The media is working itself up into a feeding frenzy again. This week’s issue: the alleged dangers of WiFi radiation. First BBC Radio news, then Newsnight, now the Daily Telegraph have all had articles from quoting from people who believe that there might be health risks from WiFi. The BBC programmes at least balanced the sillier claims by noting that WiFi power output is far lower than that from mobile phones.
The Daily Telegraph, on the other hand, had this nonsense:
However it is believed that a classroom containing 20 laptops and two routers could combine and be equivalent to the emission from a mobile phone.
Notice the use of passive voice: “it is believed”. Who believes this?
It turns out that no-one does, although the Telegraph quotes from the “Powerwatch” web site. On this site, there is a calculation designed to show that a room full of 20 computers and an access point (or two) is approaching the power output of a mobile phone call.
But this is arrant nonsense. It is nonsense because of the silly assumptions being made about average distance from antennae. It is nonsense because they first make an assumption of low power output from the phone, and it is especially nonsense because it makes no difference whether you have one computer or fifty in a room. The maximum power output is the same - capped in the UK at 100mw (but usually less than this).
Let me explain: WiFi stations are receiving a radio signal when they receive data. But if two stations transmit a signal at the same time then what occurs is a collision - the signal is garbled and not received. The solution is that WiFi uses a collision avoidance scheme to try to ensure that packets are never sent out whilst another station is already transmitting. A couple of carrier sense mechanisms are used to do this, and the result is a very low collision rate. Essentially, if one station is talking, all the other stations are quiet. Click on the thumbnail image to see how this looks in practice. Each wireless station waits a random amount of time before transmitting, and if another station transmits first, the wireless station will quietly defer to it.
Therefore the maximum power output from a room of 20 computers remains capped at 100mw. What is more, no one station is transmitting all the time. For the majority of the time, even on a congested network, each station is actually transmitting nothing.
And that brings us to the next bit of nonsense - the assumption of the amount of traffic in transit. The powerwatch page assumes more than 100% usage for periods of two hours or more in its calculations. Note that if a single wireless station on an IEEE 802.11g network were downloading constantly (and that the downstream network could maintain the same throughput), then even assuming protocol overheads reduce the actual throughput to a mere 30 Mbps, over a course of two hours, that station would download some 26 Gigabytes of data.
To put this in perspective, fairly heavy Internet users download about this quantity of data in a month. Light users (people just browsing the web, using email and similar) would not get close to this figure.
26 Gigabytes is the equivalent of about 44 CD images, or several thousand podcasts. It is a guge quantity of data - not the kind of thing people will be downloading in one sitting - and certainly not regularly.
Ah, you say - but spread between 20 computers? What classroom environment is encouraging pupils to download over a gigabyte of data per computer? None!
So the assumption of 100% utilisation is nonsense.
Next problem: The assumption is made that people are - on average - a metre from the transmitting antenna.
This is just silly. The writers seem to know that magnetic fields decrease by the square of the distance from the antenna, but they assume that on average, people are a metre from the emitted radiation. Based on the fact this is a classroom, and 20 stations are variously transmitting, the fact that some of these transmitters will be many metres away only really makes this assumption if the pupils have swallowed the transmitter.
Admittedly, I wouldn’t put this past some children - but if they have done so, they probably have worse problems to deal with than the emitted signal from the station!
No, in fact the vast majority of the transmissions come from the access point. (Remember, we download much morethan we upload) this may not even be in the class, but if it is, it will probably be several metres from even the nearest child. The law of squares tells us that these signals will be far lower than the silly assumption made on the powerwatch page and repeated uncritically by the Daily Telegraph.
As one commentator on the Telegraph page says:
“Can there ever be a better example of why the current decline of Physics teaching in schools and universities is so worrying?”
Let’s give the last word to Mike Clark, senior spokesperson for the Health Protection Agency. He has run the figures rather more intelligently than the Powerwatch pressure group, and tells us that 1 year of exposure to WiFi radiation in a classroom is equivalent to 20 minutes on a mobile phone.
Maybe we could quibble and bring that down to a month or so. So what? The worry with mobile phones is that the radiation causes excitation of water molecules in the brain near where the phone is held to the head. This causes a slight warming effect which could theoretically be a cause for concern, although there is no proof of harmful effects.
WiFi radiation - if it causes any warming at all - is so slight as to be unmeasurable.
There is no risk here.
The BBC writes:
You may not have known it, but Moore’s law appears to have reached its limits. 
The latest version of Ubuntu Linux has now been released. Feisty Fawn can be downloaded from the Ubuntu Site. You can also download Edubuntu’s Feisty Fawn release. This version of Ubuntu is loaded up with open source educational software and has a clean looking but appealing interface to it. If you want to set up a system for the kids, this is the version to use.
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