Radiation: Do Cell Phones Present a Cancer Risk?

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1. Introduction to Mobile Phones and Radiation

2. Non-Ionizing Radiation

3. Physics: The Energy of a Photon

4. Electromagnetic Effects: Heating

5. The FCC and SAR


Introduction to Mobile Phones and Radiation

When you talk on your cell phone, the phone must be communicated wirelessly with some base station that is [typically] hundreds of meters away. The phone communicates by transmitting high-frequency electromagnetic radiation. Further, since the antennas on a mobile phone are not directive, the energy is transmitted in all directions; the radiation pattern of a mobile phone is roughly omni-directional.

So, the previous paragraph states that the mobile phone can't control in what direction the transmitted energy will radiate. Since the cell phone does not know where the cell tower is, this is a good thing from a communication viewpoint. The energy goes everywhere, and so some of the energy will get to the cell tower, and the phone can communicate.

But - if the energy goes everywhere, and the phone is up against your head, your phone must also be transmitting radiation directly into your head! This most certainly is a fact. Now, the big question: should we panic or is this a non-issue? Let's examine the facts.

Non-Ionizing Radiation

The mobile phone in your pocket produces non-ionizing radiation. Sounds scary, doesn't it? Let's learn what "radiation" and "non-ionizing" actually mean before we jump off the boat.

First: radiation can be loosely defined as the existance of an electromagnetic wave. Electromagnetic waves are simply electric or magnetic fields, and these are produced from electric voltage and current. You may notice that the computer you are on is plugged into the wall, and hence, current flows on that wire. And yes-this produces radiation, although it is a very small amount. You may also know that the batteries in your remote control or on your desk have a voltage difference between the two ends: this produces an electric field, which can be absorbed by your body (radiation!). Finally, how does the sun heat the Earth? You guessed it: the heat is carried by electromagnetic waves! That is, the radiation from the sun enables life on the Earth.

Hence, let's not freak out when we hear the word "radiation". Radiation doesn't always have to deal with nuclear bombs, although much of the public associates radiation with nuclear energy and fears the word. To be more aware though, you must recognize that radiation is all around us: all the AM radio stations, FM radio stations, tv signals, gps signals, wifi, (anything energy/information that is possible to receive wirelessly) is around you all the time. And all of this radiation has been around you your entire life.

Ok, so some radiation is bad (nuclear bombs) and some radiation is relatively harmless. What distinguishes the two types? That's where we get the ionizing vs. non-ionizing radiation. Ionizing radiation means that the electromagnetic waves have enough energy to ionize particles. This means that the fields are powerful enough that they can rip the electrons off the atoms of a material - producing ions. This is the type of radiation that can damage your DNA, lead to cancer or radiation poison, etc.

So non-ionizing radiation then, is radiation that does not have enough energy to ionize particles. This means that the fields around you from non-ionizing radiation sources don't have enough energy to knock the electrons off the atoms of your body. You can still absorb energy from non-ionizing radiation, in the form of heat (if you couldn't, the sun couldn't heat the Earth!). Examples of non-ionizing radiation include cell phones, sun light, AM/FM radio, microwave ovens and tv stations.

Physics: The Energy of a Photon

Some science: the world of quantum physics shows us experiementally that electromagnetic waves behave like particles, and that particles can behave as waves. It is a messed up world. The energy (E) of a single photon (a photon is a particle that makes up an electromagnetic wave) is given by:

Energy of photon is plank's constant times frequency
[Equation 1]

In Equation [1], h is Plank's constant, is equal to 6.626 * 10^-34 Joules-seconds. The parameter f is the frequency of the radiation. Equation [1] shows us that the energy of a photon depends on the frequency of the wave. The frequency that cell phones operate at is between 700 MHz and 2100 MHz (MHz = 1 Million cycles per second). This sounds high, but note that the frequency of visible light is between 400 and 790 THz (THz = 1 Trillion cycles per second). Hence, a single photon emitted from you cell phone has a million times less energy than the light you can see from your TV, or the sun, or your monitor.

Ionizing radiation occurs when the frequency is approximately 3000 THz. At this frequency, electromagnetic energy is "UV light" - the very reason you have to wear sunblock if you go to the beach too often. The really bad stuff: x-rays, gamma rays, alpha rays, etc occur at frequencies of approximately 10^18 Hz (1 million THz), or 1 billion times more energy per photon than arises in your cell phone.

