By Steve Tawa

PHILADELPHIA (CBS) – Oil and gas companies and clean air activists got a chance to tell U.S. Environmental Protection Agency regulators visiting Philadelphia whether there should be stronger limits on “particle pollution” — the official name for soot.

Every five years, the EPA reviews standards to incorporate the most recent science to determine whether the public is protected.

The EPA is considering more protective limits of particle pollution from 15 down to 12 or 13 micrograms per cubic meter. The American Lung Association’s Kevin Stewart says that measure of the concentration of fine particles in the air includes microscopic specks of soot, metals, acid, dirt, pollen and other elements that can be inhaled and lodged deeply in your lungs:

“If you lined them up, you can fit maybe 30 of them across the diameter of a human hair.”

Representing oil and natural gas companies, Howard Feldman of the American Petroleum Institute, says the EPA has not proven a ’cause and effect’ between particulate matter levels below the current standards and health effects:

“There’s no bright lines where we could say that this number is magically safe.”