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Researchers have found evidence that our bodies produce ozone gas -- a component of smog -- that acts like internal pollution and contributes to hardening of the arteries.

In the upper atmosphere, ozone makes life possible by shielding the Earth from the sun's harmful ultraviolet rays. Closer to the ground, it irritates and damages the lungs, causing respiratory problems.

Until a year ago, researchers had no idea that it was also produced by the human immune system as part of its defence strategy. Now the U.S. and Swiss scientists who made that initial discovery have found more evidence that ozone is produced by the body, and that the highly reactive molecule is implicated in atherosclerosis.

Hardening of the arteries is a complicated disease that involves a number of factors, including the buildup of fatty deposits known as plaque, inflammation caused by the immune system, and cellular damage. The new research suggests that ozone creates toxic substances in the body that exacerbate the damage.

A year ago, U.S. researcher Paul Wentworth and his colleagues made the discovery that our bodies produce ozone when antibodies kill bacteria. Their findings challenged conventional wisdom on two fronts. Ozone was not considered a part of human biology, and immunologists did not believe that antibodies actually killed bacteria. Antibodies have been described as similar to sentinels or military intelligence -- spying on the invaders and signalling the body to bring in the big guns to destroy them.

Dr. Wentworth and his colleagues found that antibodies have a secret weapon. They produce hydrogen peroxide, the stuff that foams when poured on a wound. It is lethal to bacteria because it pokes holes in their cell walls.

They also found evidence that antibodies release small amounts of ozone at the same time as they produce hydrogen peroxide. Everybody makes ozone, Dr. Wentworth said in an interview. But the researchers had no idea what it did.

They made the link between ozone and atherosclerosis by analyzing fatty plaque from diseased arteries. They found molecules that can only be produced when ozone breaks down cholesterol. They did not find those molecules in tissue samples from healthy volunteers.

Those molecules are toxic to white blood cells, smooth muscle cells and cells in the walls of arteries. These cells surround atherosclerotic plaque.

Dr. Wentworth's theory is that when cells in our immune system try to clear away the plaque in arteries, they create ozone, which then causes more inflammation and damage.

"Ozone is damaging, and it is really a problem that we are going to have to think about in the next few years," Dr. Wentworth said. "There may be a whole slew of molecules that ozone generates that we have never thought of before."

The researchers believe ozone also plays a role in other diseases involving inflammation, including lupus and rheumatoid arthritis.

Atherosclerosis leads to coronary artery disease, and is a major cause of heart attacks and strokes. The affected arteries become brittle and narrow, restricting blood flow. The plaque can break and this can lead to blood clots.

Dr. Wentworth, who works at the Scripps Research Institute in California, says the work could lead to a new test that could detect whether hardening of the arteries is taking place by looking for the toxic compounds produced by ozone.