Secondhand smoke may damage infants' hearing
Can secondhand smoke and air pollution contribute to hearing loss in infants?
It's possible, said UCLA researchers, who found that low levels of carbon monoxide found in the environment can cause permanent hearing damage in babies.
In three studies directed by John Edmond, a professor of biological chemistry at UCLA, investigators found rat pups exposed to mild levels of carbon monoxide at levels deemed safe by the U.S. government (between nine and 50 parts per million) suffered permanent hearing dysfunction.
Carbon monoxide is an odorless, colorless, poisonous gas that is a byproduct of combustion. In the environment, it is commonly produced by tobacco smoke and auto emissions. Carbon monoxide readily replaces the oxygen in blood, and in large enough amounts (1,000 parts per million) can be fatal.
In each study, rat pups were exposed to carbon monoxide levels, ranging between 12 and 100 parts per million. The rats were in the critical early weeks of life, when they experience a brain growth spurt, the beginning of nerve development and the rapid development of the auditory pathway -- the nerves responsible for hearing.
The researchers compared these rats with control rat pups that were raised in environments without added carbon monoxide. They performed behavioral tests and brain studies to find the differences between the groups of rodents.
"Everything we have done in these experiments has been at carbon monoxide levels that exist in the environment and have been considered safe," Edmond said.
In the first study, Janet Stockard-Sullivan, an audiologist from the University of South Florida, looked at cognitive and auditory function. Her team found that while carbon monoxide exposure of less than 50 parts per million did not alter learning, nerve development in the auditory pathway was damaged.
"In rat pups exposed to low levels of carbon monoxide, the response of the auditory nerve was reduced compared with that of the rats not exposed to carbon monoxide," Stockard-Sullivan said.
Given this finding, "Secondhand smoke might have an effect on the hearing of babies and more research is needed," she said.
In the second study, a team led by Douglas Webber of UCLA examined the brains of rats to detect changes that carbon monoxide might cause in cell development in the auditory pathway. The scientists found carbon monoxide exposure at concentrations up to 50 parts per million showed a marked decrease in the number of cells expressing c-Fos, a marker of neural development.
In the third study, Ivan Lopez, an assistant professor of surgery at UCLA, and his colleagues analyzed the sensory receptor cells and their neurons in the cochlea of rat pups. Damage to hearing was seen with exposure of as little as 25 parts per million, according to the report.
"We found some damage to the neurons in the inner ear of the rats exposed to carbon monoxide," Lopez said. "There was a loss of nerve fibers that send information to the brain."
Based on the findings, Lopez believes that infants exposed to low concentrations of carbon monoxide may suffer hearing damage. Exposure can result from secondhand smoke and other environmental pollution, he said.
Edmond notes that because carbon monoxide is found in the blood and can cross the blood-placenta barrier, damage to an infant's hearing may occur in the womb.
"Since people started smoking, driving cars and living in highly industrialized areas, there has been a constant low level of carbon monoxide in the air," Edmond said. These study results suggest these low levels of carbon monoxide are not safe, but might cause permanent damage, he said.
Susan Shore from the Kresge Hearing Research Institute at the University of Michigan, said that "all the studies sound plausible, and the findings certainly have applications to human auditory pathways."
Shore said she believes the same damage to hearing caused by carbon monoxide exposure in the rats can take place in humans. She said there are ways of testing her theory in humans that are minimally invasive and safe.