Many people are scared stiff by the possibility of an explosion at the Japanese nuclear reactor. They believe that a nuclear fallout will affect life as we know it. Since, the radiation of radioactive substances takes a long time to die down to zero, and these substances can be carried by strong wind currents across large distances, the consequences might be catastrophic. Well, this is only partially correct.
First point to note – radiation from a radioactive source NEVER goes to zero. There is always something there, no matter how feeble. This is not a bad thing. This means that we already live in a bath of weak radiation. Radiation comes from various sources, like radon gas (the main source!), cosmic rays (which bombard atoms in the atmosphere), elements in soil and even food. This is completely harmless. Even we are radioactive just stand in front of a Geiger-Muller counter and hear the clicks due to the disintegration of atoms in your body! All of this constitutes the background. The crucial point here is that once the radiation from a source is close or below the background, it is harmless.
Next point, radiation flux decreases as inverse-square of the distance. Taking into account various ionization phenomenon, the decrease is even faster; making the decrease a power law in distance with some power higher than 2. This essentially means that radioactive samples high up in the atmosphere will have negligible effects on life on the ground.
For measurements, we need units. The standard units are the following:
- Gray (Gy): Unit of measurement of the amount of radiation incident on any body per unit area per unit time. 1 Gy = 1joule/1kg.
- Sievert (Sv): Unit of measurement of radiation incident on a human body per unit area, per unit time. 1 Sv = 1 Gy . W, where W=weight factor
The presence of the weight factor (W) in the definition of Sievert measures the damage done to a human body due to different types of radiation. For example, W=1 for electrons, muons, x-rays etc. These are less harmful than neutrons at 50 keV, which have W=10. Alpha particles from a close enough source can have W=20. (Higher weight=more damaging.) Weight factors also take into account the portion of the human body receiving the radiation. Bones have W=0.01, whereas gonads may have W=0.2.
1 Sv is a very high dose, so units like ‘rem’, (1 rem=0.01 Sv) or milli-Sievert (Sv) (1 mSv = 0.001 Sv) are used.
How much is safe?
There are Threshold Limit Values (TLV) lists available. These recommend that a person on average receive no more than 50 mSv per year, i.e. 5 rem. Over a five year period, a person shouldn’t receive more than 20 mSv (or 2 rem) per year. These estimates vary with location of a person and his/her occupation. Check values here.
These values are slightly misleading because it misses the important point that a large dose (say 50 mSv) delivered all at once much more damaging than the same amount of radiation spread over a long period of time, like a year.
A normal person, not working in or near a nuclear power plant, receives less than 10% of the allowed dose. So don’t worry about the radiation. It’s just not that dangerous.