The TLV Journey
Reflections on the Evolution and Significance of the TLV-CS Committee
This year we mark the 75th anniversary of the formation of the ACGIH Threshold Limit Values for Chemical Substances (TLV-CS) Committee. For many of us, the ubiquity of tools like the TLVs is often taken for granted. As we celebrate 75 years of the TLV-CS Committee, we also reflect on the hard work and dedication of the Committee’s founding members and on our responsibilities as occupational health professionals to ensure the ongoing scientific validity of this important industrial hygiene resource.
In 1941, the National Conference of Governmental Industrial Hygienists (NCGIH, later named ACGIH) approved the formation of a subcommittee to investigate and recommend a list of maximum allowable atmospheric concentrations of chemicals to ensure the ongoing health and productivity of workers. This subcommittee of the Technical Standards Committee was called the Subcommittee on Threshold Limits. Composed of William Fredrick (chair), Warren Cook, Manfred Bowditch, Philip Drinker, Lawrence Fairhall, and Alan Dooley, this group of founders was assigned the difficult task of compiling and analyzing data to form a set of Maximum Allowable Concentrations (MACs) to create a unified list of atmospheric concentrations that would protect workers and was based on science. At the time, many state and federal industrial hygiene offices did not have a unified reference list to delineate between safe and unsafe concentrations. The MAC list (and later the TLVs) would become one of the first scientific inquiries to establish these levels. In 1956 ACGIH replaced the MAC with the proprietary Threshold Limit Value (TLV), which is still used today. 
While the Committee’s main goal was to make a unified list, the early Committee members had to overcome several hurdles. The first was a lack of scientific data. Industrial hygiene was in its infancy in the early 1940s, and the concept of worker protection from chemical and material exposures was still a novelty. Very few chemicals had been scientifically evaluated with regard to workplace exposure profiles, particularly the long-term health effects at various concentrations. When scientific data wasn’t available, the Committee turned to the professional judgment and expertise of industry professionals.
The second hurdle the Committee faced was determining the definition of the Threshold Limit Value. In the early years there was significant debate regarding the extent to which the limit would or should protect workers. As related in Protecting the Health of Workers: The American Conference of Governmental Industrial Hygienists 1938–1988, in 1946 Dr. William Fredrick, the chair of the Subcommittee on Threshold Limits, wrote:

One concept is that the MAC value should represent as accurately as possible that concentration which a worker exposed for a sufficient period of time will just escape physiological or organic injury and occupational disease. A second concept is that the MAC should represent some fraction of the concentration which will injure the worker in order to allow for a margin of safety in the design of protective equipment and guard against possible synergistic effects in the case of multiple exposures. A third concept is that the MAC should perform the functions of the former concepts and in addition provide a work environment free of objectionable but non-injurious smokes, dusts, irritants and odors. 
In the intervening years, the TLV has evolved into something nearer the third concept, with the basis for each TLV stated clearly in its Documentation. Modern TLVs address prevention of adverse health outcomes including irritation of various bodily systems.
Public support for legalizing marijuana has reached an all-time high in Gallup Polling with 58 percent of Americans supporting the movement. Last January, Fox News reported the stock price for a medical marijuana machine company increased 57 percent after Colorado legalized recreational use. The state reported over $5 million in taxed and regulated sales of marijuana during the first week of legalization and more than $200 million during the first four months. - Eric Nelson and Jeremy Slagley