HOUSTON — The former smog capital of the U.S. is steadily getting cleaner.

For the third consecutive year, the Houston area has seen a drop in days with harmful levels of smog brought by the heavy industry, weather and traffic-choked highways around the nation's fourth-largest city.

Houston had a record-low 16 days that exceeded federal health-based standards for ground-level ozone in the latest smog season, according to Texas Commission on Environmental Quality data.

Not long ago, such days occurred more than 60 times a year.

"No one thought that we would see this for Houston," said Neil Carman, air quality program director for the Sierra Club's Lone Star chapter. "It's remarkable."

The cleaner air is being attributed to tighter pollution controls, enhanced regulations and more favorable weather this ozone season, the Houston Chronicle reported Monday. Some credit high fuel prices keeping more cars off the road, though there is no data to support the theory.

Days of bad ozone are down 74 percent since 1999, when Houston wrested the smog title from Los Angeles for the first time. The crown has since returned to California.

But despite the improvements, Houston isn't breathing completely easy just yet.

David Schanbacher, TCEQ's chief engineer, described the progress of the past two years as "profound" but unlikely to continue at the same rate.

Houston could also find itself out of compliance more often once the Environmental Protection Agency fully implements a set of tougher ozone standards.

The EPA determined in March that the existing standard of 84 molecules of ozone out of every billion molecules of air was no longer considered safe. Under the new limit of 75 parts per billion, Houston had 23 additional days of unhealthy ozone levels this year.

Houston has a 2019 deadline to meet the 84 ppb standard. The new limit of 75 ppb has deadlines between 2013 and 2030, depending on the severity of an area's problem.

Ozone is formed when emissions from tailpipes and smokestacks mix with sunlight. The toxic, colorless gas can damage the lungs, cause headaches and nausea and aggravate asthma.

Some environmentalists worry that Houston will be too quick to pat itself on the back for its clean-air accomplishments when more needs to done to fight smog.

"We're still a long way from 85 (ppb standard), and miles away from 75," said Matthew Tejada, executive director of the Galveston-Houston Association for Smog Prevention.