The crack of an M16 shot rings out at 156 decibels. A jet engine at takeoff blasts about 140 decibels. Submarine engine rooms drone along at 120 decibels.
Given that 85 decibels is the threshold for preventing permanent hearing loss, military service is unquestionably hard on hearing.
But what if troops could take a daily pill to protect themselves from noise-related hearing loss?
A researcher from Southern Illinois University School of Medicine is looking into the prospect, testing a common antioxidant found in fermented dairy products on the firing range at the Army's Drill Sergeant Instructor Course at Fort Jackson, South Carolina.
Kathleen Campbell, an audiologist and SIU professor, has been studying the protective properties of D-methionine — an antioxidant found in cheeses and yogurt — for well over a decade, testing its effectiveness in preventing damage caused by excessive noise and other sources.
Noise-related hearing loss occurs when cells in the inner ear, which vibrate when exposed to sound, become damaged from overstimulation. The response to the noise causes cochlear cells to release free radicals, damaging electrons which can kill off the cells.
When those cells die, a person loses the ability to hear sounds of certain frequencies.
According to Campbell, D-methionine works by neutralizing the free radicals and stimulating the body to produce glutathione, a natural antioxidant that may prevent the cells from releasing free radicals in the first place or neutralize them shortly after noise exposure.
"You can give it after the noise exposure ceases and reverse the hearing back, preventing permanent damage," Campbell said.
For the study, soldiers drink a beverage containing D-methionine before, during and after they are on the shooting range, having fired 500 rounds over 11 days.
Campbell takes a baseline hearing test from the subjects and then tests them again two weeks later.
While no results from the Fort Jackson study are available yet, Campbell has tested the compound for safety and effectiveness in humans, on cancer patients taking chemotherapy drugs known to cause hearing loss.
She also has tested how well D-methionine works in animals.
In a study using chinchillas, which have a hearing range strikingly similar to humans, D-methionine reduced hearing loss in all the animals that received it with chinchillas in the test group losing less than 10 percent of their hair cells, compared with 40 percent of the control group.
Campbell is not the only researcher to examine the potential of using medications to reduce or prevent hearing loss. Since at least 2004, researchers at Navy Medical Center San Diego and a private company, Seattle-based Sound Pharmaceuticals, have tested two other substances — N-acetlycysteine and ebselen, both which also boost the body's own natural antioxidants.
In tests involving Marine Corps recruits taking NAC during training in 2004 and 2009, the formulation yielded less-than-promising results, reducing the number of people who experienced hearing loss by about 25 percent.
But officials with Sound Pharmaceuticals, which is testing the man-made compound ebselen, say their medication reduced temporary hearing loss following exposure in 60 percent among those who took it, compared with a 20 percent reduction in the control group. Full hearing also was restored in the ebselen group more quickly than those in the placebo group (1.3 hours versus 24 hours).
Sound Pharmaceuticals research involved 83 University of Florida students listening to their iPods for four hours at levels loud enough to inhibit their hearing temporarily.
"A lot of people pooh-poohed the study, saying it was artificial. But on Feb. 27, the World Health Organization announced that 1.1 billion teenagers and young adults are at risk for noise-induced hearing loss, primarily through personal audio players," Sound Pharmaceuticals Chief Medical Officer Dr. Jonathan Kil said.
For the Fort Jackson study, participants wear standard hearing protection in addition to taking the medication.
Researchers said they don't see their medication replacing these safeguards.
"This is an adjunct. This is not in place of wearing physical hearing protection," Campbell said. "Now, one of the big advantages of pharmaceutical hearing protection is it does not cut down on situational awareness. It gives you another level of protection without sacrificing your ability to hear your surroundings."
In addition to hearing loss, the medications may also prevent the onset of tinnutis, the researchers said.
About 10 million Americans have noise-related hearing loss, according to the Centers For Disease Control and Prevention and 22 million workers are exposed to potentially damaging noise each year.
More than 800,000 veterans receive compensation from the Veterans Affairs Department for hearing-related conditions, totaling more than $1 billion a year.
Finding a solution to reduce the impact of noise-related hearing loss would benefit troops, veterans and the government, but it also would be a boon to those working in any civilian occupation subject to loud noise, in airports and mines, factories, construction sites and more.
Kil said he can see recreational applications as well. Whether its attending a NASCAR race or a rock concert, riding a motorcycle or mowing the lawn, people are exposed daily to unhealthy noise levels.
"Reducing your exposure is still the easiest way to reduce the risk of developing noise-induced hearing loss," Kil said.
These medications are still in the development phase, with Campbell expecting to analyze preliminary results of the Fort Jackson study this fall.
She declined to provide a timeline for when D-methionine would hit the market if proved effective.
Kill said he expects his company to file a new drug application with the Food and Drug Administration sometime in the next three to four years.
Campbell said she must follow strict guidelines to ensure that D-methionine passes muster by the FDA if it works.
"My priority is finding something to take care of the troops. But certainly, if we could develop this for the population in general — we live in a noisy world — that would be good," she said.