New air-quality rules being considered by the federal government could put tighter shackles on ozone, the invisible gas that chokes lungs, stops hearts and ends lives prematurely.

The U.S. Environmental Protection Agency proposed the tougher regulations the day before Thanksgiving last year, stating that recent health studies show that ozone is more dangerous than previously believed. It plans to make a final decision in October.

Although San Diego County’s air quality has consistently improved in recent decades, the region still struggles to meet the existing federal threshold and the even tougher state rules governing ozone.

Current federal law limits ozone to 75 parts per billion. But at that level, ozone can still harm the respiratory system, trigger asthma attacks and contribute to premature death from heart and lung disease, the agency said.

The proposed changes would create a new limit of 65 to 70 parts per billion. That would be stricter than California’s current ceiling of 70 parts per billion.

“The EPA by law is required to set the standard at a level that protects public health, with a margin of safety,” said Robert Reider, deputy director of the San Diego County Air Pollution Control District. “We don’t quite meet the standard, and it appears they’re going to be moving the goal post on us. … Tightening the standard will require further efforts to reduce pollution.”

Improvements in fuel formulations and emission-control devices have reduced pollution from trucks, cars, boats and trains, while controls on factories have cut industrial emissions.

County Supervisor Ron Roberts, who also serves on the California Air Resources Board, said the progressively tougher air-pollution requirements are “probably a little confusing to anybody who’s watching. We’ve had a standard we’ve been looking to meet, and we probably would in the next few years, and we’d feel good about that. So (if the new rules are finalized), we’d have to renew our effort to meet even more stringent standards.”

Ozone is a colorless gas. It occurs naturally in the atmosphere, but also forms when other pollutants released by vehicles, power plants, fuels, paints and solvents combine in the presence of sunlight. Although ozone in the upper atmosphere protects people, plants and animals from ultraviolet light, at ground level it’s toxic.

Breathing ozone can cause chest pain, coughing, throat irrigation and respiratory congestion. Over time, the chemical can reduce lung function and inflame the linings of lungs, worsening diseases such as bronchitis, emphysema and asthma.

Researchers also have found that breathing ozone may raise the risk of heart attack by inducing inflammation and increasing blood clotting.

About one in 10 school-age children have asthma, and they may suffer breathing problems, require more medication and have more emergency-room visits on days when ozone levels are high, according to the EPA. Spikes in ozone levels are associated with slightly higher death rates, particularly among older people.

As California car traffic escalated in the past century, ozone levels rose along with it. National regulators monitor average ozone levels over eight-hour periods and then look at the highest average period for each year, excluding some readings attributed to extreme weather.

Records from the state air board show that ozone concentrations surged in the 1970s and ’80s, reaching eight-hour highs of more than 300 parts per billion in 1978 and 1979.

The levels declined steadily as California adopted stricter fuel standards and put a lid on industrial emissions. But in San Diego County and other parts of Southern California, where high traffic volumes and near constant sunlight create prime conditions for ozone formation, the levels have remained stubbornly over the government-mandated limits.

“Fortunately, San Diego’s air quality is as clean as it’s ever been,” Reider said.

In 2013, the most recent year for which the state air board has posted ozone statistics, San Diego County had seven days over the national ozone limit and 28 days over the state limit. That was significantly down from the 1970s through the early 1990s, when there were more than 100 ozone violations each year.

The problem tends to be a localized clash of air chemistry and geography. In San Diego County, the majority of violations stem from the monitoring station at Alpine.

Although the rural town produces only a small share of the region’s pollution, ozone generated in coastal communities wafts eastward and runs into a wall at Alpine, where mountains and atmospheric inversion layers trap the gas in place.

And if any place within an air basin exceeds the standard, the whole region flunks for the day.

In this county, 70 percent of ozone-forming emissions comes from mobile sources including cars, trucks, buses, trains, boats and aircraft. About 14 percent originates from manufacturing plants, power plants, landfills and gas stations. And 16 percent comes from small, widespread sources such as household paints, furnaces and water heaters.

The San Diego County Air Pollution Control District has done what it can with current technology to cut emissions from commercial and industrial sources, Reider said. So the next phase likely will rely greatly on state measures to further trim vehicle emissions.

San Diego regulators also are considering measures to reduce ozone-causing chemicals in paint and convert San Diego Bay tugboats from diesel to electric shore power. They also are gradually replacing gasoline-powered taxis, cement trucks and delivery vans with electric or alternative-fuel vehicles, Reider said.

Strengthening the federal ozone standard wouldn’t eliminate all ozone-related health risks, but it could prevent nearly a million asthma attacks and up to 4,300 premature deaths, the EPA stated.

Tougher standards would mean more work for San Diego County, but the benefits could be worth it, Roberts said.

“Any of the changes in here, I see as a positive, with respect to the public health,” he said. “For people with asthma, seniors with other disord, at the end of the day, these changes are very good for us as a community.”