Our daily exposure to radiation is something we only tend to consider when a major nuclear incident reminds us of its dangers. But what is a 'safe' level? The most sensible answer is probably none, but the effects vary enormously from person to person.
There are two different measurements of radiation commonly used: the millirem (mrem) and the microsievert ( Sv), with 1mrem equalling 10 Sv. While radiation cannot be seen or felt, it is all around us and a part of our natural universe. The average annual dose per person from all sources is about 360 mrem, but it is not uncommon to receive far more in a given year, often due to medical procedures. There are standards that limit the dosage that employees may be exposed to. For example, EU Council Directive 96/29/Euratom requires an employee's exposure to be limited to 10,000 mrem in any period of five successive years, but subject to a maximum dose of 5,000 mrem in any single year.
Typical sources of radiation
To assess our individual exposure accurately is practically impossible, but there are a number of factors we can consider.
- Cosmic radiation: This is radiation from outer space and is partly blocked by the earth's atmosphere. At higher altitude the levels are higher, varying from around 25 mrem at sea level, to double that at an altitude of 1.6 km. A typical dose of radiation when flying is about 0.5 mrem per hour, due to the high altitudes involved.
- Terrestrial radiation: This is due to radioactive materials naturally found in the soil such as uranium and thorium. An average value is around 30 mrem a year, but in some places it can be as high as 1,000 mrem a year.
- Radiation in food: Foods naturally contain some radioactive elements such as potassium, resulting in an average dose of 20 mrem a year.
- Other sources: Watching television is about 1 mrem annually, a chest x-ray is about 5 mrem each time, but a computed tomography (CT) scan can be as high as 700 mrem.
Increased risk of cancer
It has been estimated that the likelihood of dying from cancer increases by 10% if a total of 250,000 mrem has been accumulated (an exposure of over 3000 mrem every year for over 80 years). There is much disagreement over how radiation measurements are calculated, what they mean in reality, what can be considered 'safe' and the level to which we should protect ourselves in our day-to-day lives. There is natural concern about radiation levels following a serious nuclear incident, but
it should always be remembered that our world is naturally radioactive and our exposure can never be completely eliminated.
Editor's note: The hazards of exposure to electric and magnetic fields from high voltage power lines are well known ashore. With increasing application of high voltage power generation, distribution and propulsion systems onboard merchant ships, ships' crews risk prolonged exposure to energy emissions when working at very close proximity to such equipment. It is suggested that more scientific studies are commissioned on this issue and ILO's system of certification of working and living spaces be expanded to include the independent measurement and evaluation of radiation from all shipboard sources (including communication system antennas), so that equipment makers and shipbuilders take all necessary measures to effectively protect ships' crews from harm.
Source: Mars/Nautical Institute