RADIATION ACCIDENTS – REDUCE RADIATION EXPOSURE

With all the terrorist activities on the rise and with availability of nuclear materials not so difficult, it is essential that adequate emergency preparedness is developed and kept in readiness. Assuming that the mass destruction is not in the mind of the terrorists, it is the use Radiological Dispersal Device (RDD), is the likely terrorist act. The RDD is mixing of any radioactive material with conventional explosives and use the same in public domain. The result is large scale contamination of the areas, which is likely to cause panic among the public and disruption of the public utilities such as transport system, food supplies, etc. One should remember that potential health effects may not be alarming in such situations.

There are some simple precautions which the members of the public should take under such situations to reduce the exposures and the health effects. Do not panic. Simply follow the instructions on the protection measures issued by the well identified government agencies responding to such a situation. Some of the simple protective actions can be:

1. Keep away from the affected area. Radiation exposure reduces with distance.
2. Hold a handkerchief, preferably wet, over the nose and mouth. Respiratory protection devices of the type used by medical staff in operation theaters are available in medical stores. This prevents inhalation of the radioactive dust which is likely to get airborne.
3. If you are indoors, remain indoors. Close windows and doors. You may have to close the common ventilation system if the incoming air is not filtered.
4. If one is coming from outside, have change of clothing including shoes, take a shower, clear your nose, wash your mouth and be indoors for further instructions, via Television/Radio from the authorities.

RADIATION HAZARDS OF AIR TRAVEL

Modern commercial aircrafts operate at altitudes varying from 8 to 16 km from the surface of the earth. Cosmic radiation coming from space is a mixture of many different types of radiation, such as protons, alpha particles, electrons and other high energy particles. The radiation is affected by the earth’s magnetic field and while entering the atmosphere, its intensity reduces due to interaction with the environment. At ground level, the cosmic radiation is mainly consists of muons, neutrons, electrons, positrons and photons. Thus, as the altitude decreases, the cosmic radiation dose also reduces. For example, the dose rate at sea level, at the top of Himalayas, and at 15 km height can be 0.03 micro Sv/h, 1 microSv/h and 10 micro Sv/h. European Commission working group ((EURADOS WG-5) reported typical per flight exposures of 67 microSv and 47.8 microSv to the aircraft crew flying from London to Tokyo and Montreal to London respectively.

This brings into focus the radiation protection aspects of airline crew and of frequently flying airline passengers. The International Commission on Radiological Protection (ICRP) recommends that the airline crew should be treated as radiation workers and for normal flying hours put up by the crew, the expected average annual dose is reported to be of the order of about 2.5 milliSv. This can be compared with the 20 milliSv, the annual average occupational dose limit recommended by the ICRP for all radiation workers. Annual dose limit recommeneded for the members of the public from man-made radiation sources is 1 milliSv.

The number of passengers flying is growing fast, rather exponentially in some countries and if all the doses of the passengers and the crew are added up (collective dose), the statistically estimated health consequences due to the radiation exposure of the group of population is of concern from radiation protection point of view. Now, compares this risk of the health consequences with the risk of potential airline accidents, the radiation exposure risk will work out to be higher!

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RADON AT HOME

Radon-222, a radioactive noble gas isotope, the daughter product of Radium-226 in the naturally occurring Uranium-238 Series has become a centre of attention in radiation protection. Similarly, another isotope of radon, i.e., Radon-220 (called thoron) is the daughter product of Radium-224 in Thorium-232 series. The radon isotopes, Rn-222 and Rn-220 are alpha emitters with half lives of 3.82 days and 55.6 seconds respectively. The decay products of the two isotopes are also short-lived, but they are particulate radionuclides of elements polonium, lead, bismuth and thallium. Official regulatory requirements in some countries call for the measurement of radon indoors before clearance of dwellings for human occupation.

Becquerel (Bq) is the unit used to express the quantity of radioactive material disintegrating at a rate of 1disintegration per second). The radon daughter concentration can also be expressed in terms of working level (WL) units. One WL corresponds to an equilibrium concentration of 3700 Bq / cubic meter of air. Sievert (Sv) is the unit used to express the dose (energy absorbed per kg - effective dose) received due to the exposure to radiation.

Being gaseous, the radon isotopes generated in the underground rocks (about 4 ppm-parts per million), in soil and from the building materials diffuses out and gets mixed up with the air in the environment. The radon gas also comes out from the cracks in the rocks and in hot spring water. Healing property of spring waters is attributed to the presence of radon. Once out in the open, the outside air provides enough dilution. Typical average concentration of radon outdoors is 10 Bq per cubic meter. It varies from place to place. Seasonal variations are seen. In the uranium mines, the concentration can be much higher and it is a potential occupational hazard for the mine workers. However, the radon concentration is much higher indoors due to limited dilution provided for the radon diffusing inside the dwellings from the building materials and the underlying soil (particularly in basement areas). The radon concentration varies with the building material used, the type of the house, quality of construction and ventilation provided. Thus, depending upon the time spent indoors, the exposure of the population can be significant, particularly in cold countries where the ventilation rate provided is low.

