If the EPA proceeds with its proposed tightening of the federal ozone standard, some parts of the country won’t be able to meet the new limit because they have high levels of “background” ozone created by wildfires and naturally occurring intrusions of ozone from the stratosphere.

That point was made repeatedly by environmental science experts who testified yesterday at a House Environment Subcommittee hearing on the achievability of the EPA’s proposed new standard.

The EPA last lowered the federal ozone standard in 2008, from 84 to 75 parts per billion, and the agency is now proposing a further reduction to 60 parts per billion.

“At a standard less than 70 parts per billion, achieving that standard over a broad portion of the western U.S. with current background ozone levels would be very difficult,” said Sam Oltmans, a senior research associate at the University of Colorado.

Furthermore, Oltmans said, “Conditions exist in other parts of the country during some times of the year when background [ozone] contributes significantly under exceedance or near-exceedance conditions.”