The key point is fiber length. Long fibers are far less likely to cause lung problems than short fibers simply because they are less likely to be inhaled.

Take asbestos, for example: It is found is short fibers in nature.

Consider, on the other hand, Kevlar and high strength fiberglass. Both are produced in fibers of unlimited length. Only when they are chopped up or abraded are short fibers released. The same is true of fibers of aluminum oxide of unlimited length, which are not only very strong, but utterly immune to oxidation.

If carbon nanotubes are produced in short lengths, they will be very dangerous. If made in long threads, there will be much less danger, at least until the products that they are used in are damaged.

David F Mayer