WiFi operates in the 2 to 5 GHz range - part of the microwave portion of the electromagnetic spectrum. This is in the same part of the spectrum where cell phones operate so I may refer to WiFi or cellphone electromagnetic radiation interchangeably. These are radio waves - no different than those used to broadcast television programs - except that they are higher in frequency. They aren't nearly as high a frequency as visible light - and no one worries about getting cancer from visible light (ultraviolet light, on the other hand, causes skin cancer - but this is the minimum energy necessary to cause ionizations that can cause breaks in strands of DNA - which is the mechanism by which cancer cells can be created). There is no credible evidence that non-ionizing radiation has any adverse health effects at all. There is no radiobiologic mechanism that could explain such an association - and absolutely no scientifically valid evidence that this has ever happened.
I have treated patients with cancer for over thirty years as a board certified radiation oncologist and I am familiar with every carcinogenic agent known to man - I'll tell you with absolute certainty that radio waves cannot harm you (unless perhaps you were in the path of a multi-megawatt microwave beam in which case they might cook you - but as far as I know, there is no likelihood that this danger even exists).
There has never been (and will never be) a randomized trial assessing the cause and effect relationship between radio frequency emissions and neoplastic disease. In order to have a randomized study, half of the randomly selected subjects would need to avoid cellphone use and that's not going to happen.
Humans have been exposed to man-made radio frequency radiation for over 100 years and we have always been exposed to microwave radiation from the Cosmos.
For example, the latency period for radiation induced malignancies is, on the average say 20 years, but epidemiologic studies of large groups of people (that only require a few thousand patients to reach stastistical significance) exposed to ionizing radiation start showing an increase above baseline by seven years. So conservatively, there should be at least a few excess cases of glioma, caused by cellular (or WiFi) electromagnetic radiation by now.
See this reference which looks at all the reported cases of gliomas caused by ionizing radiation (where we have a plausible explanation for cause and effect). Millions of people have received brain irradiation and only 73 cases of radiation induced gliomas have been reported.
Disclaimer: I found this article on the Internet. I do not have any rights about it!