A physics professor at Rice University is warning of a radioactive threat found in some kitchen countertops.

Some granite countertops contain levels of uranium high enough to be dangerous to humans, said Rice professor W.J. Llope.

Using a spectrometer, Llope tested 25 varieties of granite bought from Houston-area dealers. In some cases, he said, he found countertops that could expose homeowners to 100 millirems of radiation in just a few months �� the annual exposure limit set by the Department of Energy for visitors to nuclear labs.

Scientists at the national geophysical research institute (NGRI) of India have disturbing news for residents of Hyderabad city especially those living in rocky Banjara and Jubilee hills area. They have found that the granite rocks of Hyderabad have abnormally high concentrations of radioactive uranium and thorium compared to elsewhere in southern India. Team has measured the radioactivity of rocks from nearly 2,000 locations in the states of Karnataka, Tamil Nadu and Andhra Pradesh and nowhere did they find it to be as high as in Hyderabad.

Rocks in the western part of Hyderabad are more radioactive compared to those in the east. Rocks in the posh areas of jubilee and Banjara hills have twice as much uranium as found in Uppal in the southeastern part of the city.

The uranium content of Hyderabad granites varied from 10 parts per million (ppm) to 25 ppm in contrast to 0.23 ppm for Chennai and 1.7 ppm to 7.5 ppm for Bangalore. The thorium content of Hyderabad granites was also found to be four to five times higher than that of Bangalore. These high values of radioactive elements could pose a health hazard.

According to A. M. El Arabi, N. K. Ahmed and K. Salahel Din of Physics Department, South Valley University, Qena, Egypt, the average dose rates values for outdoor and indoor air for Elba granites of Egypt are found to be three times higher than the world average. Whereas, the corresponding average values for Qash Amir and Hamra Dome granites are five and six times higher than the world average, respectively. Thus, this information is an important alert for the local people to avoid the use of these granites in the construction of dwelling without radioactivity control.

While most experts agree that only a small percentage of granite in homes today poses any health risk, the current debate centers on identifying granite that might emit radiation and determining under what circumstances a danger occurs.

People living in granite areas or on mineralized sands receive more terrestrial radiation than others, while people living or working at high altitudes receive more cosmic radiation. A lot of our natural exposure is due to radon, a gas which seeps from the earth's crust and is present in the air we breathe.

It has been established that human exposure to radioactivity comes mainly from natural sources. The natural radiation to which the general public is exposed consists of two components, namely, internal exposure and external exposure. Internal exposure is due to the inhalation of radon gas in the air and the intake of traces of radio nuclides in food

and drinking water. External exposure arises from terrestrial gamma rays and cosmic radiation incident on the earth´s surface. In fact, only about 15% of the total effective dose is derived from cosmic radiation and about 0.6% is attributable to cosmogenic radio nuclides. The members of the radioactive decay chains of 232Th (14%), 235U and 238U (55.8%), along with 40K (13.8%) are responsible for the main contributions to the dose from natural radiation, while a more than 0.3% is due to the effect of 87Rb.

Many natural rocks contain radioactive elements such as 238U, 226Ra, 232Th and 40K. Although these radio nuclides are widely distributed, their concentrations depend on geological and geographical conditions and as such they vary from place to place.

In geology, rock is a naturally occurring aggregate of minerals. Rocks have had a huge impact on the cultural and technological advancement of the human race. Rocks especially granite have been used by Homo sapiens and other hominids for more than 2 million years. The prehistory and history of civilization is classified into the Stone Age, Bronze Age, and Iron Age. Although the stone age has ended virtually everywhere, rocks continue to be used to construct buildings and infrastructure.

But now a days rock (granite) are now seen as source of dangerous radioactivity. It is in the form of natural background radiation which affects the humans. Humans have always been exposed throughout their period of existence to naturally occurring ionizing radiation.

Geologically the term granite is placed under felsic or acidic divisions. It refers to a rock composed mainly of quartz and feldspar as essential minerals. The dark minerals like biotite, tourmaline and few of amphiboles groups, etc. occur as minor constituents of granite. Granite is the typical example of relatively coarse- grained plutonic rocks that crystallized slowly in large masses within the crust.

Granite is actually rather radioactive and has 5 to 20 times the concentration of uranium compared to other common rock types. Some health concern exists in areas that are rich in granitic terrain, as background radiation is enhanced by the presence of large granite bodies. Although the uranium is generally not concentrated enough to make granite a uranium ore, the leaching and erosion of granite has helped produce most of the uranium ore deposits around the world.

Some granites contain around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities.

Granite has been extensively used as a dimension stones and as flooring tiles in public and commercial buildings and monuments. Because of its abundance, granite was commonly used to build foundations for homes in New England. With increasing amounts of acid rain in parts of the world, granite has begun to supplant marble as a monument material, since it is much more durable. Polished granite is also a popular choice for kitchen countertops due to its high durability and aesthetic qualities.

People using granites, containing high uranium, for decorative purpose inside house may be affected with radiation.

People of Ranchi and other parts of Jharkhand state of India are frequently using polished granites for different decorative purpose without knowing how much uranium is present in the stone. Author has earlier warned the people of Ranchi about the possibility of radioactivity in Ranchi rocks. People using local granites or brought from Hyderabad, for decorative purpose, should be more cautious.

Even the granites of the Daltonganj area of Jharkhand state contain anomalous uranium values. Uranium mineralization has also been observed in the granitic rocks comprising the southern periphery of the Hutar basin of Daltonganj area. The Proterozoic granitoids, forming the provenance for the Hutar and Auranga subbasin, have been analyzed which revealed uranium content up to 520 ppm. ( Virnave, 1999).

As demand for granite has increased, exotic stones are being imported from remote corners of the world and greater scrutiny is needed. Lots of varieties of granite are sold for household use in the Jharkhand State. None of them is routinely tested for radioactivity. Even the businessman selling granites are in regular contact with the radiation.

People must go for alternative decorative stones like sandstone or marble, having low uranium, other than using radioactive granites. Even if they are using granites, their houses should be proper ventilated so that the poisonous gases can be flushed out.