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Visalia-Porterville, CA Air Quality

EPA monitor data through 2025 · Updated August 13, 2026 · Methods

In 2025, Visalia-Porterville, CA recorded 43 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. Most of those bad days were driven by ozone. Under the AQI rules that applied before 2013, the same air would have scored 15 days. Since 2000, unhealthy days are 89% lower on a consistent definition.

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.

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 Visalia-Porterville, CA's, from EPA's long-running trend monitors: 102 ppb in 1990 to 87 ppb in 2024, 15% lower.

What 2025 looked like

Median AQI 64 · 90th-percentile AQI 105 · worst day 147. Scored under current rules.

What drove the unhealthy days

Of the 43 days above 100 in 2025, 42 were driven by ozone and 1 by fine particles. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke. In the western United States, ozone is a different problem: the floor is rising, with even the cleanest summer days at high elevation now near 40 ppb, and bad years track wildfire seasons. Domestic emission cuts have less room to work against that.

The unhealthy days in 2025

Every day the index passed 100, what it reached, and which pollutant drove it. 28 of these 43 would not have counted as unhealthy under the pre-2013 rules.

Statewide context

Visalia-Porterville, CA sits in California.

The full series

Sources

• EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
• Download the full dataset for every state and metro.