London, May 16 : Forcefully flashing your teeth at work is dangerous for health, scientists have warned.

According to German researchers, forcing staff to be happy and polite all the time is likely to make them sick.

They believe most at risk are flight attendants, salesmen, call centre operators, waiters and other workers in close contact with the public.

Professor Dieter Zapf, a researcher at Frankfurt University, Germany, said fake friendliness led to depression, stress and hit the immune system.

This can trigger more serious ailments such as high blood pressure and other cardiovascular problems.

"Every time a person is forced to repress his true feelings there are negative consequences," the Mirror quoted him, as saying.

In order to reach the conclusions, for over two years, 4,000 volunteers including shop workers and flight attendants, were put in an imaginary call centre and abused by clients.

Some could answer back, while others had to stay polite and friendly.

Those allowed to stand up to clients had a rapid heartbeat for a brief period, but those who had to remain friendly found their hearts were still racing long after the client had hung up.

Researchers decided "being friendly against one's will causes nothing but stress".

Prof Zapf said: "We are all able to rein in our emotions, but it becomes difficult to do this over a protracted period as cabin attendants are forced to on long-haul flights.

"These people need space away from the passengers where they can be on their own and let feelings run free.

"We have to get away from the 'customer is king' attitude and show more respect to those in service industries."