As Americans make their annual pilgrimage to beach, pool and park in search of the perfect summer, it's important to remember one thing: The sun also fries us. During the past two decades, scientists have become increasingly concerned that the frying is getting worse and that depletion of the earth's ozone layer—which absorbs most of the sun's harmful ultra-violet rays—is responsible.

Until recently, much of the evidence about ozone depletion was ambiguous. Then, last March, an international team of 120 scientists led by Robert Watson, chief of upper atmospheric research at the National Aeronautics and Space Administration, released the most exhaustive report on the subject to date. It concluded that the ozone layer is thinning, and that production of chlorofluorocarbons—chemicals involved in the manufacture and use of foam, Styrofoam, air conditioners, aerosol sprays—is the culprit.

Watson, 40, a British-born chemistry Ph.D., spoke with correspondent Michael J. Weiss about scientists' growing concern about ozone loss.

Americans have been hearing about the depleting ozone layer for years. Why do we have more cause to worry now?

For the first time, scientists have found direct evidence that chlorofluorocarbons [CFCs] are causing ozone depletion in the upper atmosphere. For years we'd said that there was no observable long-term decrease in ozone that could be attributed to human activity. There was no direct proof. But last March we concluded that over a 17-year period, ozone has decreased by 1.7 to 3 percent in the Northern Hemisphere between 30 and 60 degrees latitude, which includes the U.S., Japan, the Soviet Union and Europe. And we couldn't find any other cause than man-made CFCs.

Why is an ozone loss of a few percentage points such a major problem?

Many reasons. Ozone acts like a protective blanket in the atmosphere, blocking out nearly all the sun's harmful ultraviolet radiation. When it's depleted, more ultraviolet rays reach the earth's surface and damage all living organisms. Ultraviolet radiation has been found to cause sunburn and skin cancer in human beings and has been linked to cataracts and the weakening of the immune system. According to the National Academy of Sciences, every 1 percent drop in ozone increases the annual number of skin cancer cases in the U.S. by 10,000 to 20,000. Because significant ozone depletion has occurred only since 1970, you wouldn't expect to see the increased cancers for 30 to 40 years.

Where do chlorofluorocarbons come from?

A variety of sources. They're employed as coolants in refrigerators and air conditioners and in foam used in car seats, in building insulation and packaging for fast food. They can enter the atmosphere when air-conditioning coils leak, when refrigeration units are broken up at waste sites, or via the propellant in aerosol sprays. If you simply break a Styrofoam cup, some CFCs escape.

What happens once they are in the atmosphere?

They trigger a chemical reaction. Ultraviolet light causes a chlorine atom to break off the chlorofluorocarbon molecule and then to attack the ozone molecule, creating oxygen and another form of chlorine ready to do more damage.

The U.S. banned the use of CFCs as aerosol propellants in 1978. Didn't that solve the problem?

Unfortunately, most other countries didn't follow our lead. We kept saying aerosol propellants are a nonessential use of CFCs, but what's nonessential to one society may not be nonessential to another. Other countries pointed out that Big Mac containers aren't exactly essential either. And the Scandinavians suggested that other countries might do without air-conditioning.

Why didn't they share our alarm?

There was a lot of debate over the seriousness of the problem. In the early 1980s, the National Academy of Sciences came out with a report saying that even if nations continued producing CFCs at the then current rate, there would be about 3 percent ozone depletion within the next century. A lot of people took that report to mean that the problem had gone away. While the academy does a good job on these assessments, it normally uses only 10 or 12 scientists to write a report. The NASA reports I've chaired usually involve between 100 and 200 scientists and tend to be much more in-depth. We predicted a depletion rate on the order of 5 percent to 9 percent during the next 100 years. The scientific community recognized that there was a lot of uncertainty about the figures and continued to be concerned.

What has happened recently that has alarmed scientists?

In 1985 Joe Farman, a scientist with the British Antarctic Survey, reported the discovery of an ozone hole over Antarctica. He found that since the late '70s, large concentrations of ozone were disappearing each Antarctic spring, from September to October.

What's occurring over Antarctica?

A good analogy would be a bathtub where you produce ozone at a constant rate, like water flowing in, but you don't quite have your plug in correctly and there's a small hole. The sun is producing ozone in the earth's atmosphere, through a very complicated process, at a given rate, but each year the leak has gotten a little bigger and the level of water in the bathtub has gotten lower and lower. We're lucky that the spread of the hole misses most inhabited countries.

Is the Antarctic hole getting worse?

Unfortunately, yes. In 1985 as much as 50 percent of the ozone had disappeared compared with levels recorded in the '70s. And last year the hole was the deepest ever; as much as 60 percent of the ozone had vanished. To me, what was worse was that the hole lasted longer, from August until early December. And it moved off the South Pole, over the Pacific Ocean toward South America. So the concern I have is over ultraviolet radiation disrupting the fish chain. It could kill the phytoplankton in the seas, which could effect the krill, and then the fish, reducing the amount people could harvest.

Isn't ozone depletion reversible? Can't the atmosphere cleanse itself over time?

Yes, but the lifetime of fluorocarbons means that it would take about 100 years. And there's nothing we can do to speed up that process. In the meantime, the hole can get only worse.

How have nations responded to the threat?

Last September, after five years of international negotiations, representatives from 24 nations met in Montreal for a United Nations-sponsored conference to draft a kind of ozone protection treaty. The Montreal accord calls for developed nations to freeze their production and consumption of CFCs at 1986 levels, and steadily to reduce them by 50 percent by 1999. In addition, the treaty allows developing countries to increase their CFC use so that they can offer basic technologies like refrigeration. The net effect will be about a 35 percent CFC reduction by the turn of the century. The accord also calls for countries to reconvene if the situation continues to worsen. I think that those are major steps forward.

What are the chances of enough nations adopting the accord? Isn't it true that only two countries—Mexico and the U.S.—have ratified it so far?

Yes. However, by this fall, it's almost certain that Canada, the Nordics, all the European countries, the Soviet Union and Japan will also ratify the treaty. And these nations produce about 90 percent of the world's CFCs. It looks like at least 20 countries will ratify it.

What can Americans do to improve the situation?

We can encourage industry to come up with CFC substitutes that are inexpensive, nonflammable and environmentally safe. I've no problem with calling for an eventual ban on CFCs. The less you put into the atmosphere, the safer you are.

What can individuals do to protect themselves?

If you're concerned about skin cancer, one way to have protection—with or without ozone depletion—is by wearing suntan lotion, sunglasses and hats. But the real long-term solution is to protect the ozone layer from further damage.