Nuclear radiation

QUESTION: Is it true that granites and marbles release small amounts of nuclear radiation?

Govada Suri, Tenali, A.P.

ANSWER: Granite and marble, like other naturally occurring substances like air, water, minerals etc contain tiny but measurable quantities of radioactive elements in different proportions.

Uranium, thorium and their daughter products like radium, polonium, radon, thoron, are generally present.

The amount of radioactivity emitted by the rocks varies from place to place. It is estimated that on an average, in each kilogram of granite, about 60 to 70 atoms of uranium and about 10 atoms of thorium and their associated elements emit radiation every second.

Further, close to 1200 atoms of potassium give out radiation each second.

Radiation emission due to uranium and potassium in marble (which is derived from limestone) is about one tenth of what is found in granite.

But, the radiation emission due to thorium in marble, is almost the same as in granite.

The radioactivity levels in granite and marble are too small to be of any concern. All construction materials, in general, give off a small amount of radon gas, which is radioactive.

The gas can accumulate in poorly ventilated buildings, usually found in countries with very cold climate. Inhaling the accumulated radon could give rise to a small degree of radiation exposure.

Therefore, several western countries have prescribed the acceptable limits for radon emanation from the building materials.

Granites from India and Egypt have very low emanation rates, and are by far the best in the world in this regard.

J. Daniel Chellappa, IGCAR, Chennai