NEW YORK - Teens exposed to higher-than-normal levels of lead are more likely to have trouble hearing, suggests a new study that links the hearing problems to lead levels well below those considered "safe" by current standards.
But other heavy metals weren't clearly tied to hearing problems, researchers said. And with lead, the report found only a small proportion of adolescents had blood concentrations that might be linked to hearing loss.
"It looks like the levels in the blood of most kids are very low and people are avoiding (heavy metals)," said study author Dr. Josef Shargorodsky, from the Massachusetts Eye and Ear Infirmary and Brigham and Women's Hospital in Boston.
He said studies in animals had suggested that exposure to heavy metals, which also include arsenic, cadmium and mercury, could cause hearing loss.
So his team consulted a nationally representative survey of adolescents who underwent hearing assessments and blood and urine tests for those metals.
Hearing loss was defined as not being able to hear sounds 15 decibels or softer, about the level of a whisper.
The current report comes from data on more than 2,500 participants ages 12 to 19. About one in five of them showed evidence of hearing loss.
Youth with the most lead in their blood -- at least two micrograms per deciliter -- were also the most likely to have hearing loss. Thirty-one per cent of them didn't pass the hearing exams, compared to 17 per cent of those with less than one microgram of lead per deciliter.
About one in 20 of the teens tested had the highest amount of lead in their blood.
Study participants with the most cadmium in their urine also appeared to have an increased risk of hearing loss compared to those with the lowest amount. But Shargorodsky said there wasn't a clear pattern and that "it's hard to say if that's real or not."
Mercury in the blood and arsenic in the urine weren't linked to teens' risk of hearing problems.
But, Shargorodsky emphasized, "Just because we don't see an association with hearing loss, doesn't mean that they're safe."
The study doesn't prove that higher lead exposure caused hearing loss in the youths, in part because it was impossible for the researchers to tell whether the lead exposure or hearing problems came first.
But an epidemiology and environmental health researcher not involved in the new study said that lead in particular has also been linked to hearing problems in older adults, and that it may cross into the brain and interfere with the transmission and processing of sound.
Sung Kyun Park, from the University of Michigan School of Public Health in Ann Arbor, said that although the amount of lead in the environment has dropped over the past few decades, the metal still shows up in the paint on some old houses, in soil and occasionally in tap water.
He and Shargorodsky agreed that at blood levels considered acceptable in U.S. kids -- under 10 micrograms per deciliter, according to the Centers for Disease Control and Prevention -- they may still be at risk.
"The current standard level is too high to protect these children...and protect (them from) hearing loss," Park told Reuters Health.
The researchers wrote in Archives of Otolaryngology-Head and Neck Surgery this week that two micrograms per deciliter of blood has been proposed as a new acceptable limit for lead in kids.
"This definitely lends credence to lowering (the current) level," said Dr. Frank Lin, who studies hearing at the Johns Hopkins School of Medicine in Baltimore but wasn't involved in the new study. "The results are quite striking."
Researchers said that while hearing loss is common in older adults, it can be especially hard on kids and teens because it's likely to interfere with their development and relationships with peers.
Shargorodsky concluded that parents should keep in mind that even at levels of lead and other heavy metals that are considered safe, those substances should be avoided whenever possible.
But, based on the findings in this study, he noted, that's already happening.
Overall, he said, "The lead levels in kids are very low, and that's a very good thing."