WASHINGTON — After the White House intervened, the Environmental Protection Agency last week weakened a rule on airborne lead standards at the last minute so that fewer known polluters would have their emissions monitored.

The EPA on Oct. 16 said it would dramatically reduce the highest acceptable amount of airborne lead from 1.5 micrograms of lead per cubic meter to 0.15 micrograms. It was the first revision of the standard since EPA set it 30 years ago.

But a close look at documents publicly available, including e-mails from the EPA to the White House Office of Management and Budget, reveal that the OMB objected to the way the EPA had determined which lead-emitting battery recycling plants and other facilities would have to be monitored.

EPA documents show that until the afternoon of Oct. 15, a court-imposed deadline for issuing the revised standard, the EPA proposed to require a monitor for any facility that emitted half a ton of lead or more a year.

The e-mails indicate that the White House objected, and in the early evening of Oct. 15 the EPA set the level at 1 ton a year instead.

According to EPA documents, 346 sites have emissions of half a ton a year or more. Raising the threshold to a ton reduced the number of monitored sites by 211, or more than 60%.

The EPA also required states to place monitors in areas with populations of 500,000 or more. But the Natural Resources Defense Council, an environmental group that pushed for tougher lead standards to protect public health, said a single monitor in a large city was different from a monitor placed near a plant.

“We don’t expect the urban monitors to be effective to get the hot spots that the site-specific monitors can get,” said Gina Solomon, an NRDC scientist and a professor of medicine at the University of California at San Francisco. “The monitoring network has a lot of gaps in it.”

Airborne lead can be inhaled, but the main way people are exposed is when they ingest it from contaminated soil — for example, when children play in a contaminated area and put dirty hands to their mouths.

The EPA originally estimated that at the half-ton annual emissions cutoff, it would need from 150 to 600 monitors, said EPA spokeswoman Cathy Milbourn. Under the final rule with the 1-ton cutoff, the requirement will be 135 site-specific monitors and 101 urban monitors in areas of 500,000 or more people, she said. There are 133 monitors now.

Milbourn said that EPA Administrator Stephen Johnson set the requirement for monitoring at sites that emit 1 ton or more of lead a year because it was “an approach that would reduce the burden to states but would still assure monitoring around those sources” that might violate the air-quality standard.

The Battery Council International, a trade group that represents U.S. lead battery makers and recyclers, told the EPA in public comments in August that the proposed half-ton threshold was “unjustifiably low.”

Milbourn said that state and local officials should monitor any site they think might violate the new EPA standard.

“In other words, states may go beyond the minimum monitoring requirements,” and EPA will help them identify sources that emit less than a ton per year but still might produce amounts of lead in the air that are higher than the rule allows, she said.

Lead in the air was greatly reduced three decades ago when the government ordered it removed from gasoline. But it is still emitted by lead smelters, cement plants and steel mills.

Scientific studies have found that lead is dangerous at much lower levels in the human body than previously thought. The studies show that children’s nervous systems are especially vulnerable, and that lead exposure can result in IQ loss and damage to many internal systems.

For a listing of plants, by state and county, that emitted between a half-ton and a ton of lead a year in 2002, click here. The list includes nine sites in Michigan. The source is the Natural Resources Defense Council, using EPA data.