Children with common gene defects face a heightened risk from passive smoking, researchers have said.
Gene defects contributed to poor lung function, the study said
A team from the University of Dundee studied 600 children and young people with asthma in the Tayside area.
They checked airway peak flows, using a blowing test, and studied genes which help eliminate inhaled toxins.
Dr Somnath Mukhopadhyay, of the Tayside Children's Hospital, said some youngsters could face extra danger of long-standing lung function decline.
Chronic exposure to second-hand tobacco smoke causes some children to develop serious chest problems like asthma, while others do not appear to be particularly affected.
The research team studied genes that help the body produce an enzyme called glutathione-S-transferase (GST), which is particularly effective in detoxifying inhaled tobacco smoke within the lungs.
But the genes which produce GST are subject to two common defects, one of which is present in about 50% of the population, with the other occurring in 12%.
Children who have either of the two defects in the GST genes are more susceptible to asthma associated with environmental tobacco smoke exposure than those with more intact GST status.
Teenagers with asthma had 15% lower peak flows if they had one of the GST gene defects and were exposed to tobacco smoke, compared to asthmatic teenagers with intact gene status, the study said.
Dr Mukhopadhyay said: "There is a risk that these children and teenagers, naturally unaware of their susceptible status resulting from this gene defect, could be undergoing a silent, long-standing decline in lung function over the years.
"This can lead to a greater risk of chronic obstructive lung disease, or COPD, in later life."
The two defects in the GST genes defined a high-risk population of young Scottish asthmatics in whom tobacco smoke was particularly harmful.
At present there is no screening for the gene defects.