Several oil wells were set on fire in southern Iraq this week, filling the air with smoke so thick satellite photos show a black smudge blowing east toward Iran.

Professor Jack Spengler at the Harvard University School of Public Health sent the first of several teams who studied the effects of the 1991 Kuwaiti oil fires. He said well fires burn extremely hot, which may actually help keep the pollution away from people. "There's a lot of buoyancy and the plume first rises well off the ground," he said. "So, ironically, there could be situations where you're getting nothing because it's well up over your head."

The plume can then disperse in the atmosphere. But if the smoke settles over populated areas it can cause problems. The oily smoke contains soot and other particles that irritate airways, making it hard to breathe. Mr. Spengler said the worst-affected would be those who already have health problems. "If someone's an asthmatic, this is certainly concentrations that would evoke asthmatic attacks. People that have, say, chronic diseases, say they have some coronary heart disease, say they have emphysema. Then they really need to get medicated, get oxygen, get out of the place," he said.

Dan Costa studies air pollution for the U.S. Environmental Protection Agency. He says besides the short-term effects, particles in the smoke may have long term effects as well. "They also contain components that, in theory at least, or in test systems, have been shown to cause cancer," he said.

Harvard University's Jack Spengler says there are more toxic chemicals found in burning crude oil smoke than in refined oil burned in cars and power plants. But it's currently unknown whether oil fire pollution can cause long term health problems. Another group at Harvard University is currently studying whether Kuwaitis suffered any extended health effects from the 1991 fires.

That year, Iraqi troops set more than 600 Kuwaiti oil wells on fire. It took firefighters more than eight months to put them all out. Mr. Spengler says the sooner the fires are put out, the less the risk to public health.