By one measure—ozone levels—air quality has improved statewide since 1980, but pockets of California continue to experience poor air quality.

Statewide, there were 22 days in 2016 where ozone levels were above the regulatory standard, according to data from the California Environmental Protection Agency’s Air Resource Board that was posted on Kidsdata.

In 1980, the state saw double the number of days above the regulatory standard for ozone.

The U.S. standard for ozone is 0.070 parts per million. Regulators consider levels above that to be potentially hazardous to health.

Elevated levels of ozone are linked to abnormal lung development, asthma and other lung diseases, according to Kidsdata.

Ozone emissions can come from vehicles, power plants and refineries, among other sources.

Between 1980 and 2016, San Diego, Ventura, Riverside and Los Angeles counties saw some of the biggest declines in ozone pollution. In 1980, all four counties recorded between 170 and 122 days where ozone levels were above the regulatory standard. By last year, the numbers had declined by nearly 100 days for each of the four counties. Ventura County, for example, saw only seven days above the regulatory standard in 2016.

San Bernardino County, meanwhile, recorded the worst ozone levels in 2016, where 106 days were above the regulatory standard. Last year the highest ozone levels were all found in a central swath of the state, from Fresno to Riverside counties.

One in 6 children in the San Joaquin Valley have asthma, the highest level in the state. The American Lung Association’s State of the Air report for 2016 reports that the counties in the San Joaquin Valley have the highest asthma rate for children in the nation. Over 550,000 Valley residents have asthma, and of those 105,000 are children.

Lucile Packard Foundation for Children’s Health is a Funder of both California Health Report and Kidsdata.