Answer to Question #12384 Submitted to "Ask the Experts"

Why do we use only the activities of radium-226 (²²⁶Ra), thorium-232 (²³²Th), and potassium-40 (⁴⁰K) to calculate the radiation hazards from building materials and to determine hazard indices such as radium equivalent? Are there any international regulations or legislation regarding this issue?

Briefly, the three radioactive elements you mentioned comprise the majority of the naturally occurring radioactive substances found in building materials such as brick, soil building materials such as adobe, and wood products. The radium equivalent was developed as a method to estimate the gamma radiation dose received from building material. The radium equivalent is defined as the radiation associated with 1 milligram (mg) of ²²⁶Ra which is equal to 370 becquerels per kilogram (Bq kg^-1). The United Nations Scientific Committee on Exposure to Atomic Radiation (UNSCEAR) in 1982 estimated that one radium equivalent will result in a radiation dose of about 1.5 millisieverts (mSv) per year to inhabitants.

The radium equivalents for ²³²Th and ⁴⁰K are 259 Bq kg^-1 and 4,810 Bq kg^-1, respectively. You can find additional information on the radium equivalent unit in the open access article by Darwish and coworkers. Although ⁴⁰K and ²³²Th comprise the majority of the radioactive materials in building materials, it is possible to calculate the radium equivalent for any radioactive substance that emits gamma radiation.

You also asked about international regulations for the use of building materials. There are no international regulations for the use of building materials; however, the International Atomic Energy Agency (IAEA) has recommendations to protect the public from indoor radiation exposures from building materials. The IAEA recommendations are found in their Specific Safety Guide SSG-32. Basically, their recommendations mirror those of the International Commission on Radiological Protection that states any intervention is justified if the radiation dose is 100 mSv per year or greater.

To summarize my response, the radium equivalent is used to estimate the radiation dose received from building materials which naturally contain radioactivity. Since ²²⁶Ra, ²³²Th, and ⁴⁰K comprise the majority of naturally occurring radioactive elements in building materials, the radium equivalent is used to evaluate these radioactive elements. Furthermore, the international community does not have any regulations limiting the use of building materials; however, the IAEA does have recommendations to limit the radiation dose, and those recommendations are that protective measures should be taken if the radiation dose is 100 mSv per year or greater.

Paul A. Charp, PhD