Preliminary findings indicate Tucson has violated the federal ozone air-quality standard for the first time. Emissions from cars, trucks and other motor vehicles are a major factor.

For the first time, Pima County is violating federal rules governing the noxious, lung-scarring pollutant ozone, local officials say.

Preliminary data show ozone levels at one of the county’s air monitors violates federal standards regulating the air pollutant. The monitor is at Saguaro National Park-East, on Tucson’s far east side.

That means that on some days in a part of the community, our air is bad enough to potentially cause major health problems for children, older adults and people of all ages with existing respiratory problems.

Legally, continued high ozone levels could put the Tucson area at risk of being designated in formal noncompliance with federal rules from the Environmental Protection Agency, a local planning agency says. That would make it harder for the region to spend federal dollars for needed transportation projects unless the air quality improves, says the Pima Association of Governments.

Here are five things to know about the area’s ozone problem and its potential implications:

1. What happened. On Thursday, the county went over the top for ozone when the level at Saguaro Park-East hit 75 parts per billion, compared to a federal standard of 70 parts per billion, over an eight-hour period.

That by itself isn’t a formal violation. Compliance is based on a three-year average of the fourth-highest reading each year, at any of the county’s eight ozone air-quality monitors.

But the 75 parts per billion was enough to push Saguaro’s three-year average to 71 parts per billion, topping the federal standard for those three years.

That reading came at a time when ozone levels were generally unusually high both in the Tucson and Phoenix metro areas due to a high-pressure weather system that was propping up temperatures and holding in pollution so it couldn’t escape.

Since July 27, ozone levels exceeded federal standards at one or more of Pima County’s eight ozone monitors three times.

In all of 2018, ozone levels have exceeded federal standards on four days at the Saguaro Park-East monitor and seven days at various other monitors, compared to five days in all 2017.

In Phoenix, whose air is far more polluted than ours, ozone levels have topped federal standards on 39 days this year, 12 days more than by the same time last year.

Pima County officials won’t know for sure that the ozone level is violating federal rules until they have validated the monitoring data, probably by spring 2019. But “our data collection process is very good, and we don’t expect it to change,” said county Department of Environmental Quality director Ursula Nelson of the preliminary finding of the ozone violation.

The last time Pima County was in violation of federal air standards was in 1999, when it violated separate standards for particles that can lodge in peoples’ lungs and cause breathing problems.

2. If you’re a kid, an older adult or suffering from asthma or other respiratory problems at any age, watch out.

High ozone levels can cause muscles in the airways to constrict, trapping air and triggering wheezing and shortness of breath, the EPA says.

Ozone can make it difficult to breathe deeply and vigorously, and can inflame and damage airways and aggravate lung diseases such as asthma, emphysema and chronic bronchitis.

It can also increase the susceptibility to lung infection, cause chronic obstructive pulmonary lung disease that can lead to other potentially fatal lung ailments and can continue damaging lungs even after other symptoms have disappeared, the EPA has said.

These effects may lead to increased school absences, medication use, visits to doctors and emergency rooms, and hospital admissions, the EPA said.

In addition, a study published this year by the National Institutes of Health concluded that in older adults, prolonged exposure to ozone pollution could accelerate cognitive declines in the early stages of dementia in those adults, noted Lana Baldwin of the Pima Council on Aging.

“Pima Council on Aging doesn’t want to be alarmist because we’ve had a few bad air days, but we are concerned that on an ongoing basis we work together as a community to improve air quality for everybody’s health,” said Baldwin, the council’s vice president of philanthropy and communications.

The Arizona Department of Environmental Quality recommends that on days when ozone levels exceed the EPA standard, active children and adults, along with those with sensitive lungs, should limit prolonged outdoor exertion. On days when ozone levels are rated moderate, or slightly to somewhat below the standard, people with sensitive lungs and most children and older adults should limit outdoor exertion.

3. Federal ozone standards are tighter, but the air is also getting a little worse.

Local environmental officials have been on the lookout for a violation since the Obama administration’s EPA lowered the standard from 75 to 70 parts per billion in 2015. The EPA’s science advisory committee concluded the previous standard was too weak to protect public health.

The tighter standard angered many business groups nationally, which said enforcing it would choke off some economic development. Many environmentalists, however, said the new standard was still too weak, since the EPA committee recommended cutting the limit to between 60 and 70 parts per billion.

