This response was prompted by the Letter to the Editor from the late Professor Warren Cook [ 19921, in which he claimed that the Threshold Limit Value (TLV) for asbestos, which was proposed in 1946 by the American Conference of Governmental Industrial Hygienists (ACGIH), was 5 mfpcuft (five million fibers per cubic foot of air) , whereas actually ACGIH had recommended 5 million particles per cubic foot of air (5 mppcuft). To the uninitiated, differentiating between particles and fibers may seem like quibbling. To those of us who were knowledgeable about asbestos hazards in 1946, stating fibers instead of particles would have made a large difference, since particles were countable by industrial hygienists, but fiber identification was still in the domain of research by just a few highly specialized laboratories. The TLV was for field testing at work sites. Particles were possible to identify. A fiber TLV was impossible to administer at the myriad places where work exposures to asbestos were already occurring by 1946. Nothing to follow should be interpreted as an attempt to disparage Professor Cook, who was a fine gentleman and a longtime friend. However, his interpretation of the TLV set forth by the ACGIH in 1946 is entirely wrong.
Most asbestos-containing products use only about 15% by weight of the asbestos, the rest being other minerals. Some of the particles in dust from such asbestos are visible to the optical microscope. Only large bundles of chrysotile fibers can be seen optically and the majority are invisible. By 1946, no one knowledgeable about the matter would have written particles for a TLV, if fibers were intended.
By 1946, Dr. Gardner of the Saranac Laboratory had established experimentally that asbestos causes harm because of the fibers in it. However, very few industrial research scientists had yet devised reliable practical methods to field test work sites by fiber counting, for lack of appropriate instruments and personnel. By contrast, particle counting had become a standardized procedure in the USA after Dr. L. Greenburg, Mr. J.J. Bloomfield, and Mr. J.M. Dallavalle (1932Dallavalle ( , 1935) ) had written specifications on how to count particles. By 1946, no one yet had published an official methodology for fiber counting that could be used practically at work sites where fibers were aerosolized when asbestos was worked with. No reliable methodology was offered by Dallavalle of the USPHS [1943] or by Drinker and Hatch, who were research professors at Harvard and Pittsburgh [ 19541. Particle counting was