Sulfur dioxide is a product of burning coal and other fossil fuels used to make electricity.  Short-term exposure to high levels of the gas can be life-threatening, but exposure to lower amounts can cause heart and lung trouble, according to the Sierra Club, a national environmental advocacy group.

Luminant, a Dallas-based energy company, owns the four power plants targeted in the EPA’s notice, three east of Dallas and one east of  Austin.  Meranda Cohn, a spokeswoman for Luminant, said that all four of the company’s plants are operating in compliance with state and federal regulations. Luminant thinks that the models the EPA used to determine the emissions levels were inaccurate, Cohn added. Cohn also raised concerns that some of the air quality models used by the EPA were provided by the Sierra Club, according to documents from the EPA.

The state of Texas has its own air quality monitors in place that show no violations in emissions, although none of those monitors were in the six counties mentioned in Wednesday’s notice, according to a letter from the state sent to the EPA in September. Instead, Luminant ran its own air quality models for those counties, but the EPA did not take those models into consideration,  Cohn said.

“The state of Texas will begin the process to evaluate whether potential control or operational changes, if any, may be necessary to demonstrate attainment and we are considering our legal options,” said Cohn, who added that the state’s objections to the EPA’s determination can be taken to court.

The EPA has had its eye on the plants for years as part of an on-going attempt to reduce air pollution in Texas. This week, after a months-long court battle, the EPA decided to withdraw another federal regulation that would have required Luminant and other companies to reduce emissions from the plants by upgrading scrubbers, air pollution control devices. The companies objected to the rule saying that it would have forced them to shut down the coal-fired power plants.

Five of the six counties, Titus, Rusk, Panola, Freestone and Anderson, are part of a sulfur dioxide non-attainment zone, where levels of the gas are higher than the EPA recommends.