Big worries about little tubes.

Carbon nanotubes could carry the same dangers to human lungs that asbestos does, says an Environmental Protection Agency researcher. Philip M. Cook of the EPA's Duluth, Minn., research laboratory and other scientists had previously linked the toxic potential of various needlelike fibers to their length, width, and shape. Fibers that damage lungs tend to fall into a size range comparable to some carbon nanotubes, an emerging family of synthetic fibers with a variety of potential industrial applications.

Cook and his colleagues have shown that short, thin mineral fibers are retained longer in the lungs than larger fibers are and that the short fibers can trigger cancer. Some commercially produced carbon nanotubes are 0.1 micrometer wide and 5 to 20 [micro]m long, just about "the asbestos-fiber size that everyone believes is toxic", notes Cook.

In some industrial applications, carbon nanotubes are being commingled with zeolites, which are natural and synthetic silica-based minerals. Cook's research has shown that at least one type of zeolite fiber is even more toxic to the lungs than is asbestos. A separate study last year found that the zeolite fibers' size and the minerals' chemistry contributed independently to toxicity, providing a double whammy.

Cook recommends caution. Until research demonstrates that nanotubes aren't toxic to people, he says, manufacturers shouldn't be commercializing them in sizes that can be readily inhaled.--J.R.