WiFi uses radio waves for communications, similar to a 2-way radio, but using different frequencies etc. Many question how bad it is for your health. Is it as bad as cell phones? How does it rate against junk food?....
WiFi radiation is well below any health regulations as set out internationally by governments. A lot can be found on the web. A good independent description can be found on Wikipedia at http://en.wikipedia.org/wiki/Wireless_electronic_devices_and_health
Basically all radiation can be bad, including the radiation from your 2 way radios which push out 20 to 25 watts. The "safety" comes in the power levels used.
Cell phones use 4 to 5 watts of power. The danger with cell phones is that they are normally pressed up against your ear, emitting radiation just centimetres from your brain. Cell phone towers push out 25 watts per channel on a tower, and there are normally many channels per tower. Tests have been made with towers, but they have been "inconclusive". But there is enough "proof" that they do have an adverse effect on health for those living in close proximity to the tower. (There is actually less radiation "under" the tower than say 500m away, as the antennas concentrate the signal horizontally in a flat band).
The power of WiFi is limited to 100 mili watt, ie 0.1 watt. Now this is 40 to 50 times less than a cell phone! And it is not being transmitted next to your head. Due to the very low power levels used, high gain antennas have to be employed to be able to connect at distance. To achieve the gain, the signal has to be concentrated in a certain direction. For example, grid antennas on repeaters which receive the signal, concentrate the signal in a narrow 8 degree by 8 degree segment. Other antennas which send the data can have a wide spread of 90 degrees, but the vertical spread is only 7 degrees.
Also important when considering WiFi radiation is that the equipment is normally far away from your person. The CPE (Customer Premises Equipment - the "radio receiver") is normally located on the outside of your house. The closest you will normally get to a WiFi source will be your wireless laptop. This will normally be at arm's length from your head. Also note that the radiation decreases as a square of the distance, ie the signal at eg 2m distance from your laptop or repeater is a quarter as strong as the signal at 1m distance away.
The worst WiFi radiation will be experienced in rooms containing multiple laptops connecting wirelessly, like in WiFi hotspots in town, or in a school classroom. The Health Protection Agency has done some tests and they claim that if a person spends one year in a WiFi hotspot, they will receive the same dose of radio waves as if they have made a 20 minute call on a cell phone.
Is WiFi 100% safe? Probably not, but it is a lot safer than most radiation out there, and safer than the junk food which is eaten. One study shows that the leakage radiation from a Microwave oven, which is declared as being safe and which is accepted as being safe by most, is a couple of magnitudes stronger than a WiFi signal. There is a lot of mis-information out there. If you search the net, you will find "proof" about just anything claiming dangers. What is interesting is that the radiation from our Sun is far more dangerous than radiation from WiFi, and that should put it in perspective.