Stagnant pollution mixture Possible
Daily PM2.5 peaked at 54.5 µg/m³ near Birmingham across 1 covered metro home to 1.2 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.9 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 2.2 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
Exact-date surface observations document smoke and persistent haze under predominantly calm or light wind. EPA data independently show a broad PM2.5 and PM10 episode accompanied by sharp carbon monoxide and sulfur dioxide increases. This supports accumulation of a combustion-associated mixture, but the available evidence cannot distinguish industrial emissions, local burning, prescribed fire, or transported wildfire smoke.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
NOAA/NWS surface observations via Iowa Environmental Mesonet · Jul 27, 1999 hypothesis
Birmingham Municipal Airport, 6.6 km from the target PM2.5 monitor, reported the official present-weather code FU (smoke) during one event-day hour and haze during nine hours. Winds were calm for 14 hours and at or below 3 knots for 20 hours.
U.S. EPA AirData hypothesis
EPA observations show median event-day PM2.5 of 48.85 µg/m³ across the available daily series, with a 54.5 µg/m³ peak and coverage from seven monitors in the comparison window. PM10 reached a 74 µg/m³ median across four monitors; carbon monoxide roughly doubled and sulfur dioxide roughly tripled relative to the preceding comparison period.
NOAA/NWS surface observations via Iowa Environmental Mesonet · Jul 27, 1999 hypothesis
BIRMINGHAM MUNI, 6.6 km from the PM2.5 target monitor, reported the official present-weather code FU (smoke) during 1 observed hour inside the catalog event window.
Inference, not direct attribution
exact_surface_smoke_haze_combustion_fingerprint_possible.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
exact_surface_smoke_haze_combustion_fingerprint_possible.v1. This inference is recorded in the catalog methodology.
Air-mass movement
The retained record does not yet show how air traveled from the proposed source to these metros. A trajectory replay or dated plume analysis belongs here.
Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.5 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.
Birmingham-Hoover, AL
Monitor comparison window 1999-07-24 to 1999-07-30 · PM2.5 target: North Birmingham
51% above nearby days
70% higher than surrounding days
51% higher than surrounding days
70% higher than surrounding days
81% higher than surrounding days
21% higher than surrounding days
193% higher than surrounding days
24% 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 |
|---|---|---|---|---|
| Birmingham-Hoover, AL | 54.5 µg/m³ | 2.5 | Jul 27, 1999 | #23 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.
NOAA/NWS surface observations via Iowa Environmental Mesonet
Birmingham Municipal Airport, 6.6 km from the target PM2.5 monitor, reported the official present-weather code FU (smoke) during one event-day hour and haze during nine hours. Winds were calm for 14 hours and at or below 3 knots for 20 hours.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The surface code establishes smoke near the monitor but does not identify whether it came from wildfire, prescribed burning, industry, or another combustion source.
NOAA/NWS surface observations via Iowa Environmental Mesonet
BIRMINGHAM MUNI, 6.6 km from the PM2.5 target monitor, reported the official present-weather code FU (smoke) during 1 observed hour inside the catalog event window.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The surface code establishes smoke at the nearby airport; it does not identify whether the source was a wildfire, prescribed burn, agricultural fire, or another local fire.
U.S. EPA AirData
EPA observations show median event-day PM2.5 of 48.85 µg/m³ across the available daily series, with a 54.5 µg/m³ peak and coverage from seven monitors in the comparison window. PM10 reached a 74 µg/m³ median across four monitors; carbon monoxide roughly doubled and sulfur dioxide roughly tripled relative to the preceding comparison period.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Pollutants were not necessarily measured at the same sites, so the fingerprint describes a metro episode rather than a collocated source profile.
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.