Japan's four mobile phone companies, responding to growing concerns about cellphone use on health, said yesterday they had agreed to team up to study the possible biological effects of exposure to radio waves.

Explosive growth in global mobile phone use has increased the public debate over possible health risks linked to the devices.

Last year an official at a World Health Organisation said a link between mobile phone use and cancer could not be dismissed without further research.

The firms -- market leader NTT DoCoMo, J-Phone, KDDI and Tu-ka Cellular Tokyo -- said that as a first step they would collaborate on a study to examine the effects of radio waves both at the cell and genetic levels.

The research, already being carried out by DoCoMo, is expected to take about four years to produce any conclusive results, a DoCoMo spokesman said.

He said actual experiments had been commissioned to Mitsubishi Chemical Safety Institute, a unit of Mitsubishi Chemical, to ensure objectivity. KDDI is Japan's second-largest mobile phone carrier, and third-ranked J-Phone is operated by Japan Telecom Holdings and its British parent Vodafone Group.

Tu-ka Cellular Tokyo, the smallest of the four, is a subsidiary of KDDI.