GREENBELT -- It lasts a second or two and makes up only a tiny fraction of the Earth's vast ocean of air, but during its fleeting life it gorges on pollutants.
This voracious scavenger is hydroxyl, a fragmentary molecule that is born when ultraviolet light collides with a mixture of water and ozone. It quickly vanishes after reacting with and destroying such noxious chemicals as gasoline fumes, smog-producing carbon monoxide or globe-warming methane.
When nature was the only source of airborne poisons, hydroxyl kept the air we breathe in balance. But for more than a century, modern industry and agriculture have been pouring more health-threatening ingredients into the atmospheric stew.
As a result, some scientists think, the beleaguered hydroxyl -- which consists of one atom each of oxygen and hydrogen -- may have too much on its plate, shortening its already brief life.
Some scientists think its concentration in the atmosphere may have been reduced by as much as 20 percent since 1850.
That could leave the fragile bubble of air surrounding the planet a bit more vulnerable to assault, like a patient whose disease-fighting immune system is out of whack.
"To the extent you have a decline in the hydroxyl radical, it would be an extremely important environmental issue," said John Hoffman, director of the U.S. Environmental Protection Agency's Stratosphere Protection Program.
"As pollutants increase, the net concentrations of OH [hydroxyl] decrease," warned a recent NASA publication. "The ability of the atmosphere to cleanse itself is reduced in what could become a destructive feedback mechanism, worsening global pollution and enhancing the greenhouse effect."
But the hydroxyl story isn't as simple as it first sounds. In fact, Anne Thompson, an atmospheric scientist at NASA's Goddard Space Flight Center, said that the saga of the chemical is a good example of why the atmosphere's mounting problems -- global warming, ozone depletion and smog -- cannot be attacked piecemeal.
"When I see how complicated and interconnected things are, I'm skeptical that any one action will solve all our problems or won't produce some unanticipated new ones," said the chemist, a pioneer in hydroxyl research.
Some hydroxyl-enhancing gases, she pointed out, contribute to acid rain, which scars forests and kills fish. Others contribute to the so-called greenhouse effect, the blanket of heat-trapping chemicals in the air that may be causing global temperatures to rise.
