Threshold Limit Value (TLV)

A threshold limit value (TLV) is a recommended limit on the amount of a dust, gas or vapor a worker may breathe, averaged over a working day. For asbestos, the American Conference of Governmental Industrial Hygienists (ACGIH) listed a limit of 5 million particles per cubic foot of air (mppcf) by 1947¹, a figure taken from a 1938 Public Health Service study². That number stood for more than twenty years and was then replaced by fiber counts that fell, step by step, from 12 fibers per milliliter to 0.2 fibers per cubic centimeter^{3, 4}. The documents in our library show how companies measured their plants against the TLV, how some of their own engineers and doctors questioned it, and how the TLV was argued over in court.

Where the number came from

In 1938 the U.S. Public Health Service published a study of three asbestos textile plants by Waldemar Dreessen and others. Because clear cases of asbestosis were found only above 5 million particles per cubic foot, the authors proposed that figure "tentatively" as a threshold "until better data are available."² At its 1947 meeting the ACGIH accepted a list of "maximum allowable concentration" values that put asbestos at 5 mppcf; the committee's chairman, Dr. L. T. Fairhall, said the values were not permanent and had to weigh health against the effect on "the manufacturing operation."¹ The Fifth Circuit later described the ACGIH as "a quasi-official body" and noted that its first asbestos guideline dated from 1947⁵.

The count was of all particles seen under a light microscope, not of asbestos fibers alone⁶. A 1965 critique by E. L. Schall pointed out that the Dreessen counts included cotton and rock dust, that most of the workers studied had worked fewer than five years, and that average counts hid peaks as high as 211 mppcf⁶. The 1946 Fleischer-Drinker survey of Navy shipyard pipe coverers found many dust counts above the 5 mppcf figure and said it was "manifestly impossible to set a threshold" for that trade⁷.

Doubts in the industry's own files

Within months of the ACGIH vote, an engineer reporting to the Asbestos Textile Institute questioned the limit. W. C. L. Hemeon's June 1947 report warned that keeping dust under 5 million particles "does not positively insure" that workers would escape asbestosis, and that very thin fibers were invisible in the counting microscope⁸. In 1951 Hugh M. Jackson, Johns-Manville's safety engineer, asked Dr. Arthur Vorwald of the Saranac Laboratory whether one million fibers per cubic foot would be a satisfactory standard, adding that "there is little concrete evidence to substantiate any particular figure."⁹ By April 1954 Jackson told J-M's engineers that the company had "tentatively established" 1 million fibers per cubic foot for asbestos in place of the ACGIH particle limit¹⁰. Minutes of a 1953 Asbestos Textile Institute committee meeting record that New Jersey was revising its own limits and might add one based on fibers¹¹.

How companies measured against it

J-M's industrial health program set out rules in 1958 for applying TLVs to mixed dusts¹², and papers given at a 1963 J-M managers' meeting listed as a goal bringing working conditions "within the safe health threshold limits."¹³ A 1974 conference of J-M industrial hygienists heard that plants concentrated on stations "over TLV at the expense of stations reported as being under TLV."¹⁴

At Union Carbide, a January 1965 memo sent sales staff a statement by Dr. C. U. Dernehl, its director of toxicology, for customers asking about asbestos and cancer¹⁵. The statement called 5 mppcf "the Threshold Limit Value for asbestos" and said no cases of asbestosis were known where exposures stayed at or below it¹⁵. A 1963 Union Carbide shop survey recalled that earlier tests of insulation blocks had found Thermobestos "exceeded TLV" when cut, while Kaylo was "within TLV."¹⁶ In later litigation Union Carbide answered that it had adhered to the ACGIH asbestos values from 1963 until 1972¹⁷.

From particles to fibers

The ACGIH's 1968 notice of intended changes proposed cutting the particle limit to 2 mppcf, with an alternative of 12 fibers per milliliter longer than 5 microns¹⁸. The proposed fiber method used membrane filters and a phase-contrast microscope at about 430 times magnification^{19, 20}. In its 1970 booklet the ACGIH kept 5 mppcf on its adopted list but gave notice of a proposed 5 fibers per milliliter²¹. J-M's own Executive Bulletin 312-3 of October 1969 told plants that new dust systems must hold asbestos below a company limit given as 2 fibers per cubic centimeter²².