“The standard is made by EPA to protect public health and the environment,” said county DEQ director Nelson. “Since we want to make sure our air quality is healthy to breathe, this sends us a message we really need to look at things we can do in our area to reduce our ozone levels.”

Overall, ozone levels in the Tucson area have risen and fallen in spells over the past two decades. In the past three years, they’ve generally risen, although this year’s levels are a bit below last year’s, after having fallen for the previous two to three years, Pima County air data show.

“Ozone is very weather-dependent. We’re seeing weather patterns more conducive to the formation of ozone recently — more hot, sunny days,” Nelson said.

Recent high-pressure weather is “kind of like a dome that keeps everything in here, so pollution can’t move out,” said Beth Gorman, a program manager for the county DEQ.

Also, when a lot of high-pressure systems come through the area, you see “fewer storms, less cloud cover, more solar radiation and worse air,” Gorman added.

For years, climate scientists have warned that continued warming would trigger an increase in ozone pollution. The latest ozone-level increases, the recent high-pressure days and the accompanying hot weather could be a symptom of climate change, said Nelson, adding that is “my speculation.”

4. Blame cars and trucks, but also trees and shrubs.

Ozone is an odorless, invisible gas. It consists of three oxygen atoms and is formed by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight, the EPA says.

Locally, a 2017 study concluded that on-road cars, trucks and other motor vehicles are the largest sources of nitrogen-oxide emissions, making up 46.5 percent of the total.

Lesser but still significant shares of those emissions come from non-road, diesel-powered vehicles, led by construction and mining equipment, and from large “point sources,” mainly the Cal Portland cement plant near Rillito and TEP’s Irvington Road Generating Station on Tucson’s south side, the report said.

For volatile organic compound emissions, what’s known as “biogenic,” or natural sources such as trees and shrubs, make up 80 percent of the total, said the 2017 emissions inventory done for the county DEQ.

That doesn’t mean county officials want to launch a tree-clearing campaign. “Trees are wonderful and they provide much-needed oxygen, shade and wildlife habitat in a desert community,” Gorman said.

Instead, people are urged to consider avoiding planting trees known to produce high levels of volatile organics.

Those include the Mexican fan palm, the lemon and weeping bottlebrush, fig trees, California and Arizona sycamores, weeping willows and five forms of oaks, including Spanish oak and southern live oak.

5. There’s potential for legal and economic fallout due to the ozone violation. But it’s hard to say how much.

The summer’s high ozone levels here came barely three months after the EPA found that Pima County was formally in compliance with its ozone standards. That was based on data from the years 2014 through 2016, the most recent available at the time, said EPA spokeswoman Margot Perez Sullivan in an email.

The EPA was noncommittal about how it might react to the newly reported violation. Sullivan said the EPA “allows states/communities to lead efforts to meet air quality standards, consistent with the Clean Air Act and cooperative federalism.”

But if we’re designated as what’s known as a “non-attainment” area, “It will be a very difficult and costly process to bring the region back to attainment status,” said Sue Cotty, an air-quality planner for the Pima Association of Governments.

The region would have to come up with a formal plan to bring the air back into compliance, with a threat of economic sanctions such as transportation funding cuts if a plan isn’t submitted.

“Motor vehicles are a major source of emissions. Regional planning efforts to improve traffic flow at congested intersections and reduce vehicle idling, such as at drive-thrus and parking areas, can reduce air pollution,” Cotty said.

Some things people can consider doing include combining errands to reduce vehicle trips, carpooling to work or other destinations, riding the bus and refueling their vehicles after dark, which results in fewer emissions, said Paul Casertano, the regional association’s transportation planning direction. The county environmental department plans to launch an anti-idling campaign soon.

In recent years, however, our travel habits have gone in the opposite direction. The annual number of vehicle miles traveled here, for instance, is now exceeding the level that existed before the 2008 economic bust caused it to drop, Casertano said.

Ridership on SunTran buses and the SunLink streetcar has also dropped in the past few years, SunTran records show.

For these trends, the county’s Nelson blames cheaper gas.

“Our surveys show it,” Nelson said. “When gas prices are $4 or more per gallon, transit ridership is much higher. When it’s down, we see transit decreases.”

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987