Wildfire smoke Agency determination
Daily PM2.5 peaked at 46.9 µg/m³ near Denver across 2 covered metros home to 3.8 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.8 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 6.9 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
Supplied qualified evidence supports wildfire-smoke as the root cause. The grouping includes only events linked by explicit shared episode evidence; otherwise this remains a single-event episode.
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 · Sep 18, 2020 official
EPA AQS carries an agency-submitted informational qualifier (IT, Wildfire-U. S.) on 4 PM2.5 samples in the candidate CBSA on 2020-09-18, 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.
National Weather Service via Iowa Environmental Mesonet archive · Sep 17, 2020 official
An archived NWS Air Quality Alert for Denver-Aurora-Lakewood, CO, issued 2020-09-17T15:10Z, explicitly identifies wildfire smoke. IEM is the archive, not the official system of record.
National Weather Service via Iowa Environmental Mesonet archive · Sep 17, 2020 official
An archived NWS Air Quality Alert for Colorado Springs, CO, issued 2020-09-17T15:10Z, explicitly identifies wildfire smoke. IEM is the archive, not the official system of record.
See every retained source and limitation ↓
Sequence: Colorado Springs (Sep 17) → Denver (Sep 18)
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 4 PM2.5 samples in the candidate CBSA on 2020-09-18, identifying wildfire smoke as affecting the samples. An informational qualifier is an official submitting-agency…
View supporting source ↗Air-mass movement
Contemporaneous reporting says thick, mainly out-of-state wildfire smoke caused unhealthy air and poor visibility in the Denver metro area under a state health advisory.
View transport source ↗Local response
EPA monitors recorded the event across 2 metros. The peak daily concentration is roughly 2.1 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 2020-09-14 to 2020-09-21 · PM2.5 target: COLORADO SPRINGS - COLLEGE COLLEGE
103% above nearby days
18% higher than surrounding days
No adjacent-day comparison
103% higher than surrounding days
No adjacent-day comparison
39% higher than surrounding days
43% higher than surrounding days
About the same as surrounding days
23% higher than surrounding days
About the same as surrounding days
18% higher than surrounding days
Denver-Aurora-Lakewood, CO
Monitor comparison window 2020-09-14 to 2020-09-21 · PM2.5 target: Globeville
97% above nearby days
About the same as surrounding days
86% higher than surrounding days
97% higher than surrounding days
86% higher than surrounding days
44% higher than surrounding days
23% higher than surrounding days
36% higher than surrounding days
About the same as surrounding days
22% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as 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.
Air-mass reconstruction
NOAA HYSPLIT traces the air arriving at each metro backward for 72 hours. Multiple arrival times and heights show whether the broad path is stable—or sensitive to assumptions about when and where the polluted layer arrived.
Air arriving near Colorado Springs
Peak-date arrivals · 2020-09-17
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
Air arriving near Denver-Aurora-Lakewood
Peak-date arrivals · 2020-09-18
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
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 | 46.9 µg/m³ | 2.1 | Sep 18, 2020 | #23 of its top days |
| Colorado Springs, CO | 29.5 µg/m³ | 2.6 | Sep 17, 2020 | #12 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 4 PM2.5 samples in the candidate CBSA on 2020-09-18, 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.
An archived NWS Air Quality Alert for Denver-Aurora-Lakewood, CO, issued 2020-09-17T15:10Z, explicitly identifies wildfire smoke. IEM is the archive, not the official system of record.
An archived NWS Air Quality Alert for Colorado Springs, CO, issued 2020-09-17T15:10Z, explicitly identifies wildfire smoke. IEM is the archive, not the official system of record.
Air Quality Unhealthy For Everyone As Wildfire Smoke Gets Worse In Colorado
Contemporaneous reporting says thick, mainly out-of-state wildfire smoke caused unhealthy air and poor visibility in the Denver metro area under a state health advisory.
Out-Of-State Wildfire Smoke Takes A Crazy Route To Colorado
Report describes West Coast wildfire smoke arriving at the Colorado Front Range and combining with in-state smoke to create unhealthy conditions.
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.