Wildfire smoke
2023 mid-May Canadian wildfire-smoke episode
Daily PM2.5 peaked at 95.6 µg/m³ near Denver across 4 covered metros home to 8.6 million people. 2 metros recorded their worst day on record.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 3.9 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 33 million cigarettes. An awareness illustration of outdoor exposure at 22 µg/m³ per cigarette (Berkeley Earth), counting only the event's ranked days; not a medical dose.
Episode peak
-- cigarettes
The smoke map's scale, replayed over this event: the surface interpolates between covered metros' daily records, framed to the affected region. Hover for a reading, click a metro to pin it to the card. Days a monitor did not report draw nothing.
What happened, in the record
Qualified supplied evidence supports wildfire smoke for this event. It is grouped with 2 other supplied events because the evidence identifies Canada as the shared origin and the affected intervals form one continuous 2023-05-18 through 2023-05-20 occurrence. Qualified supplied evidence supports wildfire smoke for this event. It is grouped with 2 other supplied events because the evidence identifies Canada as the shared origin and the affected intervals form one continuous 2023-05-18 through 2023-05-20 occurrence. Qualified supplied evidence supports wildfire smoke for this event. It is grouped with 2 other supplied events because the evidence identifies Canada as the shared origin and the affected intervals form one continuous 2023-05-18 through 2023-05-20 occurrence.
Sequence: Minneapolis (May 18) → Omaha (May 18) → Denver (May 19) → Colorado Springs (May 19)
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Denver-Aurora-Lakewood, CO | 95.6 µg/m³ | 7.1 | May 19, 2023 | Worst day on record |
| Colorado Springs, CO | 60.1 µg/m³ | 5.2 | May 20, 2023 | Worst day on record |
| Minneapolis-St. Paul-Bloomington, MN-WI | 59.8 µg/m³ | 2.7 | May 18, 2023 | #16 of its top days |
| Omaha-Council Bluffs, NE-IA | 42.9 µg/m³ | 1.9 | May 18, 2023 | #20 of its top days |
"Cigarettes, whole event" totals a metro's ranked event days at 22 µg/m³ per cigarette: what a person outdoors there breathed across the event, as the illustration. "All-time standing" is where this episode's days sit in each metro's ranked record (worst-area measure, since 1999). Full local context is on each metro's Air Records page.
The evidence
EPA AQS monitor qualifier 6
- AQS PM2.5 qualifier IT — Wildfire-U. S. EPA AQS carries an agency-submitted informational qualifier (IT, Wildfire-U. S.) on 405 PM2.5 samples in the candidate CBSA on 2023-05-18, identifying wildfire smoke as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
- AQS PM2.5 qualifier IF — Fire - Canadian. EPA AQS carries an agency-submitted informational qualifier (IF, Fire - Canadian.) on 72 PM2.5 samples in the candidate CBSA on 2023-05-18, identifying wildfire smoke as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
- AQS PM2.5 qualifier IF — Fire - Canadian. EPA AQS carries an agency-submitted informational qualifier (IF, Fire - Canadian.) on 15 PM2.5 samples in the candidate CBSA on 2023-05-20, identifying wildfire smoke as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
- AQS PM2.5 qualifier IF — Fire - Canadian. EPA AQS carries an agency-submitted informational qualifier (IF, Fire - Canadian.) on 16 PM2.5 samples in the candidate CBSA on 2023-05-19, identifying wildfire smoke as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
- AQS PM2.5 qualifier IF — Fire - Canadian. EPA AQS carries an agency-submitted informational qualifier (IF, Fire - Canadian.) on 213 PM2.5 samples in the candidate CBSA on 2023-05-19, identifying wildfire smoke as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
- AQS PM2.5 qualifier IF — Fire - Canadian. EPA AQS carries an agency-submitted informational qualifier (IF, Fire - Canadian.) on 169 PM2.5 samples in the candidate CBSA on 2023-05-20, identifying wildfire smoke as affecting the samples. An informational qualifier is an official submitting-agency flag, not a request for exclusion or an EPA-concurred determination.
NWS Air Quality Alert 1
- Archived Colorado Air Quality Alert, May 19, 2023 Repeated Colorado alerts explicitly identified wildfire smoke affecting the Colorado Springs and Denver event area. IEM is the archive, not the official system of record.
Agency and publisher pages 4
- May 18, 2023 "Smoke Front" Minnesota DNR reports that a Canadian cold front brought thick Canadian wildfire smoke into Minnesota, degrading air quality statewide; the smoke contained fine particulates.
- Nebraska Department of Environment and Energy Nebraska's regulatory material documents regional wildfire-smoke plumes over the state on May 18, 2023.
- Denver’s air quality is going to be poor most of Friday because of Canadian wildfires Reporting on a Colorado Department of Public Health and Environment advisory identifies Canadian wildfire smoke and associated fine particles as the cause of poor Front Range air quality.
- Denver among the worst air quality in the world after smoke from Canadian wildfires moves into Front Range The report reproduces CDPHE's advisory that heavy Canadian wildfire smoke was transported across northeastern Colorado, including Denver and Colorado Springs.
Status and provenance
Research preview: this record carries research-only status and its boundaries, cause, and name may change as evidence review completes. Grouping policy common_sense_root_cause_grouping.v2. The cause label reflects the assembled evidence above, not an IAQng detection formula.