Radiation is all around us all day, every day in soil, rocks and even in air and water and it sustains our lives: we are able to see because our eyes detect the radiation called "light" and infra-red radiation from the sun or from a glowing fire keeps us warm, ultra-violet radiation is also used for sterilising medical equipment.

In general radiation includes electromagnetism radiation (light and heat, microwaves, ultra-violet, X-rays and gamma ray and including also the Cosmic Microwave Background Radiation (CMBR) which is the thermal radiation left over from the Big Bang) and particles (alpha, beta and neutrons) which are emitted by some materials and carry energy. 

Although some radiation is capable of travelling large distances, it may be stopped by appropriate absorbers.
Starlight traverses galaxies, but may be stopped by a piece of paper.
Radio waves, too, are capable of travelling great distances, but may be absorbed by materials such as metals. Like light, ionising radiation travels in straight lines until absorbed or deflected. The material used to absorb ionising radiation depends on the type and energy of the radiation.
Alpha particles (helium nuclei) may be stopped by paper, beta rays (high speed electrons) are stopped by perspex, while gamma rays (like X-rays) may need lead or concrete to stop them, but can be stopped by any material, even water, providing there is enough of it

Example of alpha, beta and gamma emitters:

    α (alfa) emitter: α Americium-241 used in smoke detectors
    ß (beta) emitter: Carbon-14 used in carbon dating
    Y (gamma) emitter: Technetium 99m diagnostic medical radio

Radiation can be ionising or non-ionising.
Ionising radiation is the one that can cause damage to living tissue at high levels so it is vital to control our exposure to it.
This kind of radiation is invisible and not directly detectable by human senses, unless at very high doses.
Radiation carries energy which may damage living cells in the same way as tobacco smoke, asbestos or ultraviolet light.
If the dose is low or is delivered over a long time there is an opportunity for the body cells to repair.
There is only a very small chance that some cells may have been damaged in such a way that effects such as cancer appear in later life.

Exposure to high levels of ionising radiation can result in mutation, radiation sickness, cancer and death.
There are two kinds of radiation damage: damage to the cells of the body, which may put humans at risk (somatic effects); and damage to the reproductive cells, which may put some of our descendants at risk (hereditary effects). 

Acute radiation syndrome (ARS), also known as radiation poisoning or radiation sickness is a number of health effects which occur within 24 hours of exposure to high amounts of ionising radiation.
The radiation causes cellular degradation due to destruction of cell walls and other key molecular structures within the body.
The symptoms can begin within one or two hours and may last several months.
Relatively small doses result in gastrointestinal effects such as nausea and vomiting, larger doses can result in neurological effects and death.
Similar symptoms may appear months to years after the exposure as chronic radiation syndrome when the dose rate is too low to cause the acute form.
Radiation exposure can also increase the probability of developing different types of cancers.
Treatment of ARS is generally supportive with blood transfusions and antibiotics.

Geiger counters are necessary to measure ionizing radiation levels; the primary component of the Geiger counters is a tube filled with a gas  that conduct electricity when struck by radiation.
This allows the gas to complete an electrical circuit and usually this includes moving a needle and making an audible sound, the unit of measurement of the counter depends on the application.

A way to measure radiation is to measure the dose of radiation received and it is measured in sieverts (Sv) so saying that in Mexico City the average background radiation dose is 0.09 µSv per hour means that in 1 hour the human body absorbs 0.09 µSv.
Saying that the background radiation dose of an international air travel is 3.7 µSv does not make sense because it is necessary to specify the time we absorb that dose of radiation so the right way to say is 3.7 µSv per hour for and international air travel.

As 1 Sv represents a very large dose of radiation, smaller units are commonly used:

    - Millisieverts      (1000 mSv = 1 Sv)
    - Microsieverts    (1,000,000 µSv = 1 Sv)
    1 mSv = 1000 µSv
    1 µSv = 0.001 mSv

Dosimeters generally measure in mSv per hour and shows the radiation dose received in 1 hour. An older unit for dose the radiation level is the rem (Roentgen Equivalent in Man) and the smaller millirem (mrem) which is still used in the United States and 1 Sv = 100 rem 
Another measure for radiation are Roentgen (R) and 1 R = 0.877 rem = 0.00877 Sv





Radiation exposure depends on three factors:




Radiation comes from many sources, including the sun, rocks, air, food and they are widely used in medicine, industry, agriculture, pollution control industry and research.

These sources give us our naturally occurring background radiation dose, 80% of it comes from nature, the remaining 20% results from exposure to human-made radiation sources for example medical imaging such as X-rays, scans etc. 
The average background radiation dose in Australia is 1.5 mSv per year, in America is 6.24 mSv, in Japan is 3.83 mSv and in United Kingdom is more than 7 mSv/year.

It is hard to predict the impact of radiation on humans but around half of all those exposed to 5 Sv will die from it and almost all who receive a dose of 10 Sv will die within weeks.

During the Chernobyl disaster 400 times more radioactive material was released than at the atomic bombing of Hiroshima.
A typical dose for those workers who died within one month after the disaster was 6 Sv.

The U.S. Nuclear Regulatory Commission (NRC) requires that its licensees limit human-made radiation exposure for individual members of the public to 1 mSv per year, and limit occupational radiation exposure to adults working with radioactive material to 50 mSv per year (3-25 µSv/hr).

Living indoor increases the radiation dose received because the concentration dose of a radioactive gas called radon is increased.
Radon arises naturally from radioactive decay of uranium and thorium and it is normally present in rocks, soil, bricks, mortar, tiles and concrete.
Reducing ventilation of the house also increases radon concentrations.
Using bore water, especially in a hot shower or in thermal springs also increases the radiation dose received.

Small extra doses of radiation occur when we go high because the higher we go, the less shielding the atmosphere affords from cosmic rays that is why on a mountain top the air may be cleaner but the radiation dose is higher.
Air travel increases radiation dose and astronauts receive even higher doses.

Other common, but minor, sources of radiation are some older luminescent clocks, watches, compasses and gunsights, exit signs, certain paints and pigments, dental porcelain, fire alarms, smoke detectors and television sets.

The thermal radiation is also absorbed by objects and the amount of radiation absorbed depend on the surface of the material.
Dark surfaces absorb most of the thermal radiation they receive, silver or mirrored surfaces reflect thermal radiation.
That is why when we are in the sun we will feel the heat more if we wear dark/black clothes, it is because the transmitter radiation pourcentage is higher than the reflected radiation pourcentage.


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