The study defines "heavy cellphone use" as 15 hours or more every month for more than five years. At that level, the risk of glioma and meningioma tumors increased two to three times.
Oddly, unlike any previous study, this one showed the tumors appearing on the opposite side of the head from where participants typically held the phone.
Still, as always, this study isn't conclusive, and it found no cancer risk with moderate and light cellphone use. So, where does that leave us?
It comes down to radiation. For this general discussion, there are two main types of radiation: ionizing radiation and non-ionizing radiation. Ionizing radiation includes gamma rays, X-rays and some ultraviolet rays. These are known to damage your DNA, increasing the risk of cancer. When you get sunburned, that's thanks to ionizing ultraviolet radiation.
Non-ionizing radiation includes visible light and radio waves. Radio waves include microwaves, which is what cellphones and wireless routers use. Non-ionizing radiation doesn't damage your DNA or cells. That's a good thing, too, given how much exposure we have to things like visible light.
In extreme quantities, non-ionizing radiation can burn you - a high-intensity laser and a microwave oven are good examples. However, you don't get the same kind of damage as sunburn.