Passive smoking can cause genetic damage to sperm cells that may result in birth defects, miscarriages and other reproductive problems which make it difficult to father a healthy child, scientists have found.
A study involving laboratory mice discovered that sperm cells are vulnerable to DNA damage caused by sidestream tobacco smoke, which is composed of about 4,000 chemicals including about 60 known cancer-causing substances.
Researchers believe that similar DNA changes in boys or men exposed regularly to passive smoke could lead to reproductive problems such as infertility or a higher risk of fathering children with congenital defects.
Scientists led by Carole Yauk, of Health Canada in Ottawa, found that when mice were exposed to the sidestream smoke from a burning cigarette, they suffered a significant increase in the number of DNA mutations within the "germ cells" of the testes which are responsible for making sperm.
"Our data suggests that paternal exposure to second-hand smoke may have reproductive consequences that go beyond the passive smoker," the researchers write in the journal Proceedings of the National Academy of Sciences. They say their work on laboratory mice provides "compelling evidence" to support the argument that passive smoking should be regarded as a potential mutation-causing behaviour in human sperm cells.
"Consistent with data for first-hand smoke, male exposure to second-hand smoke before fertilisation is likely to have detrimental reproductive consequences that go beyond the passive smoker," they say.
A separate study of more than 1,500 American teenagers found that exposure to passive smoking in early childhood increased the risk of developing hearing problems that could impair a child's educational development.
It is the first time that passive smoking has been linked directly with hearing loss and will almost certainly be used to justify greater efforts to protect the estimated two million children in the UK who live in households where they are regularly exposed to the tobacco smoke of parents or relatives.
The scientists found that the degree of hearing loss was linked to the amount of nicotine breakdown products found in the bloodstream of the teenagers, indicating that passive smoking was actually causing the problem rather then merely being associated with it.
Professor Anil Lalwani, of New York University School of Medicine, said many children were exposed to second-hand smoke in the home, and discovering a connection with hearing loss in teenagers had huge health implications. "We need to evaluate how we deal with smoking in public places and at home, as well as how often and when we screen children for hearing loss," Professor Lalwani said.