A heat wave finally broke Wednesday, Aug. 15, when daytime high temperatures in Tracy dipped below the 100-degree mark for the first time since Aug. 9. But the effects of the high temperatures are likely to linger.

According to Anthony Presto, spokesman for the San Joaquin Valley Air Pollution Control District, the heat wave created the perfect crucible for poor air in the Central Valley, combining stagnant, hot conditions with emissions from tailpipes, fires and other organic compounds.

Ozone, “a corrosive gas that damages lung tissue,” Presto said, is created when volatile organic compounds such as degreasers, aerosols and livestock emissions are baked together with nitrogen oxide from internal combustion engines and fires. The high temperatures of the recent heat wave — which in Tracy peaked at 105 degrees on Saturday, Aug. 11, according to AccuWeather forecasters — were the catalyst.

“The more sun you have and the more intense around that 100-degree mark, the more ozone you get,” Presto explained.

Since the heat wave began, Presto said, air quality has been in a range that is unhealthy for sensitive people “on almost a daily basis,” potentially causing breathing and other health problems for those with asthma, heart trouble and other conditions.

Next week, Presto said, the air pollution control district could issue its first air alert of the season, encouraging people to reduce emissions so the district isn’t fined by the federal Environmental Protection Agency.

If the eight-county district does not meet a one-hour federal ozone standard, it is fined $29 million, Presto said. Drivers started paying in October 2011 for the failure to meet the standard by 2010.

Drivers foot $19 million of the fine via a $12 Department of Motor Vehicle fee, and businesses picked up the remaining $10 million tab, Presto said.

The district has made great strides in reducing the number of ozone violations, from 56 in 1996 to three in 2011. But Presto said the federal government only recognizes meeting the threshold of no more than one violation for three years in a row, but doesn’t recognize improvement.

“A majority of businesses in the San Joaquin Valley have improved a great deal and invested billions of dollars in cleaning up the valley’s air,” Presto said. “It’s really not fair for businesses to carry the burden of the entire fine after making so many improvements. A majority of our air quality problem now comes from mobile sources.”

He encouraged people to carpool, not idle their engines and combine or forgo car trips to spare the air.

“Anything you can do to reduce the amount of driving, the better,” he said.

Presto said an air alert would be in place Monday, Aug. 20, and Tuesday, Aug. 21, AccuWeather senior meteorologist Ken Clark said weather conditions could improve the situation.

On Thursday, he predicted temperatures would drop into the high 80s by the middle of next week, though temperatures over the weekend could vary by as much as 10 degrees.

It depends on the strengthening or weakening of the high pressure ridge over the western United States that caused the Central Valley’s heat wave, Clark said. It weakened slightly Wednesday and Thursday, allowing some cooler air from the coast to reach the valley, but Clark said weekend highs could range between the low 100s and the low 90s.

“It’s a very tricky forecast,” he said.

Despite August’s hotter-than-average temperatures, Clark said June, July and the first part of August were cooler overall than normal in Tracy. The same three months in 2011 were also below average in terms of daytime highs, he said.

But according to a recently released study by the Pacific Institute in Oakland, the valley could see higher temperatures on a much more frequent basis if climate change models are accurate.

Two Pacific Institute analyses of California were based on a “downscaled climate model” from the Scripps Institute of University of California, San Diego — an amalgamation of four different climate models.

The combined model shows San Joaquin County experiencing between 30 and 50 days with highs of 101 degrees or more by the end of the century, the report states. According to AccuWeather, Tracy has seen 15 days with 100-degree-or-higher heat this year.

“(Those) living in a neighborhood with high settlement density, sparse vegetation and lack of open space,” would be most vulnerable to the estimated temperature increases, the report concluded.

• Contact Jon Mendelson at 830-4231 or jmendelson@tracypress.com. • Editor's note: This story has been updated from a previous version.