cate that curls into strong, flexible, needle-like fibers and is chemically and thermally stable because of a misfit between the layers. The overwhelming bulk of asbestos used for commercial purposes is chrysotile, mined principally in Canada. Close study has shown that the incidence of mesothelioma and lung cancer among townspeople in the major chrysotile-producing town, Thetford, Quebec, is about the same as among townspeople in Zurich, Switzerland. Thetford has pervasive chrysotile dust pollution, while Zurich is essentially free of it. From these and other data the environmental hazard of chrysotile asbestos is difficult to prove.

Crocidolite is an amphibole mineral having double chains of silicate tetrahedra linked by sodium, iron, and magnesium. It was mined principally in South Africa and Australia, but its high association with lung cancer incidence in former miners, millers, and fabricators has essentially terminated its use. Amosite is another amphibole mineral, but it lacks the significant sodium or iron found in crocidolite. It too was mined mainly in South Africa, but as with crocidolite the incidence of lung cancer among workers has ended its production and use.

Epidemiological studies have shown that the amphibole asbestos is much more dangerous than chrysotile asbestos; low-to-moderate exposure to chrysotile shows minimal effects on human health (e.g., Report of the Royal Commission on Matters of Health and Safety Arising from the Use of Asbestos in Ontario). However, EPA and other regulatory agencies have assumed that all forms of asbestos are equally dangerous.

Unfortunately, the arbitrary combination of needle-like shapes and a specified fine grain size has been chosen by regulators to characterize asbestos. Not only harmless chrysotile but also a wide variety of other fine-grained, needle-like minerals that are useful and probably of negligible hazard have been nearly banished from use. Minerals also are being judged guilty by association: there is no human health study that shows that the ubiquitous nonfibrous amphiboles are harmful. Nonetheless, the Occupational Safety and Health Administration has proposed, in pending federal regulations, that dust particles of the nonfibrous tremolite, actinolite, and anthophyllite amphiboles be classified as asbestos. A ban on the use of any serpentinite rock—of which noncarcinogenic chrysotile is one type—for commercial purposes is now pending in California; at the same time, serpentine is California's state rock. It should perhaps be noted that in 1969 the common mineral quartz was placed on California's list of carcinogens because tumors were generated in rats breathing quartz dust, and the International Agency for Research on Cancer has since designated quartz as a carcinogen. Quartz, the crystalline form of silica, or SiO2, is the second most common mineral found on the surface of the Earth; it is the major component of most sands.

In view of the force of the federal initiative with regard to mineral dusts, the prognosis for the continued vitality of the U.S. mining industry is ominous. Asbestos regulations and the fear of asbestos are having a crippling effect on the talc and vermiculite industry and on important segments of the mineral aggregate industry. In the future, operations that mine rocks in which amphibole, serpentine, and possibly even quartz are found could be terminated.

Radon

Radon is an invisible, odorless, radioactive, chemically inert gas resulting from the decay of radium, which is itself produced by the radioactive decay of uranium and thorium. There are three isotopes of radon. Two of the isotopes, one with a half-life of a minute and the other with a half-life of less than 4 seconds, decay rapidly to less mobile elements before they can escape from their sites of origin. The third isotope, with a half-life of about 4 days, is the environmental hazard.

Uranium and thorium are radioactive elements contained in minerals that are unevenly distributed within rocks and soils, and some of the minerals break down through weathering. These elements and their radioactive daughter products may be leached from rocks by groundwater and transported into soils. In ordinary soils radium is usually present at a concentration of a few parts per million. If the soil is impermeable, the radon may be trapped in the ground, where it creates no hazard. If the soil is permeable and is not water saturated, the radon daughter product, in the form of a chemically inert gas, may migrate away from its original location and into houses through their foundations. Although radon is the mobile step, it is the subsequent chemically active airborne decay products of radon—radioactive polonium, bismuth, and lead—that can lodge in lung tissues, causing damage through alpha particle emissions. Radon itself is inhaled and exhaled without significant buildup.

Estimates of risk from environmental exposure to radon, like those for asbestos, are extrapolated from excess mortality detected by epidemiological studies of uranium miners, but the extrapolations are highly uncertain. The EPA estimates a 1 to 5



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