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This article warns of cell phone radiation on trains:

Train passengers exposed to cell phone electromagnetic radiation (Better Nutrition)

Passengers on packed trains and subways may be exposing themselves to electromagnetic fields far more intense than those recommended under international guidelines. The problem? Crowds of commuters using cellular phones at the same time.

When hundreds of mobile phones emit radiation, their total power is found to be comparable to that of a microwave oven or even a satellite broadcasting station, according to a recent study published in the February 2002 issue of the Journal of the Physical Society of Japan.

For the last few years I have noticed increased cell phone usage for texting, calling and 3G/EDGE/GPRS texting or browsing on trains.

When should I begin to worry? Should I move to another train car? Are underground trains (subways) with underground repeaters/cell towers more dangerous because of weaker signal strength (thus phones compensate with stronger radiation)?

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    Welcome to Skeptics.SE! According to the FAQ, Skeptics.SE is for researching the evidence behind the claims you hear or read. I have edited the question to highlight the notable claim, and reduced the focus from having people speculate or do custom calculations for your situation. – Oddthinking Jul 12 '12 at 11:11
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    Some helpful fodder for someone wanting to answer. The article is based on a PRELIMINARY CALCULATION in a LETTER (i.e. not peer reviewed) published in J. Phys. Soc. Jpn. 71, 432 (2002). Later, the authors responded to critics (that they cite - maybe there are more) with Passive Exposure to Mobile Phones: Enhancement of Intensity by Reflection, Journal of the Physical Society of Japan Vol. 75, No. 8, August, 2006, 084801 (2006) The Physical Society of Japan – Oddthinking Jul 12 '12 at 11:24
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    Thing is, that's not a one person using 200 phones simultaneously, but rather 200 people using their phones. So the absorption is distributed among them. Also a microwave oven has 0.02-0.03m³, while subway car has something like 150-200m³. And microwave ovens don't leak at all - 100% reflection, while it's obviously not true for train/subway cars. Overall this comparison with microwave oven is plain ridiculous. – vartec Jul 12 '12 at 13:19
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    "their total power is found to be comparable to that of a microwave oven or even a satellite broadcasting station" Oh, come on! It the absorbed power was comparable to a microwave oven---even in a point like was---people would be injured. If it was true over large volumes people would be cooked. So this line is pure scare story. Get them to tell you what they meant and then laugh at them. – dmckee Jul 12 '12 at 14:19
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    You would first have to identify a mechanism by which any damage was being done before you can worry about accumulation of that damage. Phone work in the microwave band, so the mechanism is heating not ionization. – dmckee Jul 15 '12 at 17:51
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+100

Simple calculations suggest this story is nonsense.

I'm not an expert in radiative physics but I can see from simple calculations that the story has strayed beyond the bounds of plausibility. The first, very simple, calculation is to look at the total power used if 200 mobile phones (a very full train carriage) were being used at the same time. I'm going to summarise from a couple of sources (the UK's Health Protection Agency has a good summary of the key issues and others are covered on www.antenna-theory.com). Point number 1 is that the worst case power transmission from a GSM phone is about 2W. So even here we can see that the total power emitted by 200 mobile phones is less than half that of a typical microwave (on the order of 1kW). Moreover, as @vartec points out in the comments, the train carriage has more than 10,000 times the volume of a microwave oven, so the specific effects on the contests will be a lot lower even if we assume all the energy is reflected back into the carriage (which is ridiculous not least because, if it were true, you wouldn't be able to use your phone in the carriage).

The second, more sophisticated, argument involves recognising that the peak power isn't the average power output. As the HPA site explains:

GSM mobile phones transmit their radio signals as 217 bursts of information every second. There is one burst every 4.6 ms (thousandth of a second) and each burst is 577 µs (millionths of a second) in duration. This means that, on average, they transmit for 1/8 of the time and their average output power is 8 times less than their peak output power.

Exposure guidelines, such as those published by ICNIRP, require exposures to be averaged over 6 minutes for comparison with their basic restrictions and it is more relevant to consider the average output power than the peak output power from phones. In this respect, GSM phones transmitting at 900 MHz and 1800 MHz have maximum time-averaged output powers of 0.25 W and 0.125 W respectively.

