Every day, we breathe in the legacy of the 1970 Clean Air Act. Thanks in large measure to Maine’s then-Sen. Edmund Muskie, that far-reaching legislation passed by a vote of 375 to 1 in the House and unanimously in the Senate (a political feat unimaginable today when, as Sen. Angus King says, Congress “can’t pass the time of day unanimously”). Since 1970, aggregate emissions from six common pollutants have fallen 68 percent, according to the American Lung Association, even as the population has grown by 52 percent and the U.S. gross domestic product has more than tripled.

For Maine, stricter air quality regulations have reduced the number of “unhealthy” air days, when high levels of ozone and particle pollution can cause people both short-term discomfort and systemic damage. Clear spikes in asthma and cardiac events occur when pollution reaches unhealthy levels, says Dr. Marguerite Pennoyer, a respiratory specialist in Scarborough. Even for healthy individuals, she adds, ozone can feel like “sunburn on the lungs,” causing inflammation and possibly long-term scarring.

People are not always aware of this danger because ground-level ozone is virtually invisible and odorless. A major component of smog, this irritating gas forms when sunlight transforms volatile organic compounds and nitrogen oxides from vehicles and power plants. Ozone is typically worst in the heat of the day, peaking in afternoon hours.

Monitoring sites track ozone year-round, and levels are typically highest between April and September when sunlight is strongest. Due to more stringent air-quality regulations, maximum daily ozone counts have fallen somewhat since 1989. In the past three years, no days in Maine have reached “unhealthy” or “very unhealthy” levels, and only a handful have been reported as “unhealthy for sensitive groups,” including children, elderly people and people with respiratory illnesses.

A downward-trending chart might seem like good news, but new medical research is finding damaging health impacts from ozone at lower and lower exposure levels. The U.S. Environmental Protection Agency has ratcheted down the safe ozone standard several times since the 1970s. In 2008, science advisers recommended lowering the standard to 60-70 parts per billion, but the Bush administration set it instead at 75 parts per billion. The Obama administration has sidestepped any subsequent reduction. A lawsuit from public health groups set a court deadline of this December for a proposed new ozone standard, to be finalized by October 2015.

Having a governmental standard not supported by medical science produces a “false sense of security,” notes Ed Miller, senior vice president for the American Lung Association of the Northeast. Data from the Maine Department of Environmental Protection shows that between 2008 and 2013, 27 days reached into the “unhealthy for sensitive groups” zone, which starts at 76 parts per billions. If the standard had been set, as scientists advocated, at 60 parts per billion, Maine would have had 144 “unhealthy” air days during those years. Until a stricter standard takes effect, people can best protect themselves by tracking ozone levels (see sidebar) and taking precautions when they are “moderate.”

Particulate matter, the other most closely tracked form of pollution, can be nearly as invisible and insidious as ozone. Fine particles of soot, aerosols, acids, dirt, metals and other substances settle deep in the lungs. What medical research has found about their effects is not encouraging, particularly for vulnerable populations like children. “Particle pollution is beginning to look more like secondhand smoke,” Miller says, “except that you can’t isolate yourself from it.” The finest particles can migrate over time into the bloodstream and damage the circulatory system.

Maine has a challenge in winter with particulate matter from wood-smoke emissions. It could achieve markedly better air quality and public health if homes and businesses reliant on wood heat upgraded to EPA-certified wood stoves and boilers, which are more efficient and hence more cost-effective. Studies show that these models are significantly cleaner-burning, reducing emissions and improving indoor air quality.

Being downwind of Midwestern coal-fired plants has proved an ongoing challenge for Maine as well. Public health advocates are optimistic that the proposed federal rules limiting carbon emissions will reduce this imported air pollution. Actions already taken by the Regional Greenhouse Gas Initiative have helped cut airborne emissions within the Northeast. The nine participating states, including Maine, sell emissions allowances to power plants and invest the auction proceeds in renewable energy and energy-efficiency programs.

When the Clean Air Act took effect more than four decades ago, Muskie acknowledged that it was an “unfinished agenda.” With a growing population and industrial base, and ever more cars on the road, the country could not keep ahead of air pollution without continued vigilance. Clean air is a classic “pay now or pay later” proposition: If we fail to invest in preventing pollution, we will pay heavily in medical costs, early deaths and diminished quality of life.

“Political power in our system is yours to use,” Muskie observed, “if you will.”