The underlying premise for such scaremongering is that a considerable percentage of the radio-frequency (RF) energy emitted by a mobile phone is not only received by base stations, but is also absorbed by the brain of the person making the call. And, as we all know from our extensive experience with microwave ovens, RF absorption can cause high enough temperature rises for use in cooking.
I trust that most people will intuitively understand that the temperature rise induced by a mobile phone is much lower than that generated by a microwave oven - but exactly how much lower is not common knowledge. I think that public concern also increased dramatically after a CNN broadcast in January 1993 suggested a relationship between the use of mobile phones and the onset of brain cancer. In the week following that programme, the stock value of all major mobile-phone manufacturers on Wall Street plummeted by 15-20%.
What's safe, what's not?
Since the 1940s, there has been an increasing amount of research conducted on the interaction between RF radiation and biological tissues.1 Usually this interaction is divided into thermal effects (a rise in temperature) and nonthermal effects. The latter includes, for example, the complex interactions between living cells and ions (calcium, potassium and so on) and the behaviour of large molecules (proteins and DNA). The term nonthermal, while correct, is also somewhat misleading, since the field strengths required to inflict nonthermal damage are already high enough to induce damaging thermal effects.
Thus, the key parameter needed to establish safety limits for RF-emitting electronic devices is the temperature rise that they induce. This parameter can be arrived at via two-stage computer modelling, which involves computation of the absorbed RF power distribution (the specific absorption rate, or SAR, measured in watts per kilogram) and calculating the resulting temperature rise.
The first of these steps is well understood and can be modelled reliably using state-of-the-art models for solving Maxwell's equations.2 The second computation, however, is extremely complicated to perform for living tissue, owing to the effects of blood perfusion and blood vessels.3 In practical terms, since it's easier to measure RF field strengths, this second step is usually neglected.
The above factors have led to the current guidelines for RF-emitting devices being based on SAR levels, rather than on the relevant temperature parameter. SAR-based safety limits are defined both for whole-body absorption and for localized absorption (the maximum average absorbed power in 10 g of contiguous tissue).
Unfortunately, there's no way to compute the temperature rise from the SAR limit for localized absorption, so the limit has been set rather low, just to be on the safe side. The European Union is considering lowering this limit even further (Interactions May 2006 p2). In my opinion, though, there's a severe side effect that arises from the use of SAR-based safety levels.
Narrowing your options
The general public should easily relate to the effects of increased temperature. Everybody knows that when your body temperature increases, you start to sweat. It's also common knowledge that our bodies have learned to deal with it: an increased temperature is only temporary and as long as it stays below, say, 40 °C it will have no long-term effects. But when it comes to more abstract concepts, such as localized and whole-body SAR, it takes an expert to understand what's really going on.
Ultimately, setting limits based on SAR rather than on the relevant factor - temperature - results in the general public being at a loss as to what's safe and what's not. This emphasis on SAR may also have led to the belief that nonthermal effects might pose a bigger threat to our health than thermal ones, even though they only become an issue at levels where thermal effects will already be significant. Some people have suggested that nonthermal effects do play a role in the wellbeing of people who are sensitive to RF radiation, but in double-blinded studies no effect could be observed.4
One may wonder then, what is the problem with setting strict SAR limits? Surely it's better to be safe than sorry? In my opinion, such an attitude is far too simplistic. I think that setting unnecessarily strict and unrealistic limits on SAR values might increase concerns about non-ionizing radiation - especially if an abstract concept such as SAR is used.
Even more problematic is the fact that if the SAR limits are too strict then they can hamper good medical practice. Certain surgical procedures are best performed under guidance from MRI. Since MRI systems use RF radiation for imaging, both the surgeon and the patient will absorb some of this RF emission. Therefore, setting too strict a SAR limit could needlessly prohibit the use of this valuable medical application.
I fully agree that one should be cautious with unknown phenomena, and that in such cases strict limits are justified. But we have been exposed to naturally occurring RF fields since the dawn of time, and to man-made RF radiation since the 1940s. We know the physics and the biology of the interaction between RF radiation and living tissue: the only effect at the current safety levels is heating. Furthermore, we know that our bodies respond appropriately to temperature rises - our whole evolution is based on this fact.
In my opinion, there is no scientifically sound reason to impose more stringent SAR limits. In fact, it's quite the opposite: it is wrong to do so as it will unnecessarily hinder technological developments that can improve lives, through communication and leisure as well as advanced medical techniques. It is good practice to keep the amount of emitted RF energy as low as is reasonably achievable - but we must stay reasonable.
If we look at the numbers involved, the maximum localized SAR in the human head due to the use of a mobile phone is about 1 W/kg for 10 g of tissue - comfortably below the safety limit of 2 W/kg. This value does seem quite large compared with the power that a human body consumes, which is about 100 W in total for a person at rest (i.e. somewhat over 1 W/kg).
However, the real issue once again is the temperature rise, which in the brain is about 0.1 °C in 1 mm3 after 33 h of continuous phone use and less than 0.05 °C after 7 h. To put things into context: compare this value with the daily temperature fluctuation of an average person, which ranges from 35.8 °C during sleep to 37.8 °C during normal activity. I think that an additional 0.1 °C will do no harm at all. And the effect on the general public of radiation from base stations is even less.
We can conclude that, as long as we don't use our mobile phones while driving, there should really be no need to worry about brain damage.
• See also Small-scale radiation check for mobile phones on physicsweb and MRI homes in on RF hot-spots on medicalphysicsweb.
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