LOS ANGELES • Teenagers aren't necessarily tuning out adults; they simply might not be able to hear them.

The proportion of American teens with slight hearing loss has increased 30 percent in the past 15 years, and the number with mild or worse hearing loss has increased 70 percent, researchers said Tuesday.

One in every five teens now has at least slight hearing loss, which can affect learning, speech perception, social skills development and self-image; one in every 20 has a more severe loss.

The authors of the report in the Journal of the American Medical Association eliminated ear infections, gunshots and exposure to loud noises in the environment as causes for the hearing loss but could not identify a specific cause. A recent Australian study, however, found a 70 percent increased risk of hearing loss associated with the use of headphones in portable music, and many experts suspect they are the primary cause of hearing loss in teens.

"Personal stereos are the most important change in the culture in the last 15 to 20 years," said Dr. Tommie Robinson Jr., president of the American Speech-Language Hearing Association. "Everybody has their own little device now, and how many times have you passed somebody and could hear their music?"

Even the relatively low level of damage found in the study can create problems. "Just because a hearing loss is slight does not mean it is insignificant, particularly when it is in the high frequencies," said Dr. Alison Grimes, manager of the audiology clinic at Ronald Reagan-UCLA Medical Center.