USEPA set a new national ambient air quality standard for ozone on Thursday that, while significantly lower than the previous standard, is at the high end of the range discussed during the regulatory development process.

USEPA set the new limit of 70 parts per billion for ground-level ozone, down from the current level of 75 parts per billion. In a draft released in 2014, the agency proposed a standard between 65 and 70 parts per billion.

Ozone is created by emissions of nitrogen oxides and volatile organic compounds. Exposure to ground-level ozone can exacerbate respiratory problems, particularly for at-risk groups including children, older adults, and people of all ages who have lung diseases such as asthma.

USEPA projects that most U.S. counties will meet the standards by 2025 with federal and state programs now in place or underway.

In announcing the new standard, USEPA Administrator Gina McCarthy said that clinical data show that 72 parts per billion “is the lowest ozone exposure that causes adverse health effects in healthy, exercising adults.”

From there, she decided to add in a “margin of safety” to protect at-risk populations, including children, the elderly and those suffering from heart and lung diseases. Setting the level at 70 parts per billion “will essentially eliminate exposures to the levels that clinical studies clearly show are harmful,” McCarthy said.

McCarthy said the final judgment was hers, and she aimed to set a standard that was “not too high and not too low. It’s very challenging. There’s no bright line.”

Over 90% of counties with ozone monitors are meeting the current 75-parts-per-billion limit, according to an EPA spokeswoman. USEPA expects that by 2025 only 14 counties of 3,000 in the US will not meet the new standard.