BackPage Primer from Environmental Building News
February 1, 2008
Good Ozone, Bad Ozone
The oxygen that we breathe consists of pairs of oxygen atoms, or O
2. Add another oxygen atom and you get O
3, or ozone. Ozone is the primary component of smog. It is created when pollutants such as volatile organic compounds (VOCs) or nitrogen oxides react with oxygen, which is triggered by sunlight and heat. These pollutants come primarily from motor vehicles and factories but also from paints, coatings, and adhesives. Ozone irritates lung tissues and can lead to serious respiratory problems. Since passage of the 1990 Clean Air Act, ozone levels in most major U.S. cities have declined measurably.
In some situations ozone can react with and neutralize other pollutants, which is why ozone is intentionally created to purify water, and why some companies even sell ozone generators as air purifiers. The scientific consensus, however, is that ozone should never be generated in air that people might breathe (see
Newsbrief: California Limits Ozone-Generating Air Purifiers).
That which is harmful up close can be essential at the proper distance; for ozone, this distance is between 10 and 20 miles (16–32 km) overhead, in the stratosphere. That’s where the “ozone layer” intercepts harmful ultraviolet (UV) rays from the sun, protecting all forms of life from cell damage, such as (in humans) sunburns and skin cancer. The maximum concentration of ozone in that layer is less than ten parts per million (ppm), but that’s 100 times more than the level that would trigger a smog alert on the ground.
The ozone layer is thinning because of certain manufactured chemicals that are stable under normal conditions—some were even used in inhalers for asthmatics. In the stratosphere, however, high-energy UV radiation breaks up these chemicals and releases atoms, such as chlorine and bromine, that destroy ozone in ongoing chain reactions, so each chlorine or bromine atom can destroy thousands of ozone molecules. This reaction is exacerbated by conditions over Antarctica where an “ozone hole” forms each spring.
In 1987 the Montreal Protocol created a plan for phasing out these ozone-depleting chemicals. Among the worst offenders are chlorofluorocarbons (CFCs), used as refrigerants R-11 and R-12, and as blowing agents in making foam products. Halons used in fire-suppression systems are similarly bad, as is methyl bromide, used to control certain insects. These chemicals are rated according to their “ozone depletion potential” (ODP), using R-12 as the reference point with an ODP value of one.
The Montreal Protocol established a class of interim chemicals that could be used during a transitional period after the worst offenders were banned. These include the refrigerant R-22, which has an ODP of 0.05. Choosing acceptable substitutes is tricky because many of these chemicals are also powerful heat-trapping greenhouse gases.
As of 2006 the production of the most problematic chemicals had been largely eliminated, and their concentration in the atmosphere had begun to diminish. The ozone layer had stopped declining and was expected to begin recovering over the next few decades.
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Illustration: Amie Walter
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