Federal rules followed. A 1969 Union Carbide plant manager's letter noted new federal standards, published in January 1969, for companies doing business with the government²³. After the Occupational Safety and Health Act took effect, J-M's G. L. Swallow told plant managers in October 1971 that stations "at but not over" the TLV would now be treated as under it²⁴. On December 7, 1971, an emergency federal standard of 5 fibers per milliliter replaced the earlier 12-fiber value³. The 1972 standard set a limit of 2 fibers per cubic centimeter to take effect in 1976²⁵. Union Carbide told OSHA in 1972 that it could meet 5 fibers but that the 2-fiber level proposed by NIOSH could not be reached "by any presently known combination of engineering installations."²⁶ In Britain, Dr. Hilton Lewinsohn noted in 1977, a 2-fiber standard had been published in 1968 and officially applied in 1970²⁷.

Lower limits and the idea of a safe level

The particle and fiber limits were not offered as lines between safe and unsafe; the ACGIH's own 1970 preface said so²¹. Lewinsohn's own conclusion in his 1977 talk was that "we do not know for sure," but that the evidence pointed toward the risk of asbestosis and lung cancer being "reduced to virtually nil" when exposure was adequately controlled; he then quoted Dr. Roach that "no exposure ... can be said to be absolutely free of risk" and that meeting a TLV is "unlikely to reduce it to zero."²⁷ Degussa, a German company that had an agreement with Union Carbide on asbestos thickening agents, wrote in 1972 that since 1968 it had become doubtful whether the accepted TLV could be considered reliable²⁸. Union Carbide's John L. Myers replied internally in 1972 that asbestos was "proven to be harmful only when TLV's are exceeded for 10-30 years."²⁹

When NIOSH proposed 0.1 fiber per cubic centimeter, the industry's trade group, AIA/NA, said there was no evidence that the existing 2-fiber limit was unsafe and that fibers could not be counted reliably at 0.1³⁰. A 1979 Union Carbide plan for its asbestos business said a NIOSH limit of 0.1 fiber/cc was "probably not attainable" by its plant or customers³¹. In 1980 the ACGIH said it would recommend separate values for chrysotile (2 fibers per milliliter), amosite (0.5) and crocidolite (0.2), and the Asbestos International Association wrote to protest that this would virtually ban amosite and crocidolite³². OSHA lowered its permissible exposure limit to 0.2 fiber per cubic centimeter in June 1986⁴, and to 0.1 in 1994.

The TLV in litigation

The limit became a defense argument early. In the Borel case file, a defendant answered that dust counts where insulators used its products had been below the ACGIH threshold limit value³³, and the defendants' joint motion argued that 5 mppcf had been regarded as a safe guideline during the plaintiff's working years³⁴. In a deposition, Dr. Cooper, whose dust research had been financed by the Public Health Service, explained that the ACGIH published a list of threshold limit values every year³⁵. The Fifth Circuit's 1973 opinion noted that none of the defendants had tried to learn whether insulators' exposures exceeded the ACGIH values, "or indeed, whether those standards were accurate or reliable."⁵

Plaintiffs' standard interrogatories later asked each defendant what year it was first advised of the ACGIH limits³⁶. In 1976 J-M told OSHA that its dust controls had aimed below the 5 mppcf limit in force before December 1971³⁷. Raybestos-Manhattan's John H. Marsh wrote in 1977 that it was "quite unfair" to expect the industry to know more about asbestos than the Public Health Service³⁸.

A TLV is a recommendation from a professional body; the limit that binds employers is OSHA's permissible exposure limit.

Related Terms

Permissible Exposure Limit, Million Particles per Cubic Foot (mppcf), Fibers per Cubic Centimeter (f-cc), Time-Weighted Average (TWA), Midget Impinger, phase contrast microscopy (PCM), Dose-Response, Cumulative Exposure, Fiber-Years, Linear No-Threshold Model (LNT), Fleischer-Drinker Study, Saranac Laboratory, OSHA, NIOSH, State of the Art Defense

