(See the updated section "Brain Cancer Trends" at the end of this article)
. . . which is to say, they are not carcinogenic at all. The analogy between the cancer risk that results from hammers and cell phones is better than you might imagine.
I only bring this up because, once again, a prominent TV doctor who apparently has no understanding of physics is pushing the idea that "we just don't know if cell phones cause cancer" and that you should go out of your way to avoid the unknown risk.
Well, Dr. Oz, that's simply nonsense. You may also just not know that gravity will continue to work tomorrow, and choose strap yourself to the ground out of an abundance of caution. Save yourself the trouble and the expense of your gravity-failure-prevention straps because, physics!
Anyone concerned about cell phones and the affect on their health should take a look at the National Cancer Institute's fact sheet. The most important of the key points on the page is this: "Studies thus far have not shown a consistent link between cell phone use and cancers of the brain, nerves, or other tissues of the head or neck."
In case that's not good enough for you, I thought I would take a moment to explain the very basic physics behind the fact that cell phone emissions can't cause cancer.
First of all, cancer can result when damaged DNA causes a cell to run amok. So for radiation to cause cancer, it has to damage DNA. I could damage your DNA with radio frequency signals like those that come from a cell phone, or like the ones that cook meat in a microwave. But in order to affect the DNA with that type of radiation, it would need to heat your cells to the point that the entire cell, and a whole bunch of nearby cells, would be severely damaged as well. That is, the radiation would have to cook your head to damage the DNA, which would destroy the cells entirely but wouldn't make them cancerous.
The reason for that is the wavelength of cell phone emissions is much larger than your cells, so the energy is deposited in a volume much larger than a cell. To damage DNA (or otherwise affect parts of the cell without totally destroying it), the wavelength must be about the size of DNA or smaller.
That's why there may be reason to worry about the potential cancer risk associated with nanoparticles. DNA in your cells is clumped in a blob about 10 nanometers across. Conceivably a nanoparticle of silver, for example, could work its way into a cell and damage the DNA, or other sub-cellular bits, without disturbing the rest of the cell. That is a recipe for cancer.
If I take the same silver and make a full sized hammer out of it, it could still damage the DNA in a cell, but it would have to smash through the rest of the cell to do it. The damaged DNA can't control a cancerous cell if the cell has been smashed to bits.
The same holds for electromagnetic radiation. In order to hit only the DNA in a cell without destroying the rest of the cell, the radiation has to have a wavelength of about the size of DNA or smaller. Because a glob of DNA in a human cell is about 10 nanometers in size, radiation must have a wavelength of 10 nanometers or less to affect it without disturbing anything else in the cell. What frequency of radiation is that?
In order to figure that out, you have to use the equation f*l=c, where f is frequency, l is wavelength and c is the speed of light.
You can rearrange the equation to solve for lowest frequency of radiation that can cause cancer
f=c/l= (300,000,000 meters/second)/.000000001=300,000,000,000,000,000 hertz = 3x10^17 hertz
That's the frequency of ultraviolet (UV) light. And as we know, you wear sunscreen because UV is carcinogenic. Visible light has frequencies that start a hundred times lower than UV, which means the wavelengths are about the size of a cell or larger. In order to damage DNA with visible light, you'd have to cook the whole cell, eliminating the possibility of a cancerous cell forming.
Microwaves ovens and cell phones put out signals that are a billion times lower in frequency than UV, and a billion times longer in wavelength - a billion too large to damage DNA without destroying the whole cell.
All a microwave or cell phone can do is heat a large group of cells. But long before your cells begin to cook, your heat sensing nerve cells would make you run from a microwave source that's intense enough to hurt you. In the same way, if someone were to tap you gently on the side of the head with a silver hammer and gradually increase the intensity of the blows, the pain would become unbearable long before you suffered much damage. And even then, you couldn't get cancer from being tapped on the head with a silver.
So there you have it cell phone radiation and hammers can't cause cancer because they're too big. UV and higher frequencies (i.e., shorter wavelengths like x-ray, gamma rays,etc.) are carcinogenic because they have wavelengths as small as DNA or smaller. I don't know if nanoparticles, like the nanoscopic silver in some sunscreens are carcinogenic, but I wouldn't be surprised if it turns out that they are.
The bottom line - and I say this with complete confidence - is that physics proves that cell phone emissions and silver hammers don't cause cancer for essentially the very same reasons. UV tanning beds and silver nanoparticles could both be dangerous, again for the very same reasons.
If you still want to waste time and money exercising an "abundance of caution" using cell phones, just in case, it's up to you. Or you could trust in science and stop worrying for nothing.
*Brain Cancer Trends (Update added August 13)
Many people who are convinced that cell phones are carcinogenic will not be swayed by the lack of a known physical mechanism for damaging DNA. For them I point out that while cell phone use has skyrocketed in the US since the phones were introduced in the 1980's, the incidence of most cancers has fallen or remained steady. In particular, here's a graph of brain cancer cases and deaths in the US since 1995.
I know some people are eager to assert the dangers of cell phones. They are going to have to find something besides cancer to shout about, because there is no correlation between cancer and cell phones despite our thirty-year-long experiment in cell phone adoption.