Hi zivija and welcome to the forums. :flower:
Wi-fi uses very low intensity radio waves with wavelengths similar to those used in domestic microwaves.
The levels of radiation given off by wi-fi are however 100,000 times lower than that used in a domestic microwave.
In addition to that, human flesh can only be effectively heated by a wavelength that is tuned to it's absorption spectrum, and there are strict guidelines in place to ensure that such absorption peaks are avoided.
To give an idea of the sort of scale of what we're talking about here, it has been calculated that sitting in a wi-fi hotspot for a year exposes an individual to a radiation dose equivalent to a 20 minute phone call on a mobile phone.
So far there's been no proven effect for such very low intensity radiation but it is however a topic that seems to continually generate unconfirmed allegations.