Carbon Monoxide May Affect Infant Hearing
Chronic Low Level Carbon Monoxide Exposure Impairs Hearing Development in Rats
Oct. 17, 2003 - Hundreds of Americans die each year and thousands are hospitalized due to carbon monoxide poisoning. In these cases, people are exposed to high levels of the toxic gas, but intriguing early research suggests that even very low levels of exposure may impair hearing development in human infants.
In a series of studies involving newborn rats, researchers found that prolonged, low-level exposure to carbon monoxide affected auditory development. The symptoms they saw in the developing rats were similar to those seen in children with a poorly understood hearing disorder known as auditory neuropathy.
Children with the disorder have normal hearing sensitivity, but they process sounds abnormally and cannot make sense of what they are hearing. They are often labeled as autistic because they fail to develop language skills, auditory researcher Janet E. Stockard-Sullivan, PhD, tells WebMD.
"We have no proof whatsoever that the rats in these studies had this disorder, but it is an interesting parallel that certainly deserves further study," she says. "These findings suggest the possibility that even levels of carbon monoxide exposure that are generally considered safe could pose a risk to babies and young children."
300 Die Each Year in U.S.
Carbon monoxide is lethal to humans at concentrations of around 1,000 parts per million (ppm) and above, but levels up to around 50 ppm are generally considered safe exposure levels for humans by the Health and Safety Executive. In the three studies, to be published in the December issue of the Journal of Neuroscience Research, rat pups were exposed to chronic and low levels of carbon monoxide between 12 and 100 ppm during a period of rapid hearing development.
They found that exposures well under 50 ppm caused measurable hearing loss and other auditory nerve developmental problems. In a separate study, they also confirmed damage to the hearing apparatus of the inner part of the ear that transmits signals to the brain with chronic and mild levels of carbon monoxide beginning at 25 ppm.
"It is too soon to tell whether these findings are significant to humans, but it is clear that we need to look more closely at carbon monoxide exposure," says investigator John Edmond, who led the research team.
Edmond says people die needlessly because they either don't know the risks or ignore them. According to figures from the U.S. Consumer Products Safety Commission, 300 fatalities occur each year from exposure to the invisible, odorless gas. The gas enters the lungs and interrupts the normal supply of oxygen in the blood, which can increase the risk of damage to the heart, brain, and other vital structures. The symptoms of carbon monoxide poisoning include headaches, dizziness, nausea, and convulsion. In people with underlying medial conditions such as heart disease it can cause chest pain. It can also cause flu-like symptoms that clear up after leaving the home. At very high concentrations it can be lethal.