Say someone found a potential association between carrying coins in your pocket and the risk of a particular type of tumor. It would set off a frenzy of activity among a group of people who were convinced that this association was real. They would lobby for a law requiring that warning signs be placed on change machines. The effect would snowball until some people would demand that the government stop minting coins.
Researchers at Wageningen University in the Netherlands have reported that radiation from WiFi networks is harmful to trees. Problems observed included growth variations, and bleeding and fissures in tree bark. The researchers exposed 20 ash trees to various sources of radiation for three months. The tress closest to the WiFi source exhibited a lead-like shine on the leaves. The researchers also found that WiFi radiation is harmful to growing corn.
Last night unfortunately I had to sleep part of the night near a wifi router, and I felt I was being fried… as if been cooked in a microwave oven (though I never experienced that). I could hardly sleep at all until I moved (into the garden). I felt like my ovaries were directly being damaged, and felt that cyst or tumour growth would start if under longer exposure.
The energy of electromagnetic radiation is determined by its frequency; ionizing radiation is high frequency, and therefore high energy, whereas non-ionizing radiation is low frequency, and therefore low energy. The NCI fact sheet Electromagnetic Fields and Cancer lists sources of radiofrequency radiation. More information about ionizing radiation can be found on the Radiation page.
Prenatal exposure to radiation from cell phones may increase the risk of ADHD and other behavior problems in children. According to a peer-reviewed Nov. 2008 study in the journal Epidemiology, exposure to cell phone radiation while in the womb "was associated with behavior difficulties such as emotional and hyperactivity problems around the age of school entry."  A Dec. 2010 study replicated those findings.  A peer-reviewed Mar. 15, 2012 study found that mice exposed to cell phone radiation in the womb "were hyperactive and had impaired memory" as adults. 
Joel Moskowitz (@berkeleyprc) of the University of California, Berkeley School of Public Health, US, says: “This is the largest technological experiment in the history of our species, with potential health risks we still know next to nothing about.” This view is shared by Denis Henshaw, professor of human radiation effects at Bristol University, UK, who said: “Vast numbers of people are using cell phones and this could be a time bomb of health problems.”