Houston, for a time, held the unwanted title of the nation's capital of bad air. These days, however, this land of unchecked growth, traffic-choked freeways and prolific smokestacks isn't even the smoggiest place in Texas.

As its best year for air quality draws to a close, the eight-county region has dropped below the Dallas-Fort Worth area and pulled even with San Antonio by measure of smog, or ozone, throughout the day.

The pollutant remains a stubborn menace for Houston, but its steady retreat is remarkable, considering it wasn't long ago when city leaders bragged about the stench from nearby oil refineries and chemical plants - calling it the smell of prosperity.

"I've got to hope that (smog levels) are going to continue to get better," said Adrian Shelley, executive director of Air Alliance Houston, an advocacy group. "But our work isn't done here."

Environmentalists and scientists warn against making too much of the accomplishment because Houston's smog levels still exceed federal health-based standards. What's more, the Environmental Protection Agency intends to propose stringent new limits by the end of the year.

The current standard, set under President George W. Bush in 2008, allows up to 75 parts of ozone for every billion parts of air. A new EPA study, which adds to a growing body of research on the health risks of lung-irritating smog, calls for a limit as low as 60 parts per billion and no higher than 70 parts per billion.

If the numbers hold, Houston will finish 2014 at 79 parts per billion - down from 118 parts per billion in the late 1990s when it had usurped Los Angeles for the smog crown. Smog levels can be fickle from year to year, so the EPA uses a rolling three-year average to determine compliance with federal ozone limits.

Houston also has seen increasingly fewer days with ozone levels above federal limits. So far, there have been 13 bad-air days in 2014 - down from 56 days three years ago.

Wet weather helped

Ozone is formed when chemicals released from tailpipes and smokestacks react in sunlight. Houston is a prodigious smog factory because of its commuting culture, heavy industry, busy port and climate.

Colder, wetter weather played a big role in short-circuiting smog-forming conditions this year, said Barry Lefer, professor of atmospheric science at the University of Houston. That's because emissions of a key ozone ingredient known as volatile organic compounds have remained fairly flat since 2009.

"Weather is probably the overriding factor" in this year's gains, Lefer said. "But it's interesting that we are seeing this long-term trend as the population grows. Every day we have more people driving cars, we have more power plants producing energy and we have more chemical plants making plastics."

The Texas Commission on Environmental Quality attributes the trend to cleaner-burning and more fuel-efficient cars, trucks and ships, new scrubbers on coal-fired power plants and infrared cameras that detect leaks of smog-forming releases from chemical plants and oil refineries.

Industrial facilities also have curbed emissions by altering the way they use open flames, or flares, to burn off pressurized gases. The reforms followed a 2011 study by the University of Texas at Austin that found small operational changes could lead to significant pollution cuts.

While weather plays an important role in ozone formation, the emissions reductions appear to be "real and sustainable," said Elizabeth Hendler, a former state regulator who now works as a consultant to industry.

Natural gas booms

Houston's hard-won gains underscore the challenges facing other Texas cities on the ozone front. Car-dependent Dallas-Fort Worth has improved, too, but not at the same pace, while the smog fight in San Antonio, which isn't known for dirty skies, resembles the early years of the campaign for clean air in Houston.

Unlike Houston, both areas are experiencing a boom from natural gas production. Studies have shown increases in smog-forming emissions from drilling, processing and transporting the gas.

The highest levels of ozone in North Texas can be found near thousands of natural-gas wells west and northwest of Dallas. The TCEQ, meanwhile, plans to add an air pollution monitor amid the drilling activity in the Eagle Ford Shale, south and east of San Antonio, at the request of local officials.

The new Karnes County monitor will be the fourth in the San Antonio area. By comparison, Houston has 21 monitors.

Even with its head start, Houston faces a tough road. The EPA expects the region to achieve the existing smog limit next year, but this target will likely be missed, the TCEQ and industry representatives said.

"That still looks like a significant challenge," Hendler said. "It looks more possible this year than it did last year, but it will require another year like this one."

The TCEQ and industry also are making the case to the EPA that tougher federal ozone limits would be unnecessary and overly expensive. The federal agency sent its proposal last week to the White House for review.

Beyond our control

Part of the challenge is that much of Houston's lingering smog problem is beyond its control. The EPA study estimates that on the region's smoggiest days, 40 percent of its ozone forms naturally or blows into the area from elsewhere.

If the EPA sets the smog limits as low as its science advisers recommend, Houston would be forced to reduce ozone-forming pollution by 75 percent, said David Brymer, chief of the TCEQ's air quality division.

"I'm sure there are other opportunities out there" for additional reductions, Brymer said, "but they're hard to find because we've already done so much."