Once again a report from the National Academy of Sciences has lowered projections of how much Earth's ozone layer may be depleted by the notorious chlorofluorocarbons. These chemicals are used in some aerosol spray cans and in refrigerators and air conditioners.
It's the second time in five years that a study by the academy has cut what had been an alarming forecast of 15 to 18 percent ozone depletion over the next century - ozone that shields us from solar ultraviolet rays. In 1982, an academy report reduced the estimated depletion to between 5 and 9 percent. The new forecast, released last week, projects between 2 and 4 percent ozone loss.
This successive downgrading of what some scientists had called a significant danger suggests that the United States acted more out of fear than foresight in banning the suspect chemicals from use as aerosol propellants. Certainly, other nations have thought the action hasty. That is why foreign aerosol products still use chlorofluorocarbons.
To begin with, the issue is not at all clear-cut scientifically. Ozone is a form of oxygen in which molecules contain three oxygen atoms rather than the normal complement of two. It is formed in the stratosphere by the action of sunlight. There, the ozone absorbs much of the incoming ultraviolet (UV) radiation in sunshine. Since UV, which causes sunburn, is considered dangerous to living tissue, any significant weakening of the ozone shield would be of concern.
There is considerable natural variation in stratospheric ozone. Also, complicated chemical processes destroy ozone at a rate that maintains a rough balance with its creation.
A decade ago, F. Sherwood Rowland of the University of California (Irvine) and Mario J. Molina of the NASA Jet Propulsion Laboratory suggested that chlorofluorocarbons (CFCs) migrate into the stratosphere to take part in the ozone-destroying chemistry. Their hypothesis - fleshed out with laboratory studies, mathematical simulations, and very limited sampling of the lower stratosphere - gradually gained enough credibility for some atmospheric chemists to consider the CFCs a likely threat to the ozone layer. In 1978 the US banned most uses of CFCs as aerosol propellants.
So far, there has been no similar move to restrict their use as refrigerants. The US Environmental Protection Agency is monitoring that issue, however. Indeed , the recent study released by the National Research Council (NRC), the operating agency of the National Academy of Sciences, was in response to EPA requests for guidance.
At no point has any expert claimed to have substantial proof that CFCs do, in fact, threaten the ozone layer. Even those experts most strongly in favor of banning the chemicals have urged this as a matter of prudence, not as a response to a clearly established hazard. They have argued that it is wiser to restrict use of the chemicals now rather than let them continue to build up in the stratosphere while awaiting convincing proof of their danger.
Ironically, ongoing research is undercutting the case for the ban rather than strengthening it. This emphasizes how poorly understood the relevant stratospheric chemistry was in the first place.
To begin with, the CFCs do not act on their own. Many dozens of other chemicals must be considered simultaneously. Present mathematical models that simulate this interaction include 150 or more reactions among perhaps 50 chemical species. The rates at which some of these reactions proceed are not even known in the laboratory, let alone under the conditions of the stratosphere.
Then there is always the possibility that chemicals or reactions not considered may play important roles. Also, the mathematical models are generally considered to be simplistic, again because of lack of adequate knowledge. Referring to such limitations, the NRC's 1982 report on the ozone threat observed that there ''are still uncertainties about the appropriateness of some assumptions common in current models. . . . It is difficult to rule out the possibility of an important role for (chemical) species not now included in models, and, if history is a guide, there may . . . be . . . surprises in this area.''
That remains so today. More is known now about stratospheric chemistry and this improved knowledge has enabled the NRC to reduce its estimate of possible ozone depletion due to CFCs. Nevertheless, even this better knowledge is still considered rudimentary. Among other things, the NRC has urged monitoring the ozone layer through two full sunspot cycles (some 22 years) to see how ozone varies naturally with solar activity.
The reduction in the NRC's estimate of CFC-induced ozone depletion does not mean there is no threat at all to the ozone shield. Nitrogen oxides from fertilizers and industrial sources also attack the ozone. But until a possible danger is more clearly identified than was that from CFCs, governments should not hastily ban a useful product.