EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Quincy, IL-MO recorded 0 days above an Air
Quality Index of 100, the level at which EPA begins warning sensitive groups, out
of 245 days with monitoring data. Most of those bad days were driven by
ozone and fine particles equally. Under the AQI rules that applied before 2013, the same air would have
scored 0 days.
Since 2000, unhealthy days are
100% lower on a consistent
definition.
Unhealthy days, 2025
0
AQI above 100
Same air, pre-2013 rules
0
for comparison with older reports
Good-air days
232
AQI 50 or below
Days monitored
245
partial coverage
Unhealthy days by year, scored two ways
EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those
changes is not comparable to one published after. Each panel below applies a single fixed rulebook to
every year, so each is internally consistent.
Why this matters.
Scored by today's rules
Scored by the pre-2013 rules
Faded columns mark years with fewer than
300 monitored days, where the count is not comparable.
Ozone since 1990
The index counts above only start in 2000, because nationwide fine-particle
monitoring did not exist before 1999 and an index without it would measure the monitoring network
rather than the air. Ozone does have a 35-year record. This is
Quincy, IL-MO's, from EPA's long-running trend monitors:
68 ppb in 1990 to
65 ppb in 2024, 4%
lower.
Peak ozone, 1990 to
2024
Annual fourth-highest daily maximum 8-hour ozone, parts per billion, averaged
across 1 long-running monitor in the
area. The dashed line is the current standard, 70 ppb. Holding the monitor set still is what makes
a 35-year comparison meaningful, and it is also why these values differ from the design value used for
compliance. Source:
EPA air quality statistics by
city.
What 2025 looked like
Air quality category
Days
Share of monitored days
Good
232
95%
Moderate
13
5%
Unhealthy for sensitive groups
0
0%
Unhealthy or worse
0
0%
Median AQI 37 · 90th-percentile AQI 49 · worst day
71. Scored under current rules.
