The researchers looked at data taken between 1988 and 1994 for the National Health and Nutrition Examination Survey conducted by the National Centre for Health Statistics. The survey collects information about the health and diet of people in the United States. As part of the survey, people give blood and fill out detailed questionnaires. Some of the children took tests that show learning ability.

Yolton's team looked specifically for levels of cotinine, a substance produced when nicotine is broken down by the body. Cotinine, found in blood, urine, saliva and hair, is considered the best marker of environmental tobacco smoke exposure. They searched for children with very low levels of cotinine in their blood -- below 15 nanograms per millilitre of blood.

The researchers ended up with 4,399 children aged between 6 and 16 who had low levels of cotinine and who said they had not themselves smoked any cigarettes for the past five days.

The more cotinine that children had in their bodies, the lower, on average, their reading, math and reasoning scores, Yolton said. "Reading was the strongest effect we saw," Yolton said in an interview.

She said other studies have shown that children who live with someone who smokes less than a pack a day have less than 1 nanogram per millilitre of cotinine in their blood. But this is enough to affect learning. "We saw pretty solid changes in cognitive scores at 0.5," she said -- half that amount.

The researchers took into account the education levels and income of the parents of the children and still saw the effect, which Yolton said suggests it is indeed something in the smoke affecting them.

MANY CHEMICALS IN TOBACCO SMOKE
"It is hard to say what is doing it because there are so many chemicals in tobacco smoke," she said.

Yolton said 43 percent of U.S. children are exposed to environmental tobacco smoke in their homes, and 85 percent of children have detectable levels of cotinine in their blood. "We estimate that more than 13 million children in the United States are exposed to levels consistent with the adverse effects seen in this study," she told the meeting, an annual conference of the Paediatric Academic Societies.

It is difficult to protect children from tobacco smoke, she added. "There was a study that looked at parents who reported they only smoked outside, and there was a definite decrease in cotinine levels in their children but it was still there," she said. Yolton said her team was testing super-efficient air cleaners in the homes of smokers to see if they helped.