Mobile phones/Wifi only emit very small amounts of radio waves, thus producing very small amounts or RF radiation and electromagnetic fields. People are always bringing out these studies but many are inconclusive, not scientifically accurate or not goverment backed, which is why there is still no official report on the dangers.
Tbh i cant see how general wifi is at all harmful, unless youve got a transmitter strapped to your head or something, as what they are concerned with is the heating up of tissue caused by long term exposure to high frequencies. But even then wifi APs and NICs still only produce minute amounts compared to radio towers.
As raymond has said, it takes a very long time and a lot of money to carry out proper tests. But by the time they come to a conclusion, things have probably changed anyway.