Historical PM2.5 records
Urban Honolulu's worst air quality days
Based on the available regulatory-monitor record since 1999, December 31, 2000 ranks as Urban Honolulu's most severe daily PM2.5 day: 130.1 µg/m³ (AQI 206, very unhealthy), about 5.9 cigarettes in a day.
- Daily rank
- #1
- Period of record
- Since 1999
- Peak daily PM2.5
- 130.1 µg/m³
- About
- 5.9 cigarettes
- Status
- Finalized
Current conditions on the smoke map → Data through Mar 31, 2026 · page built August 26, 2026
How this is measured sources, the daily measure, and what this ranking is not
Source. Daily PM2.5 from the U.S. EPA Air Quality System (AQS) , back to 1999, across 4 EPA monitors in the Urban Honolulu, HI metro area (CBSA). Days after March 31, 2026 are preliminary AirNow readings, marked provisional, and re-rank once EPA certifies them (usually the following spring).
The daily measure. "Worst area" is the metro's highest credible monitor each day, with lone bad-monitor spikes dropped (the raw single-monitor max before that filter is in the data but not charted). "Metro average" averages every monitor. The unit choice re-reads the same value as µg/m³, cigarette-equivalents (PM2.5 ÷ 22, Berkeley Earth, an illustration rather than a medical reading), or the EPA 2024 PM2.5 AQI.
Official AQI. EPA's reported metro AQI uses the single worst monitor of any pollutant, so it can run higher than the values here. The highest official daily AQI among the ranked days is 206 on December 31, 2000 (defining pollutant PM2.5).
What this ranking is not. It does not attribute days to wildfire smoke: a high-PM2.5 day can be dust, fireworks, an inversion, or industry. The wildfire-smoke event ranking below arrives with a published attribution method.
What were Urban Honolulu's worst days?
| Rank | Date | Air event | PM2.5 / cigarettes |
|---|---|---|---|
| #1 | December 31, 2000 | Fireworks EPA monitor flag | 130.1 µg/m³ · 5.9 cigs |
| #2 | December 31, 19992-day stretch | Fireworks EPA monitor flag | 105 µg/m³ · 4.8 cigs |
| #3 | December 31, 20042-day stretch | Fireworks EPA monitor flag | 102.8 µg/m³ · 4.7 cigs |
| #4 | December 31, 2003 | Other EPA monitor flag | 91.8 µg/m³ · 4.2 cigs |
| #5 | December 31, 2005 | Fireworks EPA monitor flag | 88.1 µg/m³ · 4.0 cigs |
| #6 | December 31, 20012-day stretch | Other EPA monitor flag | 87.3 µg/m³ · 4.0 cigs |
| #7 | January 1, 2000 | Fireworks EPA monitor flag | 87.1 µg/m³ · 4.0 cigs |
| #8 | January 1, 1999 | Other EPA monitor flag | 73.1 µg/m³ · 3.3 cigs |
| #9 | December 31, 20022-day stretch | Other EPA monitor flag | 57 µg/m³ · 2.6 cigs |
| #10 | January 1, 2005 | Fireworks EPA monitor flag | 54.7 µg/m³ · 2.5 cigs |
| #11 | January 1, 2002 | Other EPA monitor flag | 53.3 µg/m³ · 2.4 cigs |
| #12 | December 31, 2006 | Fireworks EPA monitor flag | 51.1 µg/m³ · 2.3 cigs |
| #13 | January 1, 2003 | Other EPA monitor flag | 46.2 µg/m³ · 2.1 cigs |
| #14 | January 1, 2023 | Fireworks Documented | 42 µg/m³ · 1.9 cigs |
| #15 | May 15, 2008 | volcanic_emissions Possible | 34.6 µg/m³ · 1.6 cigs |
What caused Urban Honolulu's worst days?
The air-event catalog has named 19 episodes affecting Urban Honolulu, 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.
- Fireworks 2024-01-01 Urban Honolulu, HI fireworks episode
- Fireworks 2023 January 1 Urban Honolulu fireworks episode
- volcanic_emissions 2012-01-03 Urban Honolulu, HI undefined episode
- Fireworks 2010-12-31 fireworks episode affecting Urban Honolulu, HI
- volcanic_emissions 2010-01-23 Kilauea volcanic emissions episode
- Fireworks 2010-01-01 fireworks episode affecting Urban Honolulu, HI
13 more, plus every named episode and its evidence is in the air events.
Urban Honolulu air season · summer so far, through July 27
Urban Honolulu's summer is the 18th-worst of 25 on record.
The summer season in every year since 1999, measured over the same window, June 1 to July 27, so each is compared at the same point. All fine particulate the metro's monitors recorded, from every source.
