Greensboro's worst air quality days

What were Greensboro's worst days?

What caused Greensboro's worst days?

The event catalog has named 8 episodes affecting Greensboro, each resolved to a cause only where retained evidence supports it: EPA monitor qualifiers, NWS alerts, agency statements, and reporting. The ranking above stays cause-neutral; this is where the causes live.

• Wildfire smoke 2023 mid-July Canadian wildfire-smoke episode Jul 17, 2023 · 40.3 µg/m³ here , #19 all-time · from Canada
• Wildfire smoke 2023 late-June Canadian wildfire-smoke episode Jun 29, 2023 · 47.7 µg/m³ here , #6 all-time · from Canada
• Wildfire smoke Mid-June 2008 Mid-Atlantic Evans Road and South One wildfire-smoke episode Jun 13, 2008 · 68.6 µg/m³ here , #1 all-time · South One Fire
• Wildfire smoke 2007 Idaho-Montana-Canadian wildfire smoke episode Aug 5, 2007 · 42.2 µg/m³ here , #13 all-time · from Idaho, Montana, and Canada
• Other 2005-08-12 other documented PM source or accumulation mechanism episode affecting Greensboro-High Point, NC Aug 12, 2005 · 39 µg/m³ here , #25 all-time
• Wildfire smoke June 25–27, 2003 southeastern U.S. wildfire-smoke episode Jun 26, 2003 · 41.3 µg/m³ here , #16 all-time

2 more, plus every named episode and its evidence is in the event catalog.

How does it compare?

The record is 31% higher than the next-worst day: 68.6 µg/m³ (June 13, 2008) against 52.3 µg/m³ (January 1, 2000).

How unusual is air like this?

Every measured day near Greensboro, by daily PM2.5; the record stands far out on the right. 36 days at "unhealthy for sensitive groups" or worse, 1 at "unhealthy", 0 at "very unhealthy" (2024 EPA thresholds, across 8,334 measured days).

Year by year (1999–2026)

Worst single day each year, by the selected measure.

Why did reported numbers differ?

During a major event, AirNow, weather apps, news reports, and neighborhood monitors can all show different figures at the same moment. All of them can be right:

• Concentration vs AQI. µg/m³ is the measurement; AQI is a 0 to 500 index derived from it. "330 µg/m³" and "AQI 500" can describe the same air.
• Hourly vs NowCast vs daily. An hourly peak, the NowCast rolling estimate apps display, and the full-day average answer different questions; this page ranks full days.
• One monitor vs a composite. A single sensor can read far above or below a citywide figure. The measure toggle above shows both framings.
• Live vs certified. Real-time feeds get corrected when EPA certifies data months later, which is why recent days here carry a provisional label.

What about indoors?

Citywide monitors can tell you what happened outside. They cannot tell you what happened inside a particular home, school, or facility: filtration, ventilation, and building operation decide that. During and after days like the ones above, outdoor particles become an indoor load question.

Check current conditions and the next clean-air window → Estimate recent filter load where you are →

Common questions

What was the worst air-quality day in Greensboro?

June 13, 2008, when daily PM2.5 reached 68.6 µg/m³ (AQI 160), about 3.1 cigarette-equivalents. It ranks first of 8,334 monitored days since 1999.

What is the highest AQI recorded in Greensboro?

On the worst day, the daily PM2.5 AQI reached 160 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 185 on July 17, 2002.

Were these days caused by wildfire smoke?

This page ranks measured PM2.5 without assigning a cause: a high day can be smoke, fireworks, dust, an inversion, or industry. A separate wildfire-smoke event ranking, using a published attribution method with its version disclosed, is the next layer of this catalog.

How often does Greensboro get air this bad?

Rarely. The record sits at the far end of 8,334 measured days; Greensboro averages about one "unhealthy" day every 28 years.

Why did different apps show different numbers that day?

Concentration (µg/m³) and AQI are different scales; hourly, NowCast, and daily values answer different questions; a single monitor differs from a citywide composite; and live feeds get revised when EPA certifies data. This page uses the certified daily record, with the provisional tail labeled.