Smoking link to hearing problems
Teenagers who smoke, or whose mother smoked in pregnancy, are at higher risk of hearing problems and understanding what is being said, a US study says.
In tests on 67 teenagers, Yale University found those exposed to smoke had trouble focusing and interpreting sounds when there was a distraction.
And the team said scans showed exposure changed the brain's white matter, responsible for transmitting messages.
The findings were reported in New Scientist magazine.
The team carried out brain scans on the teenagers and found those exposed to smoke were more likely to have more white matter.
Previous research has shown that children with overdeveloped white matter have problems transmitting and interpreting sound because the white matter it is out of sync with the rest of the brain.
The researchers believe the over-production of the white matter is caused by nicotine stimulating a chemical compound called acetylcholine.
Further evidence was also provided by the computer tests the teenagers, aged 13 to 18, completed where they were asked to recognise words while being distracted by visual images or background noise.
Among the boys who were tested, those exposed to smoke got 77% right, whereas those not exposed got 85% right.
In girls, the breakdown was 84% to 90%. The researchers said the results were "quite significant".
Lead researcher Leslie Jacobsen said: "Individuals affected will have problems in settings where there is a distraction.
"This could certainly be the case in classrooms where there may be other people talking and lots of things going on.
"Coupled with other conditions, such as behavioural disorders, this may tip a pupil towards failing at school."
David McAlpine, director of the Ear Institute at University College London, agreed the findings were interesting.
He added: "The fact that smokers show changes in this pathway means they may be less able to hear what's being said."
Richard Todd, from Washington University, added the effect on the white matter was "pretty remarkable".
"It seems the brain remains vulnerable long into adolescence."