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People who brush their teeth longer and harder than necessary are not only damaging their teeth, but also are not getting them any cleaner, a British research team has found.

Dentists have always known that overbrushing can erode both tooth enamel and gums, but have never had scientific data to determine how soft a touch you can have with a brush, while still getting the job done.

"We felt it was something we needed to establish," said Professor Peter Heasman, the research team's leader, from his office at the University of Newcastle upon Tyne in northeast England.

"We'd sort of assumed -- as most people do -- that two minutes is the right amount of time to brush, but nobody ever considers the pressure being put on."

Dentists in North America and Europe generally agree that people should brush between two and 2½ minutes to properly clean their teeth.

Prof. Heasman said his team found similar time results, but also determined that 150 grams of pressure -- or about the weight of an orange -- was the ideal amount for proper cleaning without beginning to do long-term damage.

"Abrasion due to toothbrushing is a recognized problem. You can either wear away the enamel on the teeth or wear away the root," Prof. Heasman said.

"[This study]is showing that you can brush pretty well, using fairly moderate force."

Tom Breneman, president of the Canadian Dental Association, said from his practice in Brandon, Man., that the study reinforces current dental advice. "It puts some science behind what we've been doing."

Dr. Breneman said people who take the recommended two minutes to 2½ minutes generally pace themselves better and don't press too hard.

"When you slow it down, I think your touch becomes a little bit lighter," he said. "I think it's the 30-second brushers that go in and give them a good scrub that put all the pressure on."

He also recommended people use a soft brush that will do the job, but is more gentle on teeth.

Prof. Heasman and his colleagues recruited 12 university students to clean their teeth with an electric toothbrush in front of a pressure gauge.

The gauge hooked into the brush -- provided by Philips Jordan, makers of electric toothbrushes and financial backers of the study. The gauge measured the pressure students were putting on their teeth.

Each student came in once a day over a four-day period and brushed using a different pressure. Four different testing sessions were conducted over four weeks. Students were not allowed to clean their teeth outside the lab during any of these sessions.

Prof. Heasman and his colleagues, who have done extensive research on toothbrushes in the past, measured the amount of plaque on each subject's teeth before and after each session.

Their results showed that 150 grams of pressure was the ideal level, beyond which students were not removing significantly more amounts of plaque.