Various situations of using 1 mSv/annum dose limit for assessment puroposes

The International Commission on Radiological Protection (ICRP) has set an annual dose limit of 1 mSv for the purpose of protection of members of public due to releases from nuclear facilities. This limit is over and above the background radiation exposure existing at the facility location site. The limit was set in ICRP-60 (1990) and also continued in ICRP-103 (2007) as a basic dose limit for limiting the stochastic effects. It is based on the assumption that the stochastic effects have no threshold and any dose above zero would have a proportional effect, and the effect increases proportionally as the dose increases. It uses the principle of as low as reasonably achievable (ALARA) for optimization of releases from a nuclear facility. However, it is also unequivocally stated that an exposure to an effective dose <100 mSv over a long period (>few days) has no scientific proof to show that the stochastic effect indeed exists. The proof only exists for exposures of effective doses of ≥100 mSv for 1 or 2 days, for stochastic effects. The reasons behind the setting of 1 mSv dose limit has been given in the ICRP publications but broadly it involves, the variations of natural background exposure, infants among the general public, the linear no threshold principle, the effects on the survivors of atom bomb explosion, and also epidemiological studies of radiation exposure at various nuclear facilities including the nuclear accidents. Although the limit was arrived at by ICRP basically for the control of effluent releases form nuclear facilities to the environment and exposure to member of the public and most nations follow it as a regulatory limit, there are many situations in which the annual dose limit has been applied in bringing control in them using the ICRP limit of 1 mSv dose as benchmark limit. Some of the situations are briefly given below. RADIOACTIVITY LEVELS IN CONSTRUCTION/BUILDING MATERIALS European Union in its publication 112 has laid down the formulae and principles for evaluating the effective dose based on the radioactivity content of natural nuclides of U-238 series, Th-232 series, and K-40. The evaluation is based on an annual effective dose (AED) of 0.25 mSv exposure for an inhabitant living in a concrete apartment block with average concentrations of 40 Bq/kg, 30 Bq/kg, and 400 Bq/kg of Ra, Th, and K, respectively, in building materials. When gamma doses due to building materials are limited to levels below 1 mSv/annum, Ra-226 concentration in the material is limited, and the levels of Rn-222 are unlikely to exceed 200 Bq/m3. Within the EU, AED resulting from building materials is <0.3 mSv/a, and they can be exempted from any restrictions. If the AED is in the range of 0.3–1 mSv, some kind of control on their use can be exercised. RADIOACTIVITY LEVELS IN COMMODITIES AND FOOD International Atomic Energy Agency (IAEA), in its general recommendations, states that (BSS-GSR-Part 3) regulatory bodies establish specific reference levels for exposure due to radionuclides in commodities such as food, feed, and drinking water. The AED to a representative person arising out of consumption of food containing natural and human-made radionuclides, generally should not exceed 1 mSv/annum. IAEA safety series report No. 114 gives a general methodology for the evaluation of AED to the representative person due to radionuclide intake through food, in normal conditions, other than during an emergency. It also gives the global average concentration levels of radionuclides in various food items. RADIOACTIVITY LEVELS FOR RADIONUCLIDES IN FOOD FOLLOWING NUCLEAR EMERGENCY Codex Secretariat (2011) has established guideline levels for radionuclides in food contaminated following a nuclear/radiological emergency. They have been referred as “Codex Guidance levels”. The guidance level for different radionuclides was derived using the intervention level exemption dose of 1 mSv/annum in addition to the mass of food consumed, ingestion dose coefficients, and import to production factor. Most of the human-made radionuclides, such as Cs-134, Cs-137, I-131, Sr-90, and Sr-89, have been included. RADIOACTIVITY LEVELS FOR GROSS ALPHA AND GROSS BETA IN WATER The WHO has set a reference dose level for control of exposure due to ingestion of drinking water at 0.1 mSv/annum, which is 10% of the AED limit of 1 mSv/annum. The exposure is controlled by monitoring gross alpha and gross beta radioactivity in water in terms of screening concentration levels. The derived reference levels of gross alpha activity in water are 0.5 Bq/l and gross beta activity in water is 1 Bq/l, which will independently lead to a dose of <0.1 mSv/a. The 10% of 1 mSv is taken as a guidance level, as the measurements are based on gross activity, and in case exceeding these levels further investigation of analysis for individual radionuclides is recommended. Gross alpha of 0.1 Bq/l is also set by some countries. RADIOACTIVITY LEVELS IN OTHERS Levels of radioactivity in NORM in soil and TENORM in other industries such as oil and gas, fertilizers, mining, isotopes use/production in health care also have limits directly or indirectly in terms of constraints related to the 1 mSv/annum dose limit. Thus, in a way, the dose limit of 1 mSv/annum set for a representative person for the control of effluent releases from the nuclear facilities has been/being used as benchmark dose limit for various other activities or situations.