Air pollution, a new study suggests, may play a role in hearing loss.

Laurence Fechter, a researcher at Johns Hopkins School of Public Health, discovered that laboratory animals exposed to carbon monoxide and noise lose their hearing faster than animals exposed to noise alone. Carbon monoxide is present in cigarette smoke and automobile exhaust.

Between 22 million and 24 million Americans suffer hearing loss, according to the Deafness Research Foundation -- that's one in 10 Americans.

To determine the impact of both chemicals and loud noise, Fechter exposed rats to hours of carbon monoxide and random loud noise together, carbon monoxide alone or noise alone.

The animals exposed to both noise and carbon monoxide had the greatest hearing loss. Fechter speculated that carbon monoxide reduces the amount of oxygen reaching the ears' hair cells, special cells that convert sound waves into the electrical signals sent to the brain. The cells apparently asphyxiate because the carbon monoxide cuts off oxygen supply at a time when more energy is needed by the hair cells to deal with the increased noise.

Although the total number of hair cells was counted directly, the initial tests to measure hearing loss were done like this: The animal was given a soft warning tone followed by a blast of sharp, startling noise that caused the animal to jump up in the air.

Said the Hopkins' report: "A vigorous jump indicated a hearing loss because it means that the animal had not heard the soft warning note prior to the startling noise."