The TLV Journey
Reflections on the Evolution and Significance of the TLV-CS Committee
BY SUSAN ARNOLD AND AMBER ILLIES
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.
TLVs can help an organization define an acceptable level of exposure after adapting it as appropriate to reflect work shift duration and schedules.
AN EVOLVING PROCESS Over the last 75 years, the field of industrial hygiene has been fortunate to witness a scientific and technological revolution. Increasing public awareness of exposure to industrial chemicals and materials led to the creation of public policies and spurred investment in industrial hygiene, toxicology, occupational health, environmental science, and epidemiology. Workplaces and academic institutions published more information regarding dose-response relationships for a variety of chemicals for both animal and human studies. Improvements to sampling methods and data analysis equipment allowed industrial hygienists to quantify exposures at lower levels. The availability of computers and networking capabilities has enabled us to analyze and share data at an astounding rate. 
Today, the TLV-CS Committee has greater access to scientific data, which has resulted in revisions to many TLVs. In some cases, the recommendation has been lowered 100-fold. However, this does not mean that setting TLVs is substantially easier today than in the past. As the respective scientific fields have advanced, the study methodology has become more complex, making it more challenging to conduct these studies and interpret their results. Further, the production of new chemicals and materials as well as modifications of existing materials into the nano-scale are pushing the TLV-CS Committee into new arenas. Today’s Committee faces issues similar to those encountered in the past—evaluating exposure limits for chemicals that the scientific community is on the forefront of investigating. WHAT TLVS ARE—AND ARE NOT Threshold Limit Values (TLVs) are voluntary, health-based guidelines. They are objective opinions expressed by a committee of experts who volunteer their time to establish levels of exposure below which a general population of workers may be exposed over a working lifetime with a reasonable expectation of not being adversely affected by these exposures. TLVs do not represent a consensus position that addresses all issues raised by all interested parties and are therefore different from consensus standards, where the interests of all participating parties are addressed by the number established.
TLVs are the culmination of a focused review of the peer-reviewed scientific literature and reviewed articles. In addition, ACGIH sometimes receives permission to release unpublished data or information that it considered when setting the TLV. At the top of the information hierarchy is data from human exposure and epidemiology and case studies, followed by in vivo and then in vitro studies. Human or animal studies that detail absorption into the body, distribution through the body, metabolism, and excretion from the body are also very important to the TLV process. TLVs are work products generated from rigorous evaluation of this information by Committee experts including toxicologists, epidemiologists, chemists, occupational/industrial hygienists, and occupational physicians, who review the data to identify the likely routes of exposure, mechanism of action, target organs, and manifestation of disease. From this information, the Committee establishes guidelines believed to be health protective. Since ACGIH is a scientific organization, these guidelines are not tied to any regulatory compliance obligation. Further, the TLVs for Chemical Substances and TLVs for Physical Agents Committees do not consider technological or economic feasibility when establishing a TLV.
The TLVs are intended to provide input into the risk characterization process. Assessing and characterizing risk requires an objective, quantitative benchmark, and the TLVs can serve as that health-based benchmark. More specifically, following a careful review of the TLV Documentation for a specific agent to ensure the user understands the basis on which the TLV was set, including the nature of the toxicological endpoint and biological clearance, the TLVs can help an organization define an acceptable level of exposure after adapting it as appropriate to reflect work shift duration and schedules.
TLVs do not represent a consensus position that addresses all issues raised by all interested parties and are therefore different from consensus standards.
THE DOCUMENTATION'S VALUE FOR RISK ASSESSMENT The TLV Documentation is the summary document that presents a concise review of the relevant studies used to develop the TLV, a discussion of the agent’s pathway through the body, relevant critical effects and target organs, along with other pertinent details needed to appropriately apply and interpret the TLV. For example, the substance’s physical and chemical properties are documented. For particulates, the Documentation identifies relevant particle size fraction and the affected region of the lung (inhalable, thoracic, or respirable) and presents major sources of occupational exposures resulting from the production and use of a chemical including production volumes and the number of affected workers. The relevant duration of exposure is also discussed in the context of the type of TLV set: a ceiling value, not to be exceeded; a short-term exposure limit (STEL) referring to a 15-minute duration; or an 8-hour time weighted average (TWA). Following a discussion of the basis for the quantitative TLV and any assigned notations—for example, respiratory or skin sensitizer (R-SEN, D-SEN)—the Documentation examines the limitations of the data and associated uncertainty that should be taken into consideration when using the TLV. Finally, the Documentation includes a list of dates and values of previously proposed and accepted TLVs for the substance, and references of all information utilized in the derivation and documentation of the TLV. 
