› The Jargonaut

Good Ozone vs. Bad Ozone

Ozone can be a protector or a killer. It all depends on where you find it.


Ozone (n.): an inorganic molecule made of three oxygen atoms.

The U.S. Environmental Protection Agency just proposed strict new rules limiting ozone emissions, and public health advocates are celebrating. Wait a minute—30 years ago, we were worried the atmosphere didn’t have enough ozone in it. What gives? Is ozone a good thing or a bad thing?

Although ozone is a simple chemical, made of three oxygen atoms bonded together, its relationship with human health is complicated. Depending on the molecule’s elevation, it can either be crucial in preventing skin cancer or it can exacerbate asthma, reduce respiratory efficiency, and scar lung tissue. So, it’s a bit of a Dr. Jekyll/Mr. Hyde situation.

Dr. Jekyll—Ozone in the Stratosphere

Dangerous emissions rising into the atmosphere. Projections of future calamity. An organized campaign of denial by industrial polluters. A Nobel Prize for scientists who identified the problem. Sound familiar? Yes, but I’m not talking about climate change—I’m talking about the ozone layer. If you’re over the age of 30, you remember the magnitude of the crisis. A 1980s kid couldn’t even give himself a Bon Jovi tease job without some dingus telling him, “Dude, you and your Aqua Net are totally bogarting the ozone layer.”

Truer words have never been spoken. When the sun’s rays reach the stratosphere—between 6 and 31 miles above the earth’s surface—oxygen atoms in the ozone layer combine, break apart, and recombine to absorb shortwave sunlight. That’s a nice favor for us Earthlings, because that ultraviolet sunshine is a potent carcinogen.

In the mid 1970s, a group of chemists hypothesized that certain chemicals could reduce ozone levels in the stratosphere. Most worrying among them were chlorofluorocarbon gases, which were commonly used in refrigeration and as propellants in aerosol cans of hair spray, air freshener, and other spray products. Over the next decade, atmospheric scientists confirmed their theory—the ozone layer had thinned dramatically above several regions. The “ozone hole” was especially thin over Antarctica.

The 1987 Montreal Protocol, which banned the use chlorofluorocarbons, and other international efforts have begun to remedy the problem, Scientists expect the ozone layer to return to near normal thickness by the middle of this century. But we’ve been feeling the effects of reduced ozone for years. According to the EPA, melanoma rates have doubled since 1990.

Mr. Hyde—Ozone in the Troposphere

Down here in the troposphere—the gas layer between the ground and the stratosphere—ozone forms when sunlight reacts with either oxides of nitrogen (NOx) or infamous volatile organic compounds (VOCs). These chemicals can come directly from the off-gassing of gasoline and or other solvents, but most result from combustion. The EPA estimates that motor vehicles cause 50 percent of NOx and 45 percent of the VOCs in the lower atmosphere, and most of the remainder comes from power plants and other industrial sources.

Now, just because something protects you from the sun’s harmful rays doesn’t mean you want it in your lungs. A three-oxygen molecule is nothing like its more benign two-oxygen equivalent that gives us life.

What makes ozone so dangerous to the human body is that it doesn’t like to dissolve in water. Other air pollutants, like sulfur dioxide, readily blend into the fluids in the upper respiratory system, allowing our bodies to spit them back out or send them through the digestive tract. Ozone is more likely to pass unprocessed into the lungs.

Once in your respiratory pathways, the hyper-reactivity of ozone becomes problematic. It is so reactive, in fact, that it works like a bleach and a sterilizer. In the lungs, ozone reacts with proteins and fats, and irritates our airway lining. The chain of consequences is complicated and not completely understood, but ample data show that inhalation of ground-level ozone causes coughing, burning sensations, shortness of breath, and throat irritation. Long-term exposure has been linked to chronic respiratory disease and premature death.

In short, just like Mr. Hyde, ozone makes for a better long-distance relationship.

Photo: Óscar Palop

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

Join Us