The term "radiation" is often used to scare people. Let's get it straight. There's two factors - frequency and intensity. Frequency has a far larger effect on how damaging radiation is. WiFi and other radio communications use a very low frequency - far below visible light.
Radiation that actually causes issues, could potentially cause cancer, etc., is usually ionising radiation - they have a very high frequency and can cause mutations in DNA, possibly leading to cancer (more info on that process). The frequency required to be ionising? At least 1,000,000 GHz. That's literally a 500,000 times higher frequency than what WiFi transmits on, 2.4 GHz or 5 GHz. Non-ionising radiation, which WiFi falls under, does little more than transfer heat.
Did you know light is also EM radiation? Yup. In fact, light (~500,000 GHz on the near-infrared side, ~750,000 GHz near-ultraviolet) is much closer to ionising radiation than WiFi. Sunlight actually contains some ionising radiation (UVB, UVC - UVA can also cause DNA damage, but it's not in the same way). But you're not going to hide in your house for the rest of your life, are you?
Apart from frequency, there is intensity. Non-ionising radiation can also be damaging - but this really only applies to higher intensities. And ionising radiation is not always dangerous - our bodies can cope with lower intensities, which is why we don't all die in the sun (vampires are another matter...). WiFi has a transmit power usually far under 1 Watt (I've seen figures for 200 mW). And most of that energy never reaches you - by the inverse square law, you only get about
1/distance squared of that. In layman's terms - the energy spreads equally in all directions. 10 metres away? 1/100 * 200 mW = 2 mW. That's nothing.
Microwave ovens (which operate on a similar frequency as WiFi) transmit ~1000 Watts, and it's highly focused inside that metal box. Only maybe 1 W can be released through the shielding, and even that is considered perfectly safe. To put all this in perspective, sunlight (which is a higher frequency, and therefore more energetic) is about 1000 W per square metre when it hits the ground, half of which is visible light or higher.
You might also find some interesting sources and studies cited on a similar question on Skeptics.SE.