Johns-Manville Manual of Standard Practices, Appendix II: Threshold Limit Values for 1958

Five pages of Appendix II to Johns-Manville's Manual of Standard Practices, Industrial Health section, dated September 5, 1958 (replacing the edition of December 9, 1957). It reprints the American Conference of Governmental Industrial Hygienists' threshold limit values for 1958 for gases, vapors, metals and mineral dusts, among them 5 million particles per cubic foot for asbestos. A footnote adds: "J-M standard applied to asbestos exposure is 1 million fibres per cubic foot." The introduction explains that the figures "refer to average concentrations for an eight-hour working shift rather than a maximum which is not to be exceeded even momentarily."

What it shows

By 1958 J-M measured asbestos exposure in its plants by counting fibers as well as dust particles, and set its own fiber standard alongside the published dust-count limit. The manual's introduction also says the limits "should not be regarded as fine lines between safe and dangerous concentrations." Hugh Jackson explained to the company's industrial relations managers how to apply the limits to mixed dusts (December 1958 memo), and the Manville plant's dust surveys report both kinds of count (1957–1959 summary).

In Johns-Manville's chronology

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

Appendix II from J-M Manual of Standard Practices, 9/5/58. Lists TLV's for 1958: The TLV for asbestos is 5 mppcf. with the J-M standard being 1 mfpcf. The standard refers to average concentrations for an 8-hour working shift rather than a maximum which is not to be exceeded even momentarily.