WASHINGTON - Nearly a fifth of all Americans 12 years or older have hearing loss so severe that it may make communication difficult, according to a new study led by Johns Hopkins University researchers and published Monday in the Archives of Internal Medicine.

The findings, thought to be the first nationally representative estimate of hearing loss, suggest that many more people than previously thought are affected by this condition.

Study leader Frank Lin, an assistant professor, and his colleagues used data from the National Health and Nutritional Examination Surveys (NHANES), a research program that has periodically gathered health data from thousands of Americans since 1971. The researchers analyzed data from all participants age 12 and over whose hearing was tested during NHANES examinations from 2001 to 2008. Unlike previous estimates, NHANES includes men and women of all races and ages, from cities scattered across the country, so it's thought to statistically mimic the population of the Untied States.

Using the World Health Organization's definition for hearing loss (not being able to hear sounds of 25 decibels or less in the speech frequencies), the researchers found that overall, about 30 million Americans, or 12.7 percent of the population, had hearing loss in both ears. That number jumps to about 48 million, or 20.3 percent, for people who have hearing loss in at least one ear. These numbers far surpass previous estimates of 21 to 29 million.

Hearing loss prevalence nearly doubled with every age decade, with women and blacks being significantly less likely to have hearing loss at any age. Lin and his colleagues aren't sure why these groups appear to be protected. However, he notes that the female hormone estrogen, as well as the melanin pigment in darker skin, could have a protective effect on the inner ear.