The Documentation is initially a working document developed by a member of one of the subcommittees, from which the subcommittee members rigorously debate the relevance, strengths, and weaknesses of each study, flushing out the critical target organ, effects, and point of departure (the dose at which the critical effect was observed). The document may go through many iterations before a proposed TLV is presented to the full Committee. Only after a proposed TLV is approved by the full Committee and the recommendation reviewed by ACGIH’s Board of Directors is the TLV placed on the Notice of Intended Changes list and the Documentation made available for public review and comment.
Following the appropriate comment period, any necessary revisions to the proposed TLV and supporting Documentation are completed. The Documentation is then brought before the full Committee for a vote; if approved, it goes to the ACGIH Board of Directors to be ratified. The final version of the Documentation is a concise summary of the relevant state of understanding regarding the potential for exposure, route and mode of action, basis for the TLV, limitations of the science, and associated uncertainty. This final version helps to characterize risk associated with occupational exposure. Since exposure and risk assessment are quantitative exercises, the TLVs can provide an initial quantitative value for organizations, from which considerations regarding organizational nuances such as shift durations and technical and economic considerations can be incorporated and an organizational occupational exposure limit (OEL) derived. THE FUTURE OF TLVS Participation on the TLV-CS Committee is voluntary, but candidates are vetted to ensure they are technically competent, follow best practices, and are recognized within their respective areas of expertise as subject matter experts. Participation is an honor, but also a significant commitment. These dedicated volunteers donate many hours, sometimes hundreds of hours per year, to complete their list of assigned chemicals.
To help ensure a strong supply of experts ready and able to serve in this capacity, ACGIH is establishing an internship program this year. Students in relevant graduate programs will be invited to participate on one of the subcommittees, helping to develop the initial Documentation while being mentored by dedicated Committee experts. There is much work to do, with many chemicals needing to be evaluated. We need input from exposure assessors and field hygienists, chemists, toxicologists, epidemiologists, biostatisticians, and occupational health nurses and physicians. Anyone interested in volunteering can submit resumes and obtain applications from science@acgih.org.
The Committee is planning several educational webinars and looking for opportunities to engage through a range of venues with the community of TLV users. In honor of the Committee’s 75th anniversary, and to help make the TLVs more accessible, ACGIH is considering sponsoring the development of a TLV app that would provide access to the TLVs and critical details for practitioners in the field. 
One of the areas the Committee is actively working to develop involves stakeholder input. The Committee is considering changes intended to make the public comment process more transparent and interactive. This is currently very challenging given the size and nature of this volunteer organization. The Committee values this input and is exploring some ideas on how to facilitate two-way communication. 
There is another important element of sustainable TLVs: the financial support needed to offset the cost of establishing these important guidelines. While the Committee members volunteer their time, there are expenses incurred in establishing the TLVs. A partial list of expenses includes obtaining copies of studies from journals and printing costs associated with disseminating them to the Committee, travel expenses incurred when bringing the Committees together, and editing, printing, and shipping TLVs and BEIs books and Documentation. Consider making a tax-deductible donation to ACGIH in support of the TLVs. 
All in all, it’s been an eventful 75 years. The Committee, the TLV process, and TLVs have evolved, making important contributions to the field of industrial hygiene. Congratulations to the TLV-CS Committee, and keep up the good work! SUSAN ARNOLD, PhD, CIH, FAIHA, is chair of ACGIH and assistant professor of environmental health sciences in the School of Public Health at the University of Minnesota. She can be reached at arnol353@umn.edu or (612) 624-6222. AMBER ILLIES, MS, is a student member of AIHA. She can be reached at illie016@umn.edu.


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