The decibel level in the subway is so high that daily rides could result in permanent hearing loss, a scientific investigator has found.

As little as 30 minutes of regular exposure to the noise levels in the subway system has the potential to result in serious ear damage, concludes a study by Dr. Robyn Gershon from the Mailman School of Public Health at Columbia University.

Every day 4 million people ride the New York City subway, which makes it the fifth largest in the world, behind Moscow’s, Tokyo’s, Seoul’s and Mexico City’s.

The highest noise level measured in the New York system, 106 decibels, is in Union Square station. The Occupational Safety and Health Administration limits a person’s daily exposure to this level of noise to one hour per day in the workplace. (check your ears' stress)

Lisa Tai, 44, of Queens, has been riding the subway daily for 30 years. Tai said she spent three hours a day, five days a week, on a train and worried about her hearing.

One in every 10 stations in New York showed a noise level that went above 90 decibels, which is the equivalent of listening to a stereo system with the volume fully pumped up.

Human ears should not be exposed to sounds this loud for more than 1.5 minutes a day, says Dr. Gershon. (go to interview with Dr. Gershon)

The results came after two months of research at 57 of New York's 468 subway stations. (see NYC’s noisiest subway station)

Trains are only part of the reason for the high sound levels. Other noisemakers, like sirens, speakers and subway musicians, contribute to the din.

But jumping on to a train to avoid a strident violin player will not save the ears. The noise levels measured inside the subway cars are even higher than on the platforms. One in five measurements exceeded 100 decibels on its top peak, when, for example, brakes were squealing.

The MTA contends that the study has several problems. The main criticism is that the results show only the peak noise levels on the subway stations, which do not indicate the average over time.

"You cannot just take a snap shot and make conclusions," said Charles Burrus, senior director for the city’s Environment Protection and Industrial Hygiene department.

As soon as the weather allows it, Dr. Gershon will send another team of researchers to the subway platforms. This time the goal is to discover the impact of the sound on the individual subway rider.