If the 95 degree heat didn't get to you just a little Monday, then you must not have stepped outside.

For the fourth time this season, a combination of hot, stagnant and polluting conditions made Monday a sweltering, potentially lung-irritating introduction to the work week that offered one good point -- free rides on the bus.

Monday was a so-called "Spare the Air" day, an advisory put out by the Bay Area Air Quality Management District that triggers free rides on 25 different Bay Area transit agencies, including the Napa VINE and BART.

This summer has seen the most Spare the Air days since at least 2004, when air quality officials began offering free morning commutes on hot, heavy smog days to help encourage people to use mass transportation.

There were two Spare the Air days in 2004 and only one last year.

"I think the recognition of the Spare the Air program is at an all-time high," said Aaron Richardson, a spokesman with the air quality management district. "Last year it expanded to about 20 transit agencies and this year it's 25, which is bigger than ever."

Agencies have data that show the program seems to be working. Bay Area ridership increases about 10 percent on Spare the Air days -- an additional 155,000 riders.

Locally, the last three Spare the Air days -- June 22, 23 and 26 -- created an average 13 percent increase in bus ridership for Napa VINE.

For many riders waiting to board buses in Napa, however, Spare the Air days merely mean a free ride.

"I ride everyday, so it's just another day for me," said Paul Krueger, 53. "I haven't bought a tank of gas in six years."

Ryan Mock, 20, said he enjoyed the free ride, but didn't think it made a difference to the air.

"It's pollution either way," he said. "Either my car is running or the bus is running."

Regional officials don't see it that way -- to the extent that they're willing to pay millions to keep the program going.

The last three Spare the Air days used up most of the $7.5 million allocated to pay for free rides around the Bay Area during what is typically the smoggiest season, from June to October.

Richardson said because this summer has been so bad, so fast, the Metropolitan Transportation Commission voted to spend another $5.3 million to extend the program.

Spare the Air days occur when the level of ozone reaches unhealthful levels, which is more of a problem in the Central Valley and Southern California than the Bay Area.

"Long-term exposure to ozone may create asthma in children," Richardson said.

Three things have to happen for ozone to be created in unhealthful amounts -- hot weather, little wind and lots of air pollution.

As the summer wears on, the Bay Area has been getting all three, but Napa is not bearing the brunt of it.

Measurements at the Napa County Airport show the Napa Valley as slightly below normal heat so far this summer.

But that could change this week, Allan Reppert, Accuweather meteorolgist said.

"The heat that we're seeing will make it normal or just above normal," he said, adding temperatures will drop from a high of 95 degrees Monday to highs that could reach the low 90s throughout the rest of the week.

"This is what we expect to see as we get toward the middle of summer," Reppert said.