Experts interviewed for this story say Texas doesn’t have enough data to be able to claim, with any assurance, that the air is safe.

The TCEQ relies primarily on field canister samples, on-the-ground investigations and aerial surveys with infrared cameras to detect emissions. Last summer, the agency used the cameras during two flyovers to capture hundreds of images of the Eagle Ford. A contractor then surveyed 16,015 oil and gas storage tanks and found 800 with leaks, TCEQ spokesman Terry Clawson said.

Asked how the agency dealt with the polluters, Clawson did not respond.

Scientists say that while these spot checks are important, they are no substitute for strategically placed, stationary monitors that continuously measure how air quality changes over time.

The TCEQ has only five permanent monitors in the Eagle Ford, all positioned far from the most heavily drilled areas. The Barnett Shale in North Texas, by contrast, has 35 permanent monitors, even though that field covers only about 5,000 square miles — a quarter of the area of the Eagle Ford.

“The biggest challenge with air monitoring is having the measurements in place so you can catch the times when concentrations are high,” said Rob Jackson, a Duke University scientist who studies pollution from shale extraction.

The monitor closest to the Buehrings’ house is in Floresville, 25 miles away, so it’s hard to know how the couple might be affected by the plants and wells that surround them, Jackson said. Their daily exposures would vary dramatically based on wind direction, weather patterns and the rate of emissions from each well and plant.

The TCEQ has no plans to add air monitors in the Eagle Ford, but Clawson said it has contracted with the University of Texas “to conduct mobile monitoring upwind and downwind of the Eagle Ford Shale area.”

The goal of that study, however, isn’t to measure air emissions in the Eagle Ford. Instead, it aims to figure out how Eagle Ford pollution may be affecting cities like San Antonio, just beyond the shale play.

The study’s director said the data will be gathered from the monitor in Floresville. With no wells nearby, “our sampling wouldn’t be overcome by immediate emissions,” said David Sullivan, a research associate with the University of Texas’ Center for Energy and Environmental Resources.

San Antonio’s ozone levels have violated federal standards dozens of times since the drilling began.Ozone is one of several greenhouse gases, including methane, released or created during drilling operations. Experts are particularly concerned about methane because it’s a powerful greenhouse gas and large-scale leakage could undermine natural gas’ reputation as a cleaner alternative to coal.

Even the EPA doesn’t know much about methane emissions or the other pollutants from oil and gas production. An inspector general's report last year concluded that the agency's air emissions database is incomplete and “likely underestimates” those emissions. The lack of reliable data, the report said, “hampers EPA’s ability to accurately assess risks and air quality impacts from oil and gas production activities.”

Environmental groups have tried to collect their own air-quality data in the Eagle Ford, but the process is so expensive and time-consuming that they’ve had little success.

Last March, Wilma Subra, an environmental consultant from Louisiana, and Sharon Wilson of the advocacy group Earthworks, accompanied Calvin Tillman, who runs a nonprofit called ShaleTest, as he took air samples near Mike and Myra Cerny’s one-acre tract, about a half-mile from the Buehrings.

There are at least 17 oil wells within a mile of the Cernys' small house. Their teenage son, Cameron, gets frequent nosebleeds, and the fumes make his parents dizzy, irritable and nauseous. “This crap is killing me and my family,” said Mike, a former oil company truck driver. “We went from nice, easy country living to living in a Petri dish.”

Myra complained to the TCEQ in 2012, and the agency cited Marathon Oil for operating a broken flare and failing to report thousands of pounds of unauthorized emissions at its Sugarhorn Central gas processing plant. But Marathon paid no penalty. “I feel like we’re expendable,” Myra said.

The Cernys have sued Marathon, hoping to get enough money to move away from the drilling. Marathon spokeswoman Lee Warren said in an email that the company “took corrective actions” after receiving the state citation and engaged in “good faith discussions with [the Cernys] to listen to and address their concerns.”