

The event catalog has named 6 episodes affecting San Jose, 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 2020-10-01 3-area wildfire-smoke episode Oct 2, 2020 · 79.7 µg/m³ here , #13 all-time · Glass Fire
• Wildfire smoke 2020-09-10 8-area wildfire-smoke episode Sep 11, 2020 · 123.1 µg/m³ here , #3 all-time · from Fresno, CA
• Wildfire smoke 2020-08-19 6-area wildfire-smoke episode Aug 19, 2020 · 85.3 µg/m³ here , #10 all-time · from Bakersfield, CA
• Wildfire smoke Wildfire-smoke episode, 2018-11-09 through 2018-11-20 Nov 16, 2018 · 138.4 µg/m³ here , #1 all-time · Camp Fire
• Other California Central Valley Christmas–New Year 2000–2001 aerosol episode Dec 30, 2000 · 67.2 µg/m³ here , #12 all-time · from California Central Valley
• Wildfire smoke 1999-10-20 wildfire smoke episode affecting Sacramento--Roseville--Arden-Arcade, CA, San Jose-Sunnyvale-Santa Clara, CA Oct 21, 1999 · 69.3 µg/m³ here , #19 all-time

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

How does it compare?

The record is 3% higher than the next-worst day: 138.4 µg/m³ (November 16, 2018) against 133.9 µg/m³ (November 15, 2018).

How unusual is air like this?

Every measured day near San Jose, by daily PM2.5; the record stands far out on the right. 209 days at "unhealthy for sensitive groups" or worse, 49 at "unhealthy", 2 at "very unhealthy" (2024 EPA thresholds, across 8,441 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 Jose?

November 16, 2018, when daily PM2.5 reached 138.4 µg/m³ (AQI 214), about 6.3 cigarette-equivalents. It ranks first of 8,441 monitored days since 1999.

What is the highest AQI recorded in San Jose?

On the worst day, the daily PM2.5 AQI reached 214 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 214 on November 16, 2018.

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 Jose get air this bad?

Rarely. The record sits at the far end of 8,441 measured days; San Jose averages 2 "unhealthy" days a year.

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.