High ozone levels contribute to haze in the National Park System, as this side-by-side photo from Shenandoah National Park shows/NPS file photo

A decision by the Trump administration not to delay a tightening of ozone standards was welcomed by the National Parks Conservation Association, though staff there said the administration had no choice but to reverse itself.

“It was an illegal action, so EPA did the right thing in walking it back," Stephanie Kodish, NPCA's senior director and counsel for clean air and climate, said Thursday evening. "The Clean Air Act has a pretty specific pathway forward and a pretty specific exemption for when that path can be walked away from, and nothing here met that.”

The Trump administration and some members of Congress had wanted to delay Obama-era regulations that would require states to reduce acceptable ozone levels from 75 parts per billion to 70 ppb. Congress has been considering legislation to keep the 75 ppb standard in place and delay the more stringent ozone standards established by the Obama administration until 2025.

EPA Administrator Scott Pruitt in June moved until October 1, 2018, the deadline for implementing the Clean Air Act of 2015 standards. But now he has scrapped that plan on the heels of lawsuits brought by 16 states.

"We believe in dialogue with, and being responsive to, our state partners. Today’s action reinforces our commitment to working with the states through the complex designation process,” said Mr. Pruitt on Wednesday.

At NPCA, Ms. Kodish said the more stringent 70 ppb standard is needed to protect the health of the elderly, young children "with developing lungs," and even healthy visitors to the National Park System.

“If you’re a healthy person and you happen to be in Rocky Mountain or Joshua Tree on a day when it happens to be about 70 ppb and you’re having a walk with your family and you’re having trouble walking, I‘d say it’s an issue," she said. 

The standard, the NPCA official pointed out, is "designed to protect the most vulnerable population, as well as the rest of us.”

“What’s important here is that EPA, the agency that is designated to protect public health and the environment, use the science that exists," Ms. Kodish added. "It’s the science that governs these standards, not ideology, and that’s how the EPA justified landing on that 70 ppb. That's the outer range of where the science led.”

Ozone forms "when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants" react with sunlight, according to the Environmental Protection Agency. High levels can exacerbate bronchitis, emphysema, and asthma, particularly among the young and old, people who are exercising, or those who work outdoors.

So far this year Acadia National Park's air quality has exceeded that 70 ppb limit on six days, with the maximum 8-hour average this year peaking at 98 ppb.

In California, meanwhile, Joshua Tree National Park, has exceeded the limit 33 times this year at Black Rock, and 18 days at Cottonwood Canyon, while Sequoia National Park has exceeded it on 22 days. Other parks that have surpassed 70 ppb this year include Chiricahua National Monument in Arizona, Mesa Verde National Park in Colorado, Petrified Forest National Park in Arizona, Saguaro National Park in Arizona, and Tonto National Monument, also in Arizona.

In some areas of the National Park System, such as Acadia, Zion, and Yosemite, shuttle bus systems help lower ozone levels by reducing the number of vehicles on the roads. Some parks are even installing recharging stations for visitors with electric vehicles.