A four-year long study of mobile phone base stations has found their transmissions pose no risk to human health, the mobile networks who funded the study said today.

Mitsubishi Chemical Safety Institute arried out the study for Japan's three largest carriers: NTT DoCoMo, KDDI and Softbank Mobile. The study began in November 2002.

The test involved generating signals identical to those output by WCDMA (wideband code division multiple access) base stations. WCDMA is the most common 3G phone technology and is already in use in Japan and many other countries. The signals were up to ten times more powerful than the maximum limit allowed in Japan and even so they had no harmful effects on cell samples put in their way, the companies reported on Wednesday.

The safety of mobile phone base stations and phones themselves is a hot topic among many after several previous studies and surveys have reported potential risks from wireless phone technology. In the case of base stations those potentially most at risk are people who live near antenna towers.

A widely publicised Dutch study (PDF) carried out in 2003 by the Netherlands Organization for Applied Scientific Research (TNO) found people exposed to radio fields from 3G base stations experienced unpleasant feelings including nausea, tingling sensations and dizziness but the same didn't happen with radio waves from older GSM (Global System for Mobile Communications) base stations.

The research also indicated that people close to GSM or 3G base stations perform better in cognitive tests regarding memory, concentration, reaction time and hand-to-eye coordination.

The researchers at TNO expressed surprise at their own results and noted the need for further study, but the news prompted concern from some about base stations.

Another survey completed last year with support from the Swiss Research Foundation on Mobile Communication found no effects on wellbeing and cognitive performance as a result of exposure to radio signals from mobile phone base stations.

The World Health Organization says all reviews to radio wave exposure below recommended maximum levels have concluded there is no known health risk but the agency cautions that gaps still exist in knowledge regarding RF exposure and they need to be filled before a comprehensive assessment can be made.

Results of the experiments have been submitted and accepted by the Bioelectromagnetics Journal, a peer-reviewed scientific journal, and have been registered in the World Health Organization's database, said NTT DoCoMo.