If there’s one topic likely to generate spit-flecked ire, it is the controversy over the potential health threat posed by cell phone signals.
That debate is likely to flare following the publication today of some new ideas on this topic from Bill Bruno, a theoretical biologist at Los Alamos National Laboratory in New Mexico.
The big question is whether signals from cell phones or cell phone towers can damage biological tissue.
On the one hand, there is a substantial body of evidence in which cell phone signals have supposedly influenced human health and behaviour. The list of symptoms includes depression, sleep loss, changes in brain metabolism, headaches and so on.
On the other hand, there is a substantial body of epidemiological evidence that finds no connection between adverse health effects and cell phone exposure.
What’s more, physicists point out that the radiation emitted by cell phones cannot damage biological tissue because microwave photons do not have enough energy to break chemical bonds.
The absence of a mechanism that can do damage means that microwave photons must be safe, they say.
That’s been a powerful argument. Until now.
Today, Bruno points out that there is another way in which photons could damage biological tissue, which has not yet been accounted for.
He argues that the traditional argument only applies when the number of photons is less than one in a volume of space equivalent to a cubic wavelength.
When the density of photons is higher than this, other effects can come into play because photons can interfere constructively. Bruno points to the well known example of optical tweezers in which coherent photons combine to push, pull and rotate small objects such as cells.
In this case, the force is generated when dielectric objects sit in an electric field gradient associated with the photons. More photons generate more force.
The damage that optical tweezers can do to structures in cells is well reported, he says. That’s because of the large change in refractive index at the edge of cellular structures such as vesicles, myelin sheaths and so on, and the high density of photons.
Of course, optical tweezers generally work at infrared frequencies. The question that Bruno poses is whether a similar effect could also work for microwave photons.
This boils down to two factors. The first is whether there is a high enough density of microwave photons from cellphones to generate a force capable of damaging biological tissues. The second is whether there are structures in the body with the required dielectric properties to be susceptible.
On both counts, Bruno says there are reasons to be cautious. First, the density of microwave photons from cell phones and cell phone towers is many orders of magnitude higher than 1 per cubic wavelength. For this reason alone, Bruno says the traditional safety arguments do not apply.
Second, the human body contains many structures including neurons up to a meter or so long that could be susceptible to the combined effect of many photons. Some of these structures may actually focus microwave photons, increasing the photon density inside the body.
(If you’re wondering why the concern is over cellphones and not other transmissions, it turns out that frequencies above 10 GHz tend to be absorbed by the skin while frequencies lower than 1 GHz–TV or radio transmissions say–are thought to be reflected without much energy transfer.)
So what might be a safe level of exposure? Bruno suggests that the night time background rate of microwaves might be a reasonable limit. “Unfortunately, this level is very low by cellphone-technology standards, some 8 to 9 orders of magnitude lower than common cell tower exposures,” he says.
If that is unachievable, then another choice might be about an exposure equivalent to the average thermal energy per cubic wavelength. Bruno says this would be equivalent to an exposure of about 30 picoWatts per square metre at 1 GHz. “This equates to exposure from a cell tower at a distance of a few miles,” he says.
Either way, that’s likely to generate some concern.
Bruno’s conclusion is that the the way safe dosage limits is determined is broken because it does not take this new tweezer-like mechanism into account.
That places the ball firmly into the physicists court. It may be that there are good reasons why Bruno’s tweezer mechanism does not represent a threat. If so, we can expect physicists to post a robust defence of the cell phone exposure limits.
But Bruno will need to be braced for other, mindless kinds of responses too.
Either way, sit back and watch an interesting and important debate unfold.
Ref: arxiv.org/abs/1104.5008: What Does Photon Energy Tell Us About Cell Phone Safety?
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