References

1. American Conference of Governmental Industrial Hygienists Adopts Its 1947 Limits, Including Asbestos (1947)
2. Dreessen and Others, A Study of Asbestosis in the Asbestos Textile Industry (1938)
3. UC-1432: OSHA Standards and the Safe Use of Asbestos, p. 4
4. Federal Register: Occupational Exposure to Asbestos, Tremolite, Anthophyllite, and Actinolite (1986), p. 1
5. Borel v. Fibreboard Paper Products Corporation (5th Cir. 1973), the court's opinion
6. E. L. Schall, Present threshold limit value in the U.S.A. for asbestos dust: a critique (1965)
7. Fleischer, Viles, Gade and Drinker, "A Health Survey of Pipe Covering Operations in Constructing Naval Vessels" (1946)
8. W. C. L. Hemeon, Report of Preliminary Dust Investigation for the Asbestos Textile Institute (1947)
9. H. M. Jackson to Arthur J. Vorwald Asking for a Fiber Count Standard (1951)
10. H. M. Jackson to G. S. Smith on J-M's Own Threshold Limit for Asbestos (1954)
11. Asbestos Textile Institute Air Hygiene Committee Meeting with Company Doctors on Medical Examinations (1953), p. 3
12. H. M. Jackson to All Industrial Relations Managers on Threshold Limit Values for Mixed Dusts (1958)
13. Dr. Kenneth W. Smith and C. L. Sheckler at the J-M Industrial Relations Managers' Meeting (1963), p. 11
14. Johns-Manville Environmental Control Department Industrial Hygienists Conference Minutes (1974)
15. UC-119: Union Carbide Asbestos Toxicology Reports and Related Memos, including the January 1965 memo, pp. 1–3
16. Union Carbide 63: R. J. Sexton to S. C. Giambruno, Industrial Hygiene Survey of Insulation Block Dust (1963), p. 3
17. UC-1991: Union Carbide's Response to Plaintiff's First Standard Set of Liability Interrogatories, In re New York City Asbestos Litigation, p. 34
18. UC-991: E. A. Piersall, Bureau of Mines King City Asbestos Dust Health Study (1972), p. 7
19. Lewis J. Cralley, Dust Sampling Instruments and Dust Standards in the United States for Asbestos (1969), p. 3
20. UC-1557: Bayer, Zummalde and Brown, Equipment and Procedures for Mounting Millipore Filters and Counting Asbestos Fibres (1969), p. 3
21. American Conference of Governmental Industrial Hygienists, Threshold Limit Values for 1970, with a Proposed Asbestos Limit of 5 Fibers (1970)
22. Johns-Manville Executive Bulletin 312-3 on Environmental Control Standards, Revised (1969)
23. UC-151: T. Frank Tobin to Union Carbide on Asbestos Resin and Respirators, with J. A. Riddle's Reply (1969), p. 2
24. G. L. Swallow to J-M Plant Managers on Counting Stations at the TLV as Under It (1971)
25. Asbestos Magazine, July 1972, p. 7
26. UC-2914: James W. Rawlings to the Office of Safety and Health Standards on the Proposed OSHA Asbestos Amendment (1972), p. 2
27. Hilton C. Lewinsohn, "Medical Aspects of Occupational Exposure to Asbestos" (1977), p. 18
28. UC-1578: Degussa to J. W. Rawlings on Degussa's Withdrawal of Asbestos Thickening Agents (1972), p. 4
29. UC-1523: John L. Myers to J. W. Rawlings on Degussa's Criticism of A-14 and A-28 (1972), p. 1
30. Asbestos Magazine, January 1977, p. 34
31. Union Carbide 528: Asbestos Business Component Plan Summary (1979), p. 1
32. UC-3937: Asbestos International Association Director-General's Notes for Associations (1980), p. 17
33. The Borel v. Fibreboard Case File, Pittsburgh Corning's supplemental answers to interrogatories, p. 3
34. The Borel v. Fibreboard Case File, Defendants' joint motion for summary judgment (1970), p. 47
35. The Borel v. Fibreboard Case File, Deposition of Dr. Cooper (September 20, 1971), p. 12
36. UC-1988: Union Carbide's Supplemental Responses to Plaintiffs' Master Interrogatories, Cameron County, Texas (1995), p. 59
37. Comments of Johns-Manville Corporation With Respect to Notice of Proposed Rulemaking, Occupational Exposure to Asbestos (1976), p. 25
38. John H. Marsh to Guy Gabrielson on the History of Asbestos Exposure Limits (1977)