A new study does not show any increase in brain tumors associated with increased cell phone use. This is just the latest bit of evidence of a large literature showing some mixed results, and it is far from definitive – but it is a bit reassuring.

The concern is that radiation from cell phone use, over years, increases the risk of brain tumors. I have summarized this research previously. Essentially, the evidence fails to find any increased risk of brain tumors for cell phone use of less than 10 years. For exposure of more than 10 years the evidence is less clear – an increased risk has not been definitively ruled in or ruled out, and more research is needed.

A recent review of the literature of cell phone use an acoustic neuroma (one type of brain tumor) found that the literature is simply inadequate to answer the question, and made recommendations for the type of studies that should be done. A 2008 meta-analysis of studies (since my last review) of cell phone use and brain tumors found:

We found no overall increased risk of brain tumors among cellular phone users. The potential elevated risk of brain tumors after long-term cellular phone use awaits confirmation by future studies.

So it seems the state of the research has not fundamentally changed in the last two years – for long term exposure (>10 years) we still need more study. But the fact that the data is equivocal probably means that there is no large effect. Smaller effects are harder to rule out.

Part of the problem with nailing down this question is the nature of the question itself. There are various kinds of evidence that can be brought to bare. The first is pre-clinical or biological – testing the basic science of cell phones and their biological effects, on cells or on animals. This research has failed to find a specific mechanism by which cell phones might cause brain tumors. However, it has found measurable biological effects. The most we can say at this point is that this kind of research provides some plausibility to a possible connection between cell phones and tumors, but it cannot answer whether or not cell phones have actually increased tumor risk.

The most definitive type of research would be experimental studies – studies in which things are done to subjects in a controlled manner. In experimental studies variables can be controlled for, or averaged out with randomization, and exposure can be maximally tracked. However, this type of study will never be done. It is both impractical and unethical – we cannot expose people deliberately to a potentially harmful substance to see what happens. Also – it is simply not practical to randomize people to either use or not use cell phones.

So we are left with observational studies – studies in which we simply observe what is happening or what has happened. This is the bulk of the relevant research on this question – looking at cohorts of people to see if cell phone use can be correlated with risk of developing brain tumors. This is the research I summarized above.

Observational studies are never definitive. We have to slowly build a picture of risk by looking at data from multiple angles.  We essentially triangulate risk by looking at multiple correlations.

This latest study by itself is rather weak evidence, but put into context, it adds to the growing body of observational studies and helps us build a more complete picture. This study looks at the overall incidence of brain tumors in a Swedish cohort (retrospectively). If cell phones cause brain tumors, then brain tumors should be on the rise following increased use of cell phones in the population. They looked at cohorts from the 1970s through 2003. Cell phone use sharply increased in the mid 1990s. What they found was:

During this time, the incidence rate of cancers known as gliomas increased gradually by 0.5% per year among men and by 0.2% per year among women.

For cancers known as meningioma, the incidence rate increased by 0.8% among men and, after the early 1990’s, by 3.8% among women.

This more rapid change for women was driven, the researchers say, by the 60-79 year age group.

So while there was a slow increase, likely due to other epidemiological factors, such as an aging population, they did not find any change in this rate of increase following widespread use of cell phones.

However, this study just supports the existing status of the research. It strongly supports of lack of a short term risk of cell phones, but does not really address the long term risk – > 10 years. If we take 1995 as the time when cell phone use significantly increased, then we need data from after 2005 to assess the risk of greater than 10 years of exposure, and this data only goes to 2003. So obviously we need a follow up study with data to 2008 or later. Hopefully we will have this data sometime soon.

Given the uncertainty in the literature, what should a prudent person do? There is no simple answer to this, as each person must weigh the risks of a small possible increased risk of brain tumors from long term cell phone use against the convenience of cell phones. We accept such risks all the time. Many of us accept the risks of driving for the convenience it brings, and this risk is likely much greater than that of cell phones. But you can manage that risk a bit by wearing a seat belt, driving a safe car, not driving while impaired, not using a cell phone or other distracting device while driving, and not speeding.

For cell phones there is no comparable list of easy things to do that clearly reduce possible risk. You can minimize unnecessary cell phone use. It may be useful to alternate which ear you hold the cell phone to, to spread out exposure. If you are going to have a long conversation from your home or office, use a land line. Hands free devices may be helpful, or you can hold the cell phone in front of you on speaker like a Star Trek communicator. Radiation blockers or other devices, however, are probably not helpful – there is no way to shield your head from cell phone radiation without shielding the signal the phones need to operate – so don’t fall for any magical device claims. It may also be prudent to limit or delay cell phone use in children.

It will probably be 5 years or more before we have a strong consensus on the long term risks of cell phone use. There is no way around the fact that long term risks require long term studies, and therefore take time. So be patient and stay tuned.