Where 2026 stands
The race, summer by summer
Every summer on record
| Rank | Season | Cigs | Average µg/m³ | Worst day | Unhealthy days | Coverage |
|---|---|---|---|---|---|---|
| 1 | 2010 | 28.4 cig · 11 µg/m³ avg | 11 | 18.9 | 0 | 100% |
| 2 | 2012 | 18.1 cig · 7 µg/m³ avg | 7 | 14.4 | 0 | 100% |
| 3 | 2009 | 15.9 cig · 6.1 µg/m³ avg | 6.1 | 10.8 | 0 | 100% |
| 4 | 2011 | 15.7 cig · 6.1 µg/m³ avg | 6.1 | 12 | 0 | 100% |
| 5 | 2013 | 15.4 cig · 5.9 µg/m³ avg | 5.9 | 10.4 | 0 | 100% |
| 6 | 2015 | 13.5 cig · 5.2 µg/m³ avg | 5.2 | 11.3 | 0 | 100% |
| 7 | 2018 | 12.4 cig · 4.8 µg/m³ avg | 4.8 | 10.3 | 0 | 100% |
| 8 | 2024 | 12.1 cig · 4.7 µg/m³ avg | 4.7 | 9 | 0 | 100% |
| 9 | 2023 | 11.0 cig · 4.2 µg/m³ avg | 4.2 | 9.7 | 0 | 100% |
| 10 | 2022 | 10.9 cig · 4.2 µg/m³ avg | 4.2 | 9.8 | 0 | 100% |
| 11 | 2003 | 10.9 cig · 4.2 µg/m³ avg | 4.2 | 9.1 | 0 | 100% |
| 12 | 2025 | 9.8 cig · 3.8 µg/m³ avg | 3.8 | 6.8 | 0 | 100% |
| 13 | 2019 | 9.7 cig · 3.8 µg/m³ avg | 3.8 | 6.1 | 0 | 100% |
| 14 | 2016 | 9.6 cig · 3.7 µg/m³ avg | 3.7 | 10.3 | 0 | 100% |
| 15 | 1999 | 9.6 cig · 3.7 µg/m³ avg | 3.7 | 13.6 | 0 | 100% |
| 16 | 2001 | 9.5 cig · 3.7 µg/m³ avg | 3.7 | 9.8 | 0 | 100% |
| 17 | 2014 | 9.4 cig · 3.6 µg/m³ avg | 3.6 | 7 | 0 | 100% |
| 18 | 2026 | 9.4 cig · 3.7 µg/m³ avg | 3.7 | 6.5 | 0 | 98% |
| 19 | 2020 | 9.2 cig · 3.6 µg/m³ avg | 3.6 | 6.4 | 0 | 100% |
| 20 | 2005 | 8.7 cig · 3.4 µg/m³ avg | 3.4 | 9 | 0 | 100% |
| 21 | 2021 | 8.7 cig · 3.3 µg/m³ avg | 3.3 | 12.6 | 0 | 100% |
| 22 | 2004 | 8.5 cig · 3.3 µg/m³ avg | 3.3 | 8.1 | 0 | 100% |
| 23 | 2000 | 8.4 cig · 3.3 µg/m³ avg | 3.3 | 5.8 | 0 | 98% |
| 24 | 2017 | 8.0 cig · 3.1 µg/m³ avg | 3.1 | 6 | 0 | 100% |
| 25 | 2002 | 7.7 cig · 3 µg/m³ avg | 3 | 6.5 | 0 | 98% |
What this measures, and what it does not
Each bar is the sum of Urban Honolulu's robust metro daily PM2.5 readings across the summer season through July 27, in µg/m³-days, divided by 22 µg/m³ to give cigarette-equivalents. Earlier seasons are cut at the same point in the season, so this is like-for-like rather than full-season totals. The cigarette figure is an illustration of exposure, not a medical equivalence.
It counts all fine particulate the monitors recorded, whatever produced it: wildfire smoke, traffic, industry, fireworks. It is not a smoke-only measure. The documented smoke events shown alongside explain particular spikes; they do not define the ranking, and that catalog only begins in .
Context for reading the ranking: regional fine-particulate levels have fallen substantially since the early 2000s, which is why seasons from that era rank high on season totals while recent seasons rank high on single-day peaks.
A season is ranked only where monitors reported on at least 60% of the window's days, since below that a low total would reflect missing instruments rather than clean air. Excluded on those grounds: 2006, 2007, 2008. Readings for 2026 after the certified EPA archive are provisional AirNow values and may be revised.
How does it compare?
The record is 24% higher than the next-worst day: 130.1 µg/m³ (December 31, 2000) against 105 µg/m³ (December 31, 1999).
How unusual is air like this?
Every measured day near Urban Honolulu, by daily PM2.5; the record stands far out on the right. 14 days at "unhealthy for sensitive groups" or worse, 9 at "unhealthy", 1 at "very unhealthy" (2024 EPA thresholds, across 9,300 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 Urban Honolulu?
- December 31, 2000, when daily PM2.5 reached 130.1 µg/m³ (AQI 206), about 5.9 cigarette-equivalents. It ranks first of 9,300 monitored days since 1999.
- What is the highest AQI recorded in Urban Honolulu?
- On the worst day, the daily PM2.5 AQI reached 206 (Very unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 206 on December 31, 2000.
- 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 Urban Honolulu get air this bad?
- Rarely. The record sits at the far end of 9,300 measured days; Urban Honolulu averages about one "unhealthy" day every 3 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.
Data and citation
Ranked days for both measures are downloadable, with units and status flags: honolulu.csv. Source: U.S. EPA Air Quality System (AQS) daily PM2.5, with preliminary AirNow readings for the uncertified tail. Cite as:
IAQng Air Records: Urban Honolulu, HI. iaqng.com/records/honolulu. Data: U.S. EPA AQS and AirNow. Accessed August 26, 2026.