Passive smoking may pose a different risk to African American children than their white counterparts, US research suggests.
The research looked at 220 children in the US with asthma
The study, featured in the journal Chest, examined 220 children with asthma exposed to cigarette smoke. More than half were black.
The researchers measured levels of cotinine, a substance produced by the body as it metabolises nicotine.
The black children showed significantly higher levels than the white children.
Cotinine is an indicator of nicotine being metabolised, but it does not itself have a negative effect.
However, the study does suggest that race affects how a body responds to tobacco.
Lead researcher Dr Stephen Wilson, from Cincinnati Children's Medical Center, said: "It looks as though some genes may vary. By itself, this may not matter, but it could play a role when external factors, like tobacco smoke, come into play."
The next step was to communicate the results to parents to try to encourage lifestyle changes, he said.
All of the children examined in the US research had the symptoms of persistent asthma and were exposed to at least five cigarettes per day. They were aged between five and 12-years-old.
They were tested for cotinine three times over the course of one year, using serum and hair samples.
The level of smoke in the home of each participant was also regularly measured.
Results indicated that while the African-American children spent less time exposed to tobacco smoke than the white children, the cotinine levels in their hair samples were four times higher.
Dr Somnath Mukhopadhyay, a specialist in childhood asthma at Dundee University, welcomed the research.
"It would come as no surprise to me that there are genotypic differences between these children and their Caucasian counterparts," he said.
"But research on the effects of passive smoking have tended to focus on white children. Very little has been done comparing race."
Chest is the journal of the American College of Chest Physicians.