Stagnant pollution mixture Possible
Daily PM2.5 peaked at 40.1 µg/m³ near Houston across 1 covered metro home to 7.8 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 9.8 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
A broad 13-monitor PM2.5 increase coincided with higher PM10, CO, ozone, and SO2, reduced wind, and high pressure. This supports accumulation of a mixed anthropogenic pollution burden, but the available record cannot apportion sources. Two same-day Baytown flares are retained as possible contributors only because neither incident report establishes a particulate plume or monitor impact.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
U.S. EPA AirData hypothesis
IAQng's reproducible aggregation of EPA records found a broad Houston episode across 13 PM2.5 monitors. Median PM2.5 was 32.7 ug/m3, versus 10.9 before; PM10, CO, ozone, and SO2 also rose. Median wind speed fell to 3.52 knots from 5.88 knots before the event, while pressure remained high near 1023 mb.
Inference, not direct attribution
low_wind_high_pressure_multimonitor.v1: This inference is recorded in the catalog methodology.
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
low_wind_high_pressure_multimonitor.v1. This inference is recorded in the catalog methodology.
Air-mass movement
NOAA HYSPLIT modeled 12 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 1.8 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 “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Houston-The Woodlands-Sugar Land, TX
Monitor comparison window 2001-11-15 to 2001-11-21 · PM2.5 target: Houston Texas Avenue
134% above nearby days
24% lower than surrounding days
100% higher than surrounding days
134% higher than surrounding days
100% higher than surrounding days
42% higher than surrounding days
10% higher than surrounding days
335% higher than surrounding days
99% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
24% lower 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.
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 Houston-The Woodlands-Sugar Land
Peak-date arrivals · 2001-11-18
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 |
|---|---|---|---|---|
| Houston-The Woodlands-Sugar Land, TX | 40.1 µg/m³ | 1.8 | Nov 18, 2001 | #22 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.
U.S. Coast Guard National Response Center
NRC report 586439 records a Chevron Phillips Baytown process-unit shutdown and flare on November 18, with 25 pounds of nitrogen oxide reported released to air.
Context for the event; this item does not establish the attributed cause by itself.
The report establishes a same-day industrial occurrence, not its contribution to the metro-wide daily PM2.5 response.
U.S. Coast Guard National Response Center
NRC report 586369 records an ExxonMobil Baytown compressor trip and flare on November 18, with 200 pounds of 1,3-butadiene reported released to air.
Context for the event; this item does not establish the attributed cause by itself.
The report establishes a same-day industrial occurrence, not its contribution to the metro-wide daily PM2.5 response.
U.S. EPA AirData
IAQng's reproducible aggregation of EPA records found a broad Houston episode across 13 PM2.5 monitors. Median PM2.5 was 32.7 ug/m3, versus 10.9 before; PM10, CO, ozone, and SO2 also rose. Median wind speed fell to 3.52 knots from 5.88 knots before the event, while pressure remained high near 1023 mb.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The fingerprint supports regional accumulation of a mixed pollution burden; it does not apportion sources or prove that the reported flares caused the PM2.5 increase.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
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.