On inhalation of the contaminated air, the particulate activity gets deposited in the lung and gives dose to the lung tissues. As compared to the radiation dose resulting from inhalation of unit concentration of the particulate radon daughters, the dose from inhalation of unit concentration of the inert gas is about 50 times lower. Lung cancer is the radon induced malignancy known amongst the mine workers.

Major contribution (70 to 80%) to the average dose of 2.4 milliSv per year to the public from natural sources comes from these radon isotopes and their short-lived daughter products. In some of the high background areas (HBRA), the radon concentration in dwellings may be much higher, which may call for intervention measures, such as relocation to reduce the human exposure.

The International Commission on Radiological Protection (ICRP) recommends Action Levels for regulatory intervention of 200-600 Bq per cubic meter of air with an indoor occupancy of 7000 hours in a year and an equilibrium factor of 0.4 (concentration of radon daughters divided by the concentration of radon gas). This concentration corresponds to an effective dose of 3-10 mSv.

There are various radon measurement techniques / portable monitors are available commercially for the measurement of radon accurately for the estimation of radiation dose and for regulatory compliance. Portable Alpha spectrometry based detection systems, electrets, direct scintillation based radon gas counting systems are employed for the measurement of radon in water, air and for radon exhalation measurements from soil and construction surfaces. Calibration of the systems is one of the important requirements for validation of the system operation and reliability of the data.

FEAR OF RADIATION – UNNECESSARY

The word RADIATION reminds of dropping of atomic bombs over Japan during the Second World War and the mass destruction of everything in the vicinity of the explosion sites. The effects of the exposures were short term like radiation syndrome and long term, like cancer. That is how the world today came to know about radiation. Since then, there have numerous beneficial effects and peaceful uses of radiation, which has been benefiting the society to a very great extent. Examples are: nuclear power and medical uses of radiation and radioisotopes for diagnosis and therapy. Safety records of such applications are very good as compared to other industries. In fact, other industries should emulate the safety considerations and provisions made in the nuclear industry.

Unlike in other industries, because of the fear of radiation, the effects of radiation exposure on humans are extensively studied and documented. The radioactivity measurement techniques are very well developed and at present a very small amount of radioactive material, in less than nano-gram (one-billionth of a gram) levels can be measured with required accuracy using specially designed electronic instruments. The very data on the health effects of the survivors of the bomb explosion served as the primary database for determination of the radiation exposure risk to the workers and the members of the public. We also have enough data on the health effects of low level radiation exposures of the people living in high radiation background areas (HBRA) in countries like India and China.

The truth is we are all living in this world where there exists natural background radiation, such as cosmic radiation and radiation from the earth consisting of small amounts of radioactive materials like uranium and thorium. This exposure is unavoidable. Public is unawareness of this fact. Nobody really told them or explained to them to be convinced.

However, public reacts to radiation with fear and anxiety. It is the fear of unknown. These indications need to be counterbalanced. How?
1. Two-way open communication between all the stakeholders to be enhanced to break the mutual distrust.
2. Natural background radiation levels can be displayed in public places. Follow one unit to express exposure to radiation. Too many units confuse the public.
3. Use of the term dose limit can be discontinued. It has lost its significance.
4. Improve public familiarity with respect to the effects of radiation and compare the effects with other more familiar environmental pollutants. Use the language which is easily followed by the public, in their language.
5. Educate the public what to do by themselves in case of any release of radioactivity into the environment and high radiation levels.
6. There is no point in highlighting health effects of human exposure to high radiation levels (radiation syndrome) which is likely to occur only during incidents or accidents.
7. Beneficial effects of radiation for health care (medicine) should be projected in schools and collages rather than the biological effects of radiation.
8. In general, public accepts whatever is beneficial or useful to them. Good examples are electricity and use cooking gas. Controllable risk is generally acceptable.

What is radiation hormesis

It is the hypothesis to explain the lower cancer incidences observed at higher radiation background areas, and inverse correlation observed between lung cancer mortality and indoor radon (a radioactive inert gas – decay product of a naturally occurring uranium -238 radionuclide) levels measured in some US counties. Adaptive response / enhanced immunity of the human biological system at low radiation dose levels are being invoked to explain this effect. This means radiation exposure is beneficial for health, at low doses!

If accepted, the radiation hormesis concept challenges the very basic approach of linear non-threshold model traditionally used by the International Commission for Radiological Protection (ICRP) for the dose-response relationship. The dose limits prescribed are likely to be enhanced from the existing limits of 30 mSv/year and 1 mSv/y for occupational workers and the members of the public respectively. The ICRP recommendations on dose limits are accepted by majority of the nations.