OZONE DEPLETION

Natural sources such as seawater and volcanoes release nearly 650 million tons of chlorine into the atmosphere each year. CFCs account for only 0.001% of that total. How can such a minute contribution of human activity to atmospheric chlorine be responsible for the ozone hole? Weisman seems opposed to “the scourge of so-called progress defiling the rest of the planet.”

RON M. KAGAN

Los Angeles

The writer responds: Most chemists agree that natural chlorine emissions don’t reach the stratosphere. They combine with other chemicals to form smog, or dissolve and precipitate as rain. Even the Du Pont Co. no longer disputes evidence that only non-soluble, inert CFCs make it high enough in significant amounts to confront UV. Post-eruption measurements by aircraft trace a maximum of 1% of stratospheric chlorine to volcanoes.