Wildfire smoke Agency determination
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.
Why we think this happened
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.
Agency determination A government body issued a record naming this cause for this event: an NWS air-quality alert, a NOAA Storm Events record, or a state or local official archive.
U.S. EPA Air Quality System and the submitting air monitoring agency · May 18, 2023 official
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.
U.S. EPA Air Quality System and the submitting air monitoring agency · May 18, 2023 official
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.
U.S. EPA Air Quality System and the submitting air monitoring agency · May 20, 2023 official
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.
See every retained source and limitation ↓
Sequence: Minneapolis (May 18) → Omaha (May 18) → Denver (May 19) → Colorado Springs (May 19)
Evidence path
A cause is more convincing when the record connects three things: something happened, the air mass moved toward the region, and local instruments responded. Here is how much of that chain is documented for this event.
Source or mechanism
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…
View supporting source ↗Air-mass movement
Nebraska's regulatory material documents regional wildfire-smoke plumes over the state on May 18, 2023.
View transport source ↗Local response
EPA monitors recorded the event across 4 metros. The peak daily concentration is roughly 4.3 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓Event autopsy
Before, during, and after the episode: nearby pollutant monitors and surface-weather stations show what changed. This context tests the “Wildfire smoke” explanation; it does not assign the cause by itself.
Colorado Springs, CO
Monitor comparison window 2023-05-16 to 2023-05-23 · PM2.5 target: COLORADO SPRINGS - COLLEGE COLLEGE
195% above nearby days
26% lower than surrounding days
70% higher than surrounding days
195% higher than surrounding days
70% higher than surrounding days
16% lower than surrounding days
20% lower than surrounding days
52% higher than surrounding days
16% lower than surrounding days
26% lower than surrounding days
Denver-Aurora-Lakewood, CO
Monitor comparison window 2023-05-16 to 2023-05-23 · PM2.5 target: DENVER - NJH-E
210% above nearby days
17% lower than surrounding days
90% higher than surrounding days
210% higher than surrounding days
90% higher than surrounding days
77% higher than surrounding days
12% higher than surrounding days
15% lower than surrounding days
23% lower than surrounding days
54% higher than surrounding days
About the same as surrounding days
12% lower than surrounding days
17% lower than surrounding days
Minneapolis-St. Paul-Bloomington, MN-WI
Monitor comparison window 2023-05-15 to 2023-05-21 · PM2.5 target: B.F. Pearson School
278% above nearby days
About the same as surrounding days
109% higher than surrounding days
278% higher than surrounding days
109% higher than surrounding days
37% higher than surrounding days
About the same as surrounding days
6% lower than surrounding days
12% lower than surrounding days
About the same as surrounding days
About the same as surrounding days
Omaha-Council Bluffs, NE-IA
Monitor comparison window 2023-05-15 to 2023-05-21 · PM2.5 target: ON ROOF OF GOOD SHEPARD LUTHERAN HOME
244% above nearby days
23% higher than surrounding days
81% higher than surrounding days
244% higher than surrounding days
81% higher than surrounding days
46% higher than surrounding days
26% lower than surrounding days
32% higher than surrounding days
13% lower than surrounding days
About the same as surrounding days
7% higher than surrounding days
23% higher than surrounding days
Source: U.S. EPA AirData daily observations and, where available, nearby ASOS/AWOS surface observations. Values are descriptive medians over the retained monitor set; missing measurements remain missing. Transport and source evidence remain separate layers because this instrument pattern does not assign a cause by itself.
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.
Views from the selected metro
Select a metro on the map to see nearby cameras. Where an operator archive reaches the event, drag the divider to compare two moments, adjust either date and time, or play the available event-day frames. Camera imagery is visual context, not an air-quality measurement or proof of smoke.
No useful public camera views are currently available within 80 km of this metro.
| 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.
Public evidence library
Each card states what a source supports—and what it does not. Official records, research, reporting, community observations, and transparent inferences remain visibly different kinds of evidence.
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.
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.
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.
The report reproduces CDPHE's advisory that heavy Canadian wildfire smoke was transported across northeastern Colorado, including Denver and Colorado Springs.
Agency determination A government body issued a record naming this cause for this event: an NWS air-quality alert, a NOAA Storm Events record, or a state or local official archive.
This episode was grouped from the monitor record by the catalog pipeline and given its cause from the evidence listed above. It has not been individually reviewed and signed off, so its boundaries, cause, and name may change as evidence accumulates. Grouping policy common_sense_root_cause_grouping.v2. The cause label reflects the assembled evidence above, not an IAQng detection formula. If it is wrong, tell us.