PORTAGE | BP's Whiting refinery might not emit additional particulate matter into the air.

Should a rule preliminarily adopted Wednesday by the Indiana Department of Environmental Management's Air Pollution Control Board reach final adoption, the plant would be required to reduce its air particle output by 56 percent.

IDEM Commissioner Tom Easterly said the 380-ton reduction wouldn't be noticeable to the naked eye.

Each particle is "one-seventh the width of a human hair," Easterly said. "You can't see it."

It may be noticeable, though, when people breathe. Easterly said the specific particulate matter the rule limits can penetrate the lungs, exasperating respiratory ailments.

The plant, however, might already be in compliance, as some sources of pollution are no longer in use, rendered obsolete because of changing technology and a shift in fuels the processes.

"Using the cleanest fuel is what we prefer," IDEM Assistant Commissioner Dan Murray said.

The rule change approved Wednesday calls for particulate matter changes for 72 sources around the state and an additional 32 sources in Lake County.

Despite the changes at BP's Whiting refinery, Murray said preliminary readings from this summer show ozone levels exceeding the standard at the Whiting High School monitoring site.

"Everything should be better, but it's not," Easterly said, noting that in 2005, a study found that more than half of the ozone concentration at the Whiting site came from Illinois.