With "radiation" is it assumed what is called EM radiation, occurring mainly natural but also manmade.
Wavelengths of interest are from a few times of earth diameter down to short UV(100nm).
Especially, in an era where lots are heard about cell phone and dect cordless phones radiation.
"Heard" is poor knowledge. A basic course in radio technology can give better insight.
There exist no serious research result that shows anything else then a heating effect when a EM wave is absorbed in human body or absorbed in anything else.
It is not likely that it will ever come any proof of anything else as it would be just as sensational as saying that there exists effects which not requiring energy to happen, which would violate physical laws.
How EM waves behaves are well-known and follow strictly mathematical laws, with minimum space for personal estimations.
Even if EM-waves are a assumed to be ether or waves with or without a photon mass is practical result the same.
Currently accepted basic energy unit for EM waves is Joule.
One definition for 1J is when an electric current of one Ampere passes through a resistance of one Ohm for one Second.
So far very basic, but to calculate if an EM-wave can be harmful for the easiest affected particle in human body, an electron, do we need two more parameters.
It is Planck constant and Einsteins first relativity theory. Then can we calculate what physical effect that would happen if an EM wave would pass our body.
Extreme cases are also easy to calculate. If 100% of energy of an EM-wave would pass our body, without any absorption, would nothing at all happen inside the body, as no effect can happen without energy.
Opposite, absorption is almost 100% for frequencies above 2 GHz. At 2.5 GHz is typical attenuation through a human torso around 100 dB, which is almost total absorption.
Still, even at very high frequencies can some waves pass through human body.
As example, pointing a strong flashlight inside your hand palm and most of its energy will be absorbed but a small amount of light can visible be seen passing through the hand.
Lower frequencies will pass human body more easily. Reason is same as for any radio-receiver, our biggest antenna, actual body length, is too ineffective due to the length of emitted wavelength.
If any energy should be absorbed, must something act as receiving antenna. If it not exist any antenna, will no physical reaction happen and the EM wave will either pass without any attenuation or be reflected.
For lower frequencies then visible light, such as 2.5 GHz emitted from a microwave oven is it mainly long chains of albumin in skin and flesh that act as antennas. Pure fat have low amount albumin and is less good as antenna.
Often assumed is that it is water in body that act as antenna/absorber but water is a relative poor absorber at 2.5 GHz. A glass of pour water in microwave oven will be heated, but same wave must pass several times, reflecting against walls as only minor amount of energy is absorbed each time.
At lower frequencies then 2.5 GHz, already at 1 GHz can much more of an EM wave pass deeper in human body or even pass through thinner body parts.
At 100 MHz will most of an EM wave pass a human body with minimal absorption.
Instead do human body at these lower frequencies start acting as an electric conductor. As an example, if touching an FM receiver with poor antenna, can reception be improved, as human body then acts as an extender of antenna => conductor, but some absorptions for same freq. can also occur if body is placed between transmitter and receiver.
At frequencies such as AM at a few MHz, is human body an even better conductor but body is then too short to be able to act as an antenna of value. These EM waves will therefore pass a human body with almost no attenuation at all => less heating effect.
If a lossless conductor surface size relative wavelength is big enough will it reflect EM waves, without any absorption.
Whatever frequency, human body is never a effective reflector but at 1THz and upward do it reflect a bit better, and at frequencies for visible light do human skin, very wavelength depending, either reflect or absorb waves.
If reflection or absorption will occur for a particular wavelength is depending on size and structure of molecules in our skin.
As these structures in skin antennas varies in size between different people will some people seems to have different skin color.
What we sees as skin color is a result of amount of wide band of emitted white light that either is reflected or absorbed by skin. A painted wall may look to have a red color, and it is then because all other colors are absorbed by wall, but red is reflected.
High levels of EM waves may cause burning wounds but nothing else from medical view.
What actually happening is that EM waves causes electrons in our body to increase its vibration. This effect is measured as an increased temperature.
Concentrated levels of EM waves radiated from a strong source can be seen as a lot of streamed photons. A single photon can not create changes in a atom in human body such that an electron can be moved from its position, because of amount of energy in the photon that it would require not is possible until at very high frequencies which starts above low UV-light. UV light of higher frequencies can emit photons with such high energy-level that it can move an electron out of its normal position. This kind of radiation of UV-light is a known reason for skin cancer.
If a single photon not can move an electron, does it not change anything from chemical or medical view, except for a minor vibration in electron due to the collision.
