Why Bad Heat = Bad Air

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As if the stifling, tripe-digit temperatures gripping much of nation weren’t bad enough, the heat wave is also contributing to dangerously high levels of air pollution—especially around the cities of the Northeast and the Mid-Atlantic region. The Environmental Protection Agency’s (EPA) air quality rankings range from 0 to 500—500 being the worst—and the air quality index (AQI) for New York City, where I live, is 110, which means that people with respiratory problems and the very young and very old should limit their time outdoors.

It’s worse further south—both Baltimore and Washington have AQIs over 150, which means that even healthy adults and children should avoid exertion outdoors. (Not that many people could handle outdoor exercise in this heat even if the air were cleaner.)

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When we talk about “air pollution,” we’re usually referring to a number of different possible pollutants, all mixing up in the air. Particulate pollution (particles of soot and dirt from coal combustion, diesel engines or fires), along with chemicals like carbon monoxide or sulfur dioxide, all add to air pollution.

But the extreme heat adds an additional factor, intensifying what’s known as “ground-level ozone.” In a blog post, Rich Kassel of the Natural Resources Defense Council explains why ozone gets worse as the temperature rises:

Ground-level ozone (or “smog”) is formed when hydrocarbon and nitrogen oxide pollution from vehicles, power plants and other combustion combines in sunlight and heat.

So, if there’s more sunlight and heat, there will be more ozone in the air.  Today was hot, and tomorrow will be hotter.

As you can imagine, ozone levels have been too high for comfort at many points up and down the eastern seaboard during this week’s heat wave.

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It’s a little confusing. Isn’t ozone a good thing, keeping us safe from ultraviolet rays? It is—but only when that ozone is high in the stratosphere. Closer to the ground, where we actually breathe, ozone is real health threat, especially for children, the elderly and those with respiratory problems. Asthma victims can be particularly sensitive—while air pollution doesn’t necessarily cause asthma, it can certainly make life hell for those who suffer from it. Other studies indicate that long-term exposure to ozone in childhood can lead to decreased lung function as an adult, and ozone also leads to an estimated $500 million in crop losses each year as well. Ozone actually seems to restrict breathing pathways, as the EPA points out in a guide:

The major effect is thus restrictive rather than obstructive in nature and reflects itself in decreases in forced vital capacity (FVC), FEV1 and other spirometric measures that require a full inspiration. Observed changes in breathing pattern to one with more rapid shallow breathing may also be a manifestation of C-fiber stimulation and may be a protective response to limit penetration of ozone deep into the respiratory tract. It is likely that these lung function changes and respiratory symptoms are responsible for observations that short-term ozone exposure limits maximal exercise capability.

You can also see ozone in a way that you can’t for many other air pollutants. That hazy hot sky above cities like New York and Washington is due to ozone-related smog.

When temperatures decrease next week, the air quality should improve as well. Obviously there’s not much we can do to reduce the heat—though getting a grip on carbon emissions would obviously be a nice start—but we can reduce the toxic emissions that lead to air pollution. In the case of ozone, that means cutting back on emissions of nitrous oxides and volatile organic compounds, which mix with sunlight to create ozone. Major sources include gasoline exhaust, chemical facilities—and power plants, especially those that burn coal. Efforts like the Sierra Club’s Beyond Coal campaign—which I wrote about yesterday—are meant to fight this kind of pollution.

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As it happens, the EPA is set to release what are likely to be newer, tougher standards on ozone pollution. Quick back story: the Clean Air Act requires the EPA to reexamine air quality standards every five years, to keep them up to date with the latest science. In 2008, President George W. Bush’s EPA set a new ozone standard of 75 parts per billion—significantly higher than the levels suggested by the agency’s own scientists, who would have put the limits between 60 and 70 ppb. That standard was challenged in court by environmental and public health groups, and now Obama’s EPA Administrator Lisa Jackson has said she expects to set the limit between 60 and 70 ppb—in line with her own scientists.

Business leaders are screaming bloody murder about the proposed ozone standards, claiming that it will cost some 7 million American jobs as manufacturers flee the country. The EPA, for its part, estimates that compliance costs could range from $19 to $90 billion a year for industry, while providing public health benefits—in the form of fewer deaths, hospitalizations and lost work days—worth between $13 and $100 billion. Setting standards at the low-end of the scientific range could save some 10,000 lives a year. Public health groups say the costs are more than worth it, as they told President Obama in an open letter released yesterday:

Throughout the years, the Clean Air Act has built a solid science and health foundation, making it one of the nation’s most effective public health laws. By saving millions of lives, avoiding millions of cases of pollution-related illness, and preventing millions of lost work days, the Clean Air Act has also been an economic success story. The total benefits of the Clean Air Act between 1990 and 2010 exceeded total costs by an estimated 35 to 1.

On a day like today—when you can almost taste the acid of the ozone on your tongue—those costs seem like a bargain.

UPDATE [7/22/11 2:00 PM]: Of course, as the weather warms thanks in part to climate change, ozone-related pollution will likely worsen as well. A recent study by the Union of Concerned Scientists, a research-focused environmental think tank, could result in 2.8 million additional serious respiratory illnesses by 2020. All told, that could cost the economy more than $5 billion. Check out a PDF of the full study here.

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Bryan Walsh is a reporter at TIME. Find him on Twitter at @bryanrwalsh. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.