Another factor is also relevant here. GSM phones don't usually transmit at close to full power. Again the HPA summarise the reality well:

A key feature of mobile phone technology is that a mobile phone does not operate with a fixed output power level when a call is made. The maximum power output from a GSM mobile phone is around 2 W peak, but this can reduce in a sequence of 15 steps down to around 2 mW during calls, a power reduction factor of 1000.

So the typical power output is likely (crudely) 1,000 times lower than the reported peak (factor of 10 for time averaging factor of 100 for not always using maximum power) even ignoring the volume issue.

And then we have to consider where the power is absorbed. The antenna-theory site summarises part of the issue like this (not using quite the numbers or adjustments above):

The antenna is radiating, but less than half the power will be directed at your head - most radiates in all directions away. In addition, the antenna efficiency will be 50% for a good antenna that is held directly up against a head (the head actually detunes the antenna and makes it less efficient). Hence, of the 0.5 W of output power the phone transmits, there is a loss of at least 50% for the antenna efficiency, and at least 50% for the radiation that is not directed to your head. Hence, we can safely take 0.125 W (=0.5*0.5*0.5) as an upper bound for the power absorbed by your head.

This argument needs to be extended a little to cope with the 200-users-on-a-train scenario. But the key point it that the relevant thing for the amount of energy you will absorb is related to the distance from the transmitter and the cross sectional area of whatever is absorbing the radiation. If you are not holding the phone to your head, the possible absorption is much, much lower than the loss factor of 80% you get when you are.

So, again very crudely, if we start with a peak power output of 400W (200*2W) we should realistically factor in reductions of 10,000 (volume), 1,000 (average power) and perhaps 5 (cross section for absorption) to give a reduction of 50 million on the 400W initial estimate (which is half the typical Microwave's output). Or about 4 millionths of a typical microwave's intensity per unit volume. And the biggest contributor will be the phone held next to your head which isn't going to dump much more than 0.1W into you. If you are on a train outdoors where sunlight is shining through the window it might be worth putting this in context as antenna-theory does:

Is this a lot? Well, it is tough to say without comparison to something else. Let's take our good friend, sunlight on the Earth's surface. The power density of sunlight is roughly 1.35 kW/m^2 (killiWatts [sic] per square meter). Now, the bigger your head is, the more energy your head absorbs from the sunlight. Let's say you have a standard adult head, which we'll approximate with a circular cross section of radius 4". Your head would then be roughly 0.0324 square meters in cross section. As a result, the power absorbed by your head will be roughly 1.35*0.0324 = 0.0438 kW = 43.8 W.

You might also want to consider the fact that that the 40-odd watts of sunlight contains ionising UV which is a known carcinogen.

  • While I agree the answer conclusion is probably right, I think it is not good by current standards, as it is theoretical. Such answers are nowadays not only downvoted, but often removed.. – Suma Feb 22 '13 at 8:19
  • @Suma I don't think the answer is pure theory. The key points were based on pointing out how things actually work in practice in mobile telecoms to counter the theoretical view of the original question. We could do better if someone got an RF detector and walked around a crowded train filled with people on the phone. That hasn't, AFAIK, been published. Maybe we should call Mythbusters, it would be an easy if unspectacular experiment to do. – matt_black Feb 22 '13 at 9:04
  • This answer is good and correct, but a more comprehensive answer would need to include similar numbers for 3G, HSPA, and LTE communications. These are less passive than older generation technology and thus can emit significantly more power. Obviously still nowhere near enough to be even plausibly dangerous, but just considering 2G technologies is outdated now. – Phoshi Feb 23 '13 at 15:55
  • @Phoshi Modern phones still contain GSM RF capability and the newer technologies 3G and LTE tend to be more efficient and use less RF power. So I don't think the answer would be materially changed (though happy to revise if a source for 3G and LTE power transmission is pointed out). – matt_black Feb 24 '13 at 19:46
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    @matt_black Hm, that sounds plausible. In lieu of finding any actual hard numbers on power usage I suppose there's not much to do but mark it down as "There's probably a difference one way or the other, but it isn't orders of magnitude so it won't make a difference to the answer", then! – Phoshi Feb 24 '13 at 22:51

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