Interestingly, the study of all this stuff was mathematically characterized by Einstein in 1905, which yielded him his Nobel Prize in Physics in 1921! Basically, you cannot ionize atoms by increasing the intensity (power) of a low-frequency wave. This limits the damage that can be caused by non-ionizing waves.

Electromagnetic Effects:  Heating

So, if non-ionizing electromagnetic radiation does not rip apart molecules, how does it affect things? The most direct effect: heat. Your microwave heats food by using non-ionizing radiation at roughly 2.45 GHz (the same as your WIFI signals). Why does this work?

Water molecules (and also those of fats) have an intrinsic dipole moment. This just means that molecules like water have an intrinsic positive side and negative side, known as an "electric dipole". If you remember some of your chemistry, the oxygen molecule "borrows" an electron from two hydrogen atoms to complete its valence ring. The hydrogen atoms are then slightly positive, while the oxygen (with the extra negatively-charged electrons) is slightly negative. The result is that a water molecule is electrically equivalent to a dipole, which is a positive charge separated from a negative charge:

electric dipole describing heating via electromagnetic radiaton

Figure 1. A Water Molecule is Electrically Similar to a Positive and Negative Charge Separated by a Small Distance.

Now, the electric fields associated with non-ionizing electromagnetic radiation oscillate back and forth in time. They exhibit a force on these molecules, which can flip them front and back. Since heat (or temperature) is just a measure of how fast molecules within a substance are moving, the energy in the electromagnetic radiation is converted into heating of the water molecules. Hence, by applying non-ionizing radiation to a water-based substance, we can heat it up. And that is how a microwave oven works.

The next question we should ask in determining if cell phones are safe or dangerous is: how much do our phones heat up our brains? Are we being cooked like Hot Pockets in a microwave when we talk on the phone? Let's look at the numbers by comparing the cell phone to sunlight.

A cell phone has a maximum output power of about 1 Watt [W]. In actual operation, the peak output power is less than 0.5 Watts. [Technicality: Some people will quote 1-2 W for peak output power. However, this is for signals that are not transmitted more than they are transmitting. Taking the average over a brief period, say 250 milliseconds, the peak output power is less than 0.5 W.] Now, suppose this antenna is right up against your head. The antenna is radiating, but less than half the power will be directed at your head - most radiates in all directions away. In addition, the antenna efficiency will be 50% for a good antenna that is held directly up against a head (the head actually detunes the antenna and makes it less efficient). Hence, of the 0.5 W of output power the phone transmits, there is a loss of at least 50% for the antenna efficiency, and at least 50% for the radiation that is not directed to your head. Hence, we can safely take 0.125 W (=0.5*0.5*0.5) as an upper bound for the power absorbed by your head.

Is this a lot? Well, it is tough to say without comparison to something else. Let's take our good friend, sunlight on the Earth's surface. The power density of sunlight is roughly 1.35 kW/m^2 (killiWatts per square meter). Now, the bigger your head is, the more energy your head absorbs from the sunlight. Let's say you have a standard adult head, which we'll approximate with a circular cross section of radius 4". Your head would then be roughly 0.0324 square meters in cross section. As a result, the power absorbed by your head will be roughly 1.35*0.0324 = 0.0438 kW = 43.8 W.

Hence, a cell phone cannot transmit more than 0.125 Watts of energy to your head, while sunlight will transmit 43.8 W. This means that sunlight heats your head 350 times faster than a cell phone. In fact, since sunlight is a much higher frequency than cell phones, the sunlight energy is more dangerous (see Equation [1]). The conclusion: The heating effects of a cell phone are negligible!

Incorrect Objection: "But sunlight is harmful, as you can get cancer from sunlight." This is false. Skin cancer from sun exposure is caused by exposure to UV light, which is a type of ionizing radiation. Visible light from the sun and radiation from a cell phone are non-ionizing forms of radiation, and therefore do not exhibit the harmful effects similar to UV light.

The FCC and SAR

To legally sell a cell phone in the US, the FCC limits the radiation you can receive from the phone when it is placed directly against your ear. How exactly is this specified?

