"...it is possible to select the results of individual research studies in support of a variety of opinions [regarding exposure to RF energy]; which may range from no risk of health effects on the one hand, to a clear need to reduce current exposure limits on the other."
"...there is emerging evidence that long-term frequent use of cellphones may be associated with an increased risk of tumours on the side of the head where the cellphone is used. This is an active area of research and additional studies may confirm or refute this association."
"...Wi-Fi exposure are not only well within recommended limits, but are only a small fraction (less than 1%) of what is received during typical use of cellphones."
In other words, it isn't clear whether wi-fi is harmful, but cellphones, however harmful they may be, are almost certainly much worse.
[ Quotes from "Wireless Technology and Health Outcomes: Evidence and Review: Are there human health effects related to the use of wireless internet technology (Wi-Fi)?" from the Ontario Agency for Health Protection and Promotion ]