The word radiation covers a lot of ground but in the nuclear context we’re talking about ‘ionising radiation’, which in sciency terms basically means streams of invisible waves or particles with sufficient energy to knock electrons out of atoms, creating ions, resulting in chemical changes to materials at the molecular level.
Essentially we're concerned with the three main types of naturally occurring, non-ionising radiation, designated Alpha, Beta and Gamma.
Alpha particles are the biggest and composed of two protons and two neutrons but they are also the weakest and cannot penetrate more than a few centimetres of air and even have trouble getting through a few sheets of paper. However, the size of Alpha particles can make them rather dangerous, especially at very close quarters and in high concentrations. Because the particles are so big they are more likely to interact with and cause chemical and biological changes to whatever they happen to smash into.
Beta particles are essentially loose electrons or positrons, they have more energy and penetrating power than Alpha particles and can punch through a few millimetres of wood or metal. But like all radiation, if there’s enough of it, and in the wrong place for long enough it can change or damage whatever it comes into contact with.
Gamma particles (or waves) are essentially high-energy photons -- the same stuff as light and the gamma radiation family also includes X-Rays. Gamma radiation has no charge but it packs a lot of energy. However,because the particles are so small they can pass right through matter without hitting anything, but once again if there’s enough of it then it can and will cause chemical and biological changes
Ionising radiation sounds like a really bad thing but the fact is we are being constantly bombarded with natural radiation, from the food we eat -Brazil nuts are notoriously radioactive - rocks and minerals, water,air, cosmic rays from outer space and a wide range of consumer products in and around the home, like smoke alarms and fluorescent lamps to old clocks and watches with luminous hands. Even our own bodies are naturally radioactive and it's not just the nicotine and tar in tobacco that can kill you. In the past sixty years there has been a small but measurable increase in the background radiation from such things as nuclear power generation, fallout from nuclear weapons, moreover your exposure to ionizing radiation is significantly increased if you have an X-Ray, travel regularly by air or undergo radiological treatment.
The point is the human race has been exposed to low level doses of radioactivity since the year dot. We have evolved to deal with it, it may even have had a part to play in the evolutionary process, it’s when we get too much of it that things can go wrong.
Scientists can say with some certainty how much radiation will kill you outright, and the sort of exposure that will make you sick but these tend to massively large doses that we are never likely to encounter in our normal day-to-day lives. When it comes to very low levels, exposure no one can say exactly how much is bad for you.
SO HOW MUCH IS TOO MUCH?
no-one knows. It depends on a multitude of factors including age,gender,
genetic makeup, where you live, what you eat and needless to say, which experts you talk to. There are clear beneficial effects and
controlled exposure to high doses of ionising radiation is a cornerstone of
Medicine; some even suggest that small doses can be
good for you,therapies, like those offered in this Radon Mine are claimed to
date back 6000 years.
it is generally accepted that most of us, on average, receive a dose of between
100 and 200 millirem (1- 2 millisievert) of radiation per year (the millirem is
a measurement of absorbed radiation dosage and100 mrem = 1 millisievert). We
live to tell the tale because almost all of it comes from natural sources and
it is a part of our normal environment.
Around 70 to 80 mrem comes from
purely natural sources, as much as 11% of that from naturally-occuring
materials in our own
bodies. Apparently you will receive a 2 mrem
dose sleeping next
to someone for 8 hours... The rest is man-made with the bulk of
it coming from hospital and dental X-Rays and medical treatments, 4 - 6 mrem
comes from nuclear power plant emissions and leakage and fallout from nuclear
weapons plus 1 to 2 mrem from consumer products, such as smoke detectors and so
internationally agreed limits for exposure to radiation for those working in
the nuclear industry is a maximum whole body dose of 5,000mrem (or 5 rem) per
year. For the rest of us exposure levels are significantly lower. A chest
X-Ray, for example typically gives a surface dose of 50 to 80 mrem, abdominal X-rays
can be up to 600mremwhilst a full body CT Scan can be as high as 1000 mrems.
For the record, a whole body dose of 500 rem will be enough to kill you,usually
within one to two weeks whilst a dose of 100 rem will cause severe radiation