The Environmental Protection Agency’s air pollution estimates from oil and gas drilling have not kept up with the boom in production and are likely too low, the agency’s internal watchdog says.

The Environmental Protection Agency’s air pollution estimates from oil and gas drilling have not kept up with the boom in production and are likely too low, the agency’s internal watchdog says.

EPA Inspector General Arthur Elkins, Jr. reported Thursday that EPA has only limited direct measurements of toxic air pollution from oil and gas drilling procedures. Scant state data have undercut the reliability of pollution data from the National Emissions Inventory it uses for potential regulations.

“Because so few states submitted data for this sector, we believe the NEI likely underestimates oil and gas emissions. This hampers EPA’s ability to accurately assess risks and air quality impacts from oil and gas production activities,” he said.

He said about half of the emissions factors that EPA has assigned to production operations are rated below average or unrated because of insufficient or poor data. “Limitations in EPA’s air emissions data for a number of oil and gas production pollutants have contributed to emission factors of questionable quality,” Elkins noted.

He also reported that EPA has yet to put a strategy into place to improve its data, which are used by states for permits, enforcement and pollution inventories. Oil and gas operations release nitrogen oxides, air toxics, volatile organic compounds, soot and methane.

The oil and gas industry has said the EPA has overestimated emissions of methane and argued that they already were working to reduce pollution, without the agency’s intervention. Industry representatives did not offer any immediate comment Thursday.

Elkins said EPA “did not anticipate the tremendous growth of the sector, and previously only allocated limited resources to the issue.”

The agency’s work in the area is overseen by Assistant Administrator for Air and Radiation Gina McCarthy, who is thought to be a top candidate to become the next agency administrator.

The report comes less than a year after EPA imposed air pollution controls on hydraulic fracturing of oil and gas wells. The regulations require capture or flaring of escaping gases until 2015, when only capturing would be allowed.

EPA said the regulations will reduce release of 95 percent of smog-forming volatile organic compounds, sulfur dioxide and air toxics from wells, and have the side benefit of preventing the release of methane, a powerful greenhouse gas.

In a letter to Elkins, McCarthy agreed with his recommendations to devise a comprehensive strategy for strengthening EPA data and emissions factors.

She disagreed with a recommendation that EPA do more to ensure state data submissions, contending states do so when data is available. McCarthy also disagreed with a recommendation that EPA provide default calculations for states to use in determining oil and gas non-point emissions, on the basis that EPA is already assisting states through another actions.