WASHINGTON (Reuters) - The head of the U.S. federal agency in charge of mine safety said on Thursday he has no plans to fast-track new limits for coal miner exposure to silica dust because he believes exposure rates are already falling.

FILE PHOTO: Homes built on a former surface strip coal mine sit above piles of coal in Norton, Virginia, U.S., May 18, 2018. Picture taken May 18, 2018. REUTERS/Brian Snyder

The comments come as the national coal miners and steel workers unions urge the Trump administration to regulate silica on the basis of research showing it is causing a resurgence of black lung disease among coal miners in central Appalachia here.

“We have increased our sampling, we have lowered the average exposure and we continue to do that every day,” Zatezalo told a Congressional committee hearing, adding that setting an “emergency standard” that set limits on the amount of crystalline silica miners can be exposed to would be “uncalled for.”

Government research and reports from black lung disease clinics in West Virginia, Virginia and Kentucky show the incidence of black lung rebounding despite improved safety measures adopted decades ago that had almost eradicated the progressive respiratory disease.

Researchers, including at the National Institute of Occupational Safety and Health, believe the resurgence is due to an increase in quartz rock blasting to reach deeper mine seams, a practice that produces large amounts of silica dust.

In 2016, MSHA put out a standard to reduce respirable coal dust but did not set any limits for exposure to silica, which is finer and more dangerous to miners’ lungs.

The Occupational Safety and Health Administration, which regulates a slew of industries but not mines, set a new silica standard that same year, cutting the permissible exposure limit to half of what it used to be, but MSHA did not follow suit.

Zatezalo said his agency has been conducting since last July a “request for information” to gather the latest information on available safety technology and equipment before it can proceed with a new silica standard.

Lawmakers, scientists and union representatives at Thursday’s hearing took issue with MSHA’s pace.

“We have to act now. We have convincing scientific evidence to prove that,” said California Congressman Mark Takano, a Democrat.

United Mineworkers President Cecil Roberts also said at the hearing that MSHA needs to act urgently. “We can all act like we need more time. But coal miners don’t have any more time,” he said.