Cell phones worldwide must pass SAR requirements. SAR is "Specific Absorption Rate", which is a measure of the energy absorbed in your head when the phone is transmitting maximum power. SAR is a measure of absorbed energy over a small amount of tissue (1 gram in the US, 10 grams in Europe). Mathematically, SAR is written as:

sar - specific absorption rate
[Equation 2]

Don't worry if this doesn't make sense; I'll explain Equation [2] simply after I define what all the parameters are. conductivity is the electric conductivity of material in which the SAR is measured (the brain tissue, typically). Conductivity is measured in units of Siemens/meter. Electric conductivity is a measure of how easily electric current flows through a material; in this case electric conductivity can be thought of as how quickly a material converts an electric field into heat.

Also in Equation [2], density is the mass density of the material being measured. The density is measured in kg/m^3 [kilograms per meter cubed] or grams/cm^3. E(r) is the electric field as a functin of position, where r is a position vector. Finally, V is the volume over which the SAR is measured.

Now, Equation [2] can be re-stated in plain English. Equation [2] states:

SAR is a measure of the average heat absorbed over a specified region, divided by the weight of the material in that region.

Hence, SAR is measured in W/kg (Watts per kilogram) or mW/g. For more information on how this is done in certification labs, see the SAR measurements page.

The FCC in the US sets the strictest SAR limit in the world. This limit is that for a mobile phone transmitting maximum power at any frequency, the SAR cannot exceed 1.6 mW/g (1.6 W/kg). They furthur require that the "averaging" in Equation [2] be performed over a region of 1 gram.

Fun Fact: [Europe requires the SAR to be measured over 10 grams and has a limit of 2.0 mW/g. Not only is Europe's limit higher numerically, but averaging over a larger volume makes the SAR spec much easier to pass. This is because the electric field dies off quickly as it enters human tissue. As a result, averaging over a larger volume reduces the average Electric field, making the specification much easier to meet! Hence, the US SAR spec is significantly more stringent than in the rest of the world!]

It is commonly stated that the FCC has determined that a phone that passes these exposure limits is safe. This is false. The FCC has no idea what the safe limit is. The fact of the matter is that no study has confirmed a link between mobile phone radiation and cancer. Many studies have attempted, none have proven a link. All studies that have shown a link have later been falsified.

Possibly you think the last paragraph is bogus. Suppose then that the FCC knew that 1.6 mW/g was safe. Then all the scientists doing studies would therefore know they are wasting their time. But it is not a proven fact, and so they continue to look. How then, you might ask, did the FCC come up with a SAR spec of 1.6 mW/g over 1 gram of tissue as a safe measure?

They guessed.

The FCC (or any scientific body) has no evidence that exposure to mobile phones leads to any biological impairments. As such, they took an arbitrary measure, and went with it. The problem is this: you cannot definitely prove no correlation exists. Modern mobile phones haven't been around long enough to study someone with 30 years of exposure. Consequently, while there is no evidence of any link between phones and cancer, the "we don't know for sure" argument will resonate with the media and the uniformed. This fact coupled with the knowledge that "cell phones radiate directly into your head" and an ignorant public (and equally ignorant) news media have made this non-issue much larger than it is.

In 2011, the World Health Organization released a note labeling mobile phones as a "carcinogenic hazard". They are quick to state that because mobile phones are a relatively new technology, it will take years before the long term effects are truly known. Hence, the WHO has classified cell phones as a carcinogenic hazard, chiefly because they do not have the data that cell phones are not a cancer risk.

Ultimately, there is little reason to fear the radiation from cell phones. If you are concerned, simple methods to drastically reduce SAR exposure include using a wired headset (which cuts SAR to practically 0), hold the phone an inch from your ear or use the speaker phone, or send text messages.

Some people are comparing mobile phones companies to the cigarette makers of the 21st century - selling a product they know causes cancer. There's one little flaw with that argument: the exposure risk can be reduced to zero via simple means, stated in the previous paragraph. If the companies were at all concerned, or had any information or reason to believe there was a link, why would they not just provide a wired earpiece with each device, thus eliminating the health risk to the customers and the financial risk to the companies?

In closing, I'll state that the author uses a mobile phone without restriction, and pays no mind to the hysteria that erupts over mobile phone radiation from time to time. As always, the best defense mechanism is knowledge. If the media and the WHO declare something is dangerous, educate yourself about the argument and figure out if it holds water. You'd be surprised what a little thought can do to a world of ignorance.

For more extended reading, check out these books (the 3rd and 4th books below are there in case you want to read the counter argument - that cell phones are not safe):


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