If there are a lot of photons bombarding the electron doesn't change anything as it is the individual mass of each photon relative the electron that counts.
It is called the photoelectric effect.
Formula from
about.com:
hν = W + E
where
h is Planck's constant
ν is the frequency of the incident photon
W is the work function, which is the minimum energy required to remove an electron from its electron layer.
Assume that you is worried about body heating due absorption of EM energy emitted from surrounding radio transmitters.
The good news is that human body has inbuilt a lot of temperature sensors reacting on minimal temperature changes. Less then 0.1 degree change of internal body temperature is easily detected by these sensors and starts a lot reactions to compensate for that.
If too hot blood, and it will flow nearer skin for better cooling. Increase skin moisture is another trick to increase cooling effect.
If opposite if temperature is too low, will body increase energy burning, reduce blood circulation and concentrate blood to circulate around vital organs. Whiter fingers is a visible sign of this.
Our dominating EM wave source is the sun. Radiates with about 1kW/squaremeter a sunny day. Human body expose about that size when unshaded. Most of that energy is absorbed but for a limited time can human body compensate an still keep same internal temperature.
A typical indoor light source is about 60W resulting in ~1W/squaremeter depending on size and shape of the room.
A cellphone does effect human body less due to its lower frequencies and less power lesvel.
Amount of energy absorbed when a cellphone is transmitting at full power is about one tenth compared to the energy absorbed from the lamp, for a typical talk position.
It assumes then that the phone is held close to body so that at least 50% of its energy is absorbed in human body.
Main radiators of EM waves in a house are typical heating elements and stove. Radiation from these are easy detectable as heat in human cells and radiation levels can be rather high.
We can even detect direction of a such heatsource from long distance by using our heat sensors.
At body local strong radiation sources can for example be a bed-lamp, if placed so close to your head, that you can feel the increased heat when light is on.
Returning to cellphones, main part of its radiation is already inside phone converted to heat due to electric losses. A big amount of loss is due to the background lightning of the screen.
Some phones can become real hot when surfing, and even worse if phone is held near head, and outer part of phone mostly is covered by a hand, as hand then will act as heat isolator, so that there not is any natural way for this heat to be exchanged to surrounding cooler air.
Remaining heat absorber is then the human body that is holding the phone.
Main frequency emitted from phone is infrared wavelength and a minor amount around 1-2 GHz. Peak power in GHz range is 2 Watt but average power is less then 10mW if reception not is too bad.
It is likely that audio amplifier in the phone consumes/emits similar amount of power during a phone-call.
Total can a modern phone emit up to 10W when in heavy use.
Human body need a lot of radiation that it can absorb, it will else die within minutes. If environment is colder then body temperature must heat be created inside body as compensation and protection suh a thick clothes can stop heat leaking from body to the colder environment.
Earth acts both as radiator and absorber keeping temperature (radiation) in a comfortable level even at night. Partly can we compensate living in colder areas by internally in our body increase creation of radiation by eating/burning more food.
Improve our radiation reflectors (clothes) is another alternative to gain some heat.
Keeping an active cellphone near body is another way to keep warmer, even if it is a marginal effect. Human body can receive or emit max around 1kW. It occurs when receiving sunshine or emit that amount of effect due to heavy work during short periods.
In rest is emitted effect around 100W.
Radiation from a cellphone will not make any bigger difference in total sum of radiation sources needed to keep body alive
A cellphone have a frequency range that have less possible photon-energy then visible light.
Human body length and internal structures makes it less effected by lower frequencies. This is bad if our body needs more heat, as we hardly can be heated by sitting near an AM radio station.
If you want to avoid EM-waves, is cellphone adding marginal amount compared to other surrounding sources, but do not use bed-lamp as it is magnitudes more powerful. Avoid any heat-source including earth/sun.
Actually, only way way to avoid radiation from sun, direct or indirect, is to travel real long away in space. Hiding in shadow of a atmosphere-less planet is almost as good. Effective temperature is then just a few degrees above 0K.
Temperature is a way to measure amount of EM radiation. Without EM-radiation is temperature 0K.
Whatever level of radiation/temperature your body need to keep alive, a cellphone will add very marginal amount of radiation.
A cellphone will shorten many lives due to environment impact when produced and recycled.
Not to forget, traffic accidents caused by people that think that they only partial need to concentrate on driving a car, while texting or checking email.
A link about required level of photon energy to kick an electron
Recommended book:
Modern physics for scientist and engineers.