Dr. Kenneth Smith to Detroit Edison Company on Thermobestos, Mineral Wool and Asbestos (1959)

A two-page letter of February 19, 1959, from Dr. Kenneth W. Smith, Johns-Manville's medical director, to the Detroit Edison Company, which had asked about health hazards of some J-M products. Smith said Thermobestos insulation was mostly calcium silicate with about 10 percent asbestos fiber, and that since the asbestos limit was 5 million particles per cubic foot, "dust levels approaching 50 million particles per cubic foot would not contain an excessive amount of asbestos fibre." He repeated that J-M's mineral wool caused no lung disease, and that although long inhalation of asbestos "may cause some pulmonary changes," J-M had "developed no new cases of asbestosis for many years" in its mines and mills.

What it shows

The letter applied the Threshold Limit Value (TLV) for asbestos to a product's whole dust by its asbestos content, which allowed ten times the asbestos figure. Its mineral wool and asbestos paragraphs repeat, nearly word for word, what Smith had told other customers in 1958 (A. O. Smith Corporation, Aluminum Company of Canada). The same year, J-M's own manual set a company standard for asbestos of 1 million fibers per cubic foot (1958 manual).

In Johns-Manville's chronology

Johns-Manville's own chronology of what the company knew describes this document as follows:

Letter from Kenneth Smith to Detroit Edison Company, 2/19/59. discussing the potential risks of Thermobestos, rock wool, and asbestos fiber. Since Thermobestos is composed of approximately 10% asbestos fiber, Smith recommends that the appropriate TLV should be set at 50 mppcf.