Now that a giant city-wide Wi-Fi network is being launched, paranoid hypochondriacs are coming out of the woodwork to proclaim that these devices will have unpredictable health effects, for the same reasons that cellphones cause brain cancer.

Are these risks serious? Technically, nobody knows for sure. Studies of cellphone use haven’t found a definitive link between long-term use and any cancer, but cellphones have only been in widespread use for about a decade.

But here’s what we do know:

First, apologies for the technical nature of this explanation, but you’ll need to learn a little bit about electromagnetic radiation.

The spectrum 101

Everything that transmits a radio wave does so on a particular frequency, whether it’s over-the-air television signals, broadcast AM/FM radio, cellphones, walkie-talkies, cordless phones, satellite signals or remote controls. All the frequencies combined form the electromagnetic spectrum, which goes from low-frequency shortwave radio to microwave to infrared, visible and UV light and eventually X-rays and gamma rays. The latter categories are dangerous in high doses because the rays penetrate the skin and start messing around with your cells, but frequencies below visible light don’t have that problem because they are non-ionizing.

To prevent interference between two groups trying to use the same frequency (close enough that they can hear each other), governments allocate spectrum for specific purposes and assign licenses to companies to use specific frequencies or frequency bands.

Industry Canada, which regulates the allocation of spectrum, has a chart on its website (my favourite chart ever) that tells you what frequencies are allocated to what type of services.

As you can see from the chart, every bit of usable radio frequency, from 9 kilohertz to 275 gigahertz, is already assigned to one or even multiple purposes. This makes it difficult for new services like cellphone providers or cordless device makers to avoid interfering with existing services.

There are, however, areas of the spectrum where people can play around without as much restriction. These bands, called “ISM bands“, are allocated to uses where the purpose isn’t communication. Because of that, there’s no problem with interference. (However, devices using this band for communication have to accept interference from ISM devices.)

One of these bands is at 2.4 GHz, which is pretty ideal for high-bandwidth short-distance radio communication like cordless phones and home wireless networks. So companies started making low-power devices that uses this band to communicate over short distances. Because they are low-power and the signal doesn’t carry far, there isn’t much interference, and coordination isn’t necessary.

So what’s special about 2.4 GHz? It’s the frequency used by microwave ovens.

Microwave ovens work by bombarding food with electromagnetic radiation, which causes water molecules to vibrate and heat up. A metal cage prevents the radiation from escaping the microwave and heating up your face.

It’s all about power (and distance)

The difference between microwave ovens and 802.11 WiFi is power. Microwaves require a lot of it: between 500 and 2,000 Watts. But wireless routers use maybe 10 Watts less than 1W. And they’re not always-on in the same way microwaves are.

With the cellphone debate, the argument is that long-term exposure to a heat source can cause adverse effects. There’s no question that some EM radiation is absorbed by the brain when using a cellphone. And that causes a slight temperature increase. But then so does putting your head on a pillow.

Though they operate at about the same power levels, there’s a big difference between cellphones and WiFi transmitters: cellphones are a potential danger because the antenna is inches from your brain. Unless your face really is glued to your laptop, this isn’t the face for WiFi. The amount of energy absorbed decreases exponentially with the distance to the source of radiation, so even an inch or two makes a big difference.

Electromagnetic radiation has been with us for decades, and isn’t going away soon. There’s no conclusive evidence that non-ionizing radiation at low power has any adverse health effects, and until some shows up, there’s really no sense in worrying.

(Note: It should be noted that the network being created in Montreal is actually WiMAX and not WiFi, which operates on licensed frequencies near the ISM bands, but the potential effects are the same.)

UPDATE (Sept. 11): A response to the Gazette article points out that you’ll get more EM radiation from standing out in the Sun. Though, funny enough, aluminum foil hats would be pretty effective at blocking the radiation from getting into your head.