Correspondence Address:
Pushparaja
Editor, Radiation Protection and Environment, C/o Radiation Safety Systems Division. Bhabha Atomic Research Centre; Mumbai 400 085
India

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The raw materials commonly used in the construction industry contain various amounts of natural and traces of man-made radionuclides depending upon their place of origin. Building materials originating from rock and soil contain mainly natural radionuclides of uranium ( 238 U), thorium ( 232 Th) series, and radioisotope of potassium ( 40 K). Traces of 137 Cs are often found in building materials from the environmental releases from nuclear facilities and due to fallout from testing of nuclear devices. Industrial by-products/residues such as ash from fossil-based power plants are being increasingly used to make bricks or as additive in the concrete due to economic and environmental reasons. The radionuclides present in the coal get concentrated in the ash and it becomes a source of radiation exposure due to the enhanced levels of radionuclides. The ash is also used for landscaping, as bulk material under roads and taken to dumps.

Radiation exposure due to building materials can be divided into internal exposure and external exposure. The external radiation exposure is caused by the gamma-emitting radionuclides, starting with 226 Ra in 238 U series, 232 Th, and gamma photon from 40 K. The internal (inhalation) radiation exposure is due to 222 Rn, and marginally due to 220 Rn, and their short-lived decay products exhaled from the building materials into the room air, which get inhaled by the residents. The Radon isotopes are the gaseous, alpha-emitting decay products in uranium and thorium series. Thus, it becomes necessary that the activity concentrations of these nuclides in the final product should be measured or assessed reliably. The concentration of the natural radioactivity in building materials can be measured and quantified using gamma ray spectrometry with HPGe detector. Some studies on measurement of natural radioactivity are reported in the current issue of RPE. The absorbed dose rate at a height of 1 m above the ground level in air was measured (as about 80 nGy/h) due to the concentrations of the natural radionuclides in soil.

The Radium Equivalent (Ra eq ) concept allows a single index or number to describe the gamma ray output from different mixtures of uranium (taken as 226 Ra), thorium, and 40 K in a material. The Ra eq in Becquerel per kilogram (Bq/kg) can be calculated knowing the activity concentrations of 226 Ra, 232 Th, and 40 K. It is thus possible for a country to place a single regulatory limit on the Ra eq activity in building materials.

The maximum Ra eq activity limit of 370 Bq/kg is set by the Organization for Economic Co-operation and Development (OECD). It is necessary that measurements of activity concentrations in building materials should be made with appropriate equipment which has undergone approved calibration and quality assurance programs.

Radon in dwellings can be a significant source of radiation exposure and is of health concern (one of the causes of lung cancer). Separate limitations, in terms of reference levels, for radon or thoron exhaling from building materials should be considered where internal dose evaluations based of the ICRP recommendations show that building materials may be a significant source of indoor radon or thoron. A concept of activity concentration index is used when more than one radionuclide contributes to the dose.

As a good international practice, exposures due to building materials should not be disregarded from the radiation protection point of view. Controls should be based on a dose criterion which can be established considering overall national circumstances. Like the European Union standards, doses exceeding 1 mSv in a year should be taken into account from the radiation protection point of view. Controls can be based on a lower dose criterion, in the range 0.3-1 mSv a year.