22 May 2007

Mast debate

A recent BBC Panorama documentary has suggested that wifi Internet might be a major health hazard. Scary quotes about chromosome damage and radiation exposure have appeared all over the Internet.

Unfortunately, the documentary’s conclusions are junk science.

Let’s start off by noting the inverse square law, a piece of basic physics which applies to electromagnetic radiation exposure. Basically, the strength of a signal varies in proportion to the distance squared.

The people who put together the documentary measured the wi-fi signal at a distance of 1m, and the cell phone tower signal at a distance of 100m. From their measurements, they concluded that the wifi signal was “three times the highest level of the mast”.

Well, no, it wasn’t. Because the cellphone signal was measured 100x further away, it was attenuated by a factor of 100×100 = 10,000×. So an accurate quote would be that the wi-fi signal was “three times the level of the cell phone mast divided by 10,000”. Not as exciting, though, is it?

You might argue that it’s reasonable to measure at different distances because people don’t tend to sit close to cell phone masts, but they do tend to sit close to wi-fi equipment. However, think for a moment about how a cell phone works. Yes, the mast transmits a signal to your phone, which is 10,000× weaker by the time it gets to you. However, you don’t just listen to your phone; hence, it must also transmit your voice back to the network. And the same physics works the other way: the signal your phone transmits is 10,000× weaker by the time it gets back to the mast.

So as you might guess, the radiation your phone emits is much, much more powerful than the radiation that reaches you from any nearby mast. That’s the radiation levels the BBC program should have been measuring and comparing with wi-fi.

While raw power is measured in watts, the relevant measurement for assessing radiation danger levels is the Specific Absorption Rate or SAR, which is measured in watts per kilogram. An adult’s body has much more bulk to dissipate the electromagnetic field, hence it’s less susceptible than (say) a lab rat’s body.

The US limit on radiation from consumer mobile phones is 1.6W/kg. (That’s lower than the European limit, so we’ll take that as our guideline.) The limit for devices like wifi is a mere 0.08W/kg. (Figures are in FCC OET Bulletin 56.)

Those are the maximums. The actual SAR ratings of common mobile phones are well documented. A value of 0.9 is fairly normal, with few phones below 0.5. So already, it’s clear that the average mobile phone actually exposing you to 0.9W/kg is likely far more dangerous than the 0.08 W/kg theoretical maximum allowed for devices like wi-fi.

Let’s look at some actual figures for wi-fi output compared to phones. I haven’t managed to find SAR ratings for wi-fi (if you have any, let me know), so we’ll have to compare power output in both cases. Peak power output from a phone is around 2W, with the average being around 250mW, according to a handy page from the Australian Radiation Protection and Nuclear Safety Agency.

Coincidentally, 250mW is the absolute maximum power output you can get from my wi-fi router (a Linksys WRT54GS). But to get that, you have to hack the firmware. The default power output for the router is around 20mW, 100× less than the phone. Now add in the fact that mobile phones are held against your head, whereas your wifi antenna is likely at least 30cm away from you at all times, and invoke the inverse square law again. The end result is that the electromagnetic radiation you get from wifi is a tiny fraction of that which you get from mobile phones.

As a UK Health Protection Agency scientist puts it in The Times, “a year sitting in a classroom near a wireless network is roughly equivalent to 20 minutes on a mobile.”

Aha, you say—what if you don’t use the mobile phone much? I’m afraid you still get irradiated. As you travel around, the signal from the nearest cell will get weaker. The phone checks signal strength every 7 seconds. If it drops off too far, the phone sends out an “I’m here!” transmission in order to locate another cell.

Given that each cell covers a square km or two, or as little as a few blocks in cities, taking a quick drive across town can involve your phone transmitting dozens of times. So wi-fi is a pretty negligible concern compared to carrying a mobile phone, let alone using one.

Wi-fi and phones aren’t the only sources of electromagnetic radiation, though. Wi-fi operates at 2.4GHz, which just happens to be the same frequency as your microwave oven. In fact, you may have noticed that your wi-fi signal strength is lower if you’re cooking something in the microwave, especially if your laptop is in the kitchen.

You might wonder why wi-fi operates at the same frequency as microwave ovens. Well, microwave ovens operate at 2.4GHz because that’s the frequency that’s best for heating up water molecules. For the same reason, it’s a bad frequency for long distance telecommunications through damp air, so it hadn’t been grabbed for any major commercial purpose. Hence, it was declared as free unlicensed spectrum for local low-power radio. This lack of regulatory hurdles led to innovation such as cordless phones, wireless video surveillance systems, and (eventually) wi-fi.

This also means that the effect of microwave exposure at wi-fi frequencies is simply heat. It’s not like nuclear radiation, it doesn’t mutate your genetic material; it simply warms up your water molecules a bit. From a scientific perspective, people are having a hard time coming up with theories to explain why localized warming of the body might cause damage. (In fact, it’s reported anecdotally that sailors on night watch on deck during WW II would stand in front of the radar in order to keep warm. They got hundreds of times the electromagnetic radiation warming you could ever get from a phone, yet they apparently didn’t suffer major damage.)

But let’s head back to the kitchen. Microwave ovens are allowed to leak up to 5mW/cm² at 5cm distance. A leaky oven may expose you to 0.256W/kg, at the same 5cm distance, according to measurements of leaky microwave ovens from the Australian Radiation Protection Agency. So at typical watching-lunch-rotate distance, it’s about the same level of danger as the radiation from your wifi router. So if you’re worried about wi-fi, you should be worried about your microwave oven too.

But there’s a much larger source of microwave radiation in your life. It’s called the sun. Summer sunlight at ground level can be up to 100mW/cm² of electromagnetic radiation. So standing outside on a sunny day irradiates you with 20× the radiation of a leaky microwave or wifi router, and a good chunk of it is microwave frequency.

So if you’re worried about electromagnetic radiation, perhaps the rational thing to do is what us computer scientists do—stay inside and browse the Internet via wi-fi, but never emerge blinking into the daylight…

Meanwhile, there are a growing number of people who believe that they are sensitive to low levels of microwave radiation like that found in wi-fi and sunlight. They call the phenomenon “electrosensitivity”. They claim that wi-fi and mobile phones give them headaches, make them nauseous, and so on, after just a few minutes. So, what’s the evidence?

Well, so far there have been at least 7 separate scientific trials in which allegedly electrosensitive people were asked to tell researches whether a mobile phone signal was present. In proper double-blind trials, “electrosensitive” people were unable to detect a mobile phone signal even after 50 minutes of continuous exposure. (Update: Here’s a very recent one.)

And even if they could have detected the signal—which they couldn’t—that wouldn’t have proved that the signal was responsible for their reported symptoms.

So if you believe wi-fi or mobile phones are making you ill, please do see a doctor—specifically, a psychiatrist. You may have a psychosomatic illness, you may be schizophrenic and believe you are picking up radio waves with your teeth, but you are not being made sick by people’s wireless Internet. If you truly believe you can tell when a mobile phone or wi-fi system is transmitting, sign up for a research study and prove it. You’ll be the first.

The sad thing is, though, that there are crackpots in even the highest levels of government. So expect to see more scare stories about wi-fi in the next few years.

I’m old enough to remember that back in the 80s, the scare story was about overhead power lines. They were making us nauseous, giving us cancer and leukemia, causing headaches. Funny how that risk seemed to vanish.

© mathew 2017