The Finnish study found that those who had regularly used mobile phones for more than ten years were 40 percent more likely to develop brain tumors known as gliomas on the side where they hold their phones.

Gliomas are the most common types of brain tumors, which begin in glial cells. Glial cells are a type of cell that surrounds nerve cells and holds them in place. Glial cells also insulate nerve cells from each other, according to the National Institute of Cancer.

Previous studies are inconsistent in terms of the effect of mobile phone use on the risk of brain tumors.   Results vary with factors including, but not limited to the selection of subjects, duration of phone use, type of tumors, and consideration of other risk factors.

Studies that did not include subjects who used mobile phone for more than ten years often failed to find an association between use of mobile phone and elevated risk of brain tumor. And those studies that included subjects who used mobile phone for more than ten years were likely to find the association.

The current finding, published Jan. 17, 2007 online in the International Journal of Cancer, is believed to be the second study to clearly link an increased risk of specific types of brain tumors to long-term use of mobile phone.

In the current study, Anna Lahkola from the radiation and Nuclear Safety Authority in Finland and colleagues from other institutions compared 1,521 mobile phone users who had had gliomas with 3,301 controls without tumors.

The researchers found that the association between use of mobile phone and gliomas became significant only after the duration of use, years since first use, cumulative number of calls or cumulative hours of use were considered.

Among those who used a mobile phone for more than ten years, the risk for developing a glioma on the side of the head where they held their handset was nearly 40 percent higher than average users.

The finding meant that there is a possibility that long term use of mobile phones may raise risk of brain tumors although the researchers say a firm conclusion could be drawn only after further study is conducted.

A 2006 study led by researchers at the Swedish National Institute for Working Life suggested that the risk of malignant brain tumors linked to use of mobile phone could be much higher.

According to the study published in the International Archives of Occupational and Environmental Health, those who used mobile phone including cell phones and cordless telephones for more than 2,000 hours in their life time may be 240 percent more likely to have a malignant brain tumor or brain cancer on the side of the head the phone was used.

In the study, Lennart Hardell, Michael Carlberg and Kjell Hansson Mild analyzed pooled data from two case-control studies they did earlier of 905 cases of malignant brain tumors diagnosed between 1997 and 2003, and thousands of controls aged 20 to 80 years.

The researchers found that those who used analogue cellular phones for more than 2,000 hours in their lifetime were 590 percent more likely to develop malignant brain tumors compare with controls.  

For those who used digital cellular phones for more than 2000 hours, the risk was raised by 270 percent and for those who used cordless phones, the risk was increased by 130 percent compared to controls.

With all types of phones combined, the risk was raised by just 140 percent.   In other words, those who used any type pf mobile phone for more than 2,000 hours were 240 percent more likely to develop brain cancer.

The highest risk was associated with use of analogue phone. The association between phone use and risk of brain cancer was most significant in those who used mobile phones for 2,000 hours in their lifetime.

Other than the duration of phone use, latency would also make a difference.

For all malignant brain tumors with latency period of longer than 10 years and in the highest exposure group, cumulative use of analogue cellular telephones for more than 85 hours, the risk was raised by 200 percent.  

For digital phones, cumulative use of digital phones for more than 64 hours raised the risk by 180 percent compared to 230 percent for use of cordless telephones for more than 195 hours.

Considering the same side of the head exposed to radiation (ipsilateral exposure), the risk of high-grade astrocytoma, a type of glioma, was raised by 140 percent for analog phones, 130 percent for digital phones, and 100 percent for cordless telephones.

The risk was highest among those users who were younger than 20 years, according to the researchers.

Hardell et al. suggested that the reason some early studies failed to establish an association between elevated risk of brain cancer and use of mobile phone was because they did not consider long-term use.   They said some of previous studies on brain tumors from USA (Muscat et al. 2000; Inskip et al. 2001), Denmark (Johnansen et al. 2001), and Finland (Auvinen et al. 2002) did not report any case with more than ten-year latency period for use of cellular phones.

The risk became evident when studies included subjects who used mobile phone for more than ten years.   In a 2005 Swedish study by Lonn et al., an increased risk of glioma was raised by 60 percent and the risk of meningioma, a type of brain tumor, was increased by 30 percent for the same side exposure for more than ten years.  

But Lonn et al. also found on the other side of exposure, the risk of brain tumors was somehow decreased.   Hardell et al. suggested that there were some problems with the methodologies used in Lonn et al.’s study.  For instance, the number of cases was not the same as recorded in the Swedish Cancer Registry.

Contradictory results were derived from Lonn et al.'s study, part of the WHO Interphone study. Actually, all the results from the WHO Interphone study, according to Hardell et al., led to the same conclusion that use of mobile phone seemed to reduce the risk of glioma and high-grade glioma.

Hardell te al. suggested that the methodologies used by the Interphone study must have problems because microwave radiation is unlikely to protect against malignant brain tumors from a biological viewpoint.

However, the Interphone study did show a statistically significant increased risk for acoustic neuroma for more-than-ten-year exposure to a cellular phone on the same side. The risk was raised by 80 percent.

A scientist affiliated with foodconsumer.org said that results from those short term studies should not be extended to those who used mobile phones for more than ten years because brain tumor may not develop within ten years after exposure to cell phone radiation.  

The latency of brain cancer may explain why short term studies failed to find an association between an elevated risk of brain cancer and use of mobile phone.

Further readings:

Inskip PD et al. (2001) Cellular telephone use and brain tumors. New Engl. J. Med. 344: 79-86.

Jonansen C et al. (2001) Cellular telephone and cancer -a nationwide cohort study in Denmark. J. Natl. Cancer Inst. 93: 203-207.

Muscat JE et al. (2000) Handheld cellular telephone use and risk of brain cancer. JAMA 284:3001-3007.

Lonn S et al. (2004) Mobile Phone use and the risk of acoustic neuroma.   Epidemiology 15: 653-659.

Lonn S. et al. (2005) Long-term mobile phone use and brain tumor risk. Am J. Epidemiol. 161: 526-535

Auvinen A et al. (2002) brain tumors and salivary gland cancers among cellular telephone users. Epidemiology 13: 356-359

Christensen HC et al. (2005) Cellular telephone and risk for brain tumors. A population-based, incident case-control study. Neurology 64: 1189-1195