Dust Agency determination
Daily PM2.5 peaked at 63.5 µg/m³ near Phoenix across 1 covered metro home to 5.2 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.3 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 12 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 frozen evidence supports dust as the broad cause for this time-bounded 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.
National Weather Service via Iowa Environmental Mesonet archive · Jul 7, 2018 official
An archived NWS Air Quality Alert for Phoenix-Mesa-Scottsdale, AZ, issued 2018-07-07T21:52Z, explicitly identifies windblown or transported dust. IEM is the archive, not the official system of record.
GeoHealth · Jan 1, 2025 official
The study identifies July 8, 2018 as a Greater Phoenix convective dust event with exceptionally high daily PM10, supporting dust as the event driver; it does not separately quantify the PM2.5 share.
Maricopa County · Jul 9, 2018 corroborates
The county relayed ADEQ's PM10 dust and PM2.5 smoke advisory immediately following the July 8 dust event.
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
An archived NWS Air Quality Alert for Phoenix-Mesa-Scottsdale, AZ, issued 2018-07-07T21:52Z, explicitly identifies windblown or transported dust. IEM is the archive, not the official system of record.
View supporting source ↗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
The county relayed ADEQ's PM10 dust and PM2.5 smoke advisory immediately following the July 8 dust event.
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 “Dust” explanation; it does not assign the cause by itself.
Phoenix-Mesa-Scottsdale, AZ
Monitor comparison window 2018-07-05 to 2018-07-11 · PM2.5 target: HIDDEN VALLEY
155% above nearby days
6% higher than surrounding days
201% higher than surrounding days
155% higher than surrounding days
201% higher than surrounding days
About the same as surrounding days
40% lower than surrounding days
68% higher than surrounding days
13% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
6% 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.
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 Phoenix-Mesa-Scottsdale
Peak-date arrivals · 2018-07-08
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.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Phoenix-Mesa-Scottsdale, AZ | 63.5 µg/m³ | 2.9 | Jul 8, 2018 | #21 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.
An archived NWS Air Quality Alert for Phoenix-Mesa-Scottsdale, AZ, issued 2018-07-07T21:52Z, explicitly identifies windblown or transported dust. IEM is the archive, not the official system of record.
The study identifies July 8, 2018 as a Greater Phoenix convective dust event with exceptionally high daily PM10, supporting dust as the event driver; it does not separately quantify the PM2.5 share.
Arizona Department of Environmental Quality issues high pollution advisories for July 9–10, 2018
The county relayed ADEQ's PM10 dust and PM2.5 smoke advisory immediately following the July 8 dust event.
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.