Health, Safety, Security and Environment

Overview of TLV and BEI and how they can make a big difference to workplace safety.

6 min read

Safety professionals deal with hazards every day. That’s a given. We delve into the world of laws, acts, regulations, and standards as a matter of course, and use what we find to build systems that keep our employees, workplaces, and the public safe.

We also deal in a world of guidelines, codes, and practice, all meant to help us control health hazards, risk, and loss.

When it comes to chemical substances and physical agents that can harm, injure, or kill, all safety professionals need to be very well versed in Threshold Limit Values (TLV) and Biological Exposure Indices (BEI).

Takeaway: The Threshold Limit Value is not an absolute demarcation but a guide. Different people can react differently to the same level of exposure.

In this article, I’ll give you an overview of TLV and BEI and why knowing them can make a big difference to workplace safety.

Industrial Hygiene

A bit of a disclaimer before we get started.

You might not be an industrial hygienist, but you will need to consult with them or work with them to ensure your workplace is truly safe.

And understanding industrial hygiene means understanding the technical language that comes with it. Here are just a few terms you need to know when it comes to TLVs:

  • Time-weight average (TLV TWA)
  • Short-term exposure limit (TLV STEL)
  • Ceiling limit (TLV-C)
  • Excursion limits
  • PPM
  • mg/m3

Yes, that means there is some math involved. That’s the nature of the subject. And on top of that, most of the industrial hygiene world speaks in metric.

What Is Threshold Limit Value?

Threshold Limit Values and Biological Exposure Indices are health-based values. TLVs are based on airborne concentrations of chemical substances.

TLVs are guidelines set by the American Conference of Governmental Industrial Hygienists (ACGIH) and represent the level of exposure the organization believes most workers can experience without adverse health effects. Airborne concentrations of a substance that exceeds its TLV are, therefore, considered harmful to an average person.

TLV is generally expressed as a time weighted average (TWA) or short term exposure limit (STEL) in parts per million.

(Learn more about Exposure Limits )

Why a Guide for TLV? Why Not a Standard?

That was one of the first questions I asked when I was studying Industrial Hygiene in university. The answer was an eye opener and it explained a lot of the things I had seen in the workplace.

The answer I got is that threshold limit values represent a scientific opinion, one based on a review of the existing peer-reviewed scientific literature.

An opinion. That’s what TLV is. But it’s a really, really good opinion about the safe levels of exposure to chemicals and physical agents found in the workplace.

The reason we’re dealing with an opinion is because the people are affected very differently by exposures of the same level.

Think about allergies, for instance. Cat dander might make you sneeze, make your eyes water, cause you to break out in hives. But that doesn’t mean the cat inherently has these effects – someone else could sleep with the cat resting against their head and have no allergic reaction whatsoever.

People are different, and the way they react to substances can be different. That’s why these values and indices are guidelines, not standards.

Exposure Effects Can Happen Below the TLV

Keep the fact that people have different susceptibilities in mind. Some individuals can experience adverse health effects when exposed to an airborne concentration of a chemical substance or dermal exposure well below the TLV.

This can be due to lots of different factors, like age, genetics, lifestyle choices, substance use, prescription medications, and pre-existing conditions like asthma or heart disease.

The frequency and severity of the exposure can also have this result. Just like an allergy, you can become sensitized to a chemical, chemical mixture, or physical agent over time.

Also consider work schedules, work conditions, and temperatures and air pressure outside of the normal range. For example, some asthmatics who are not normally sensitive to small amounts of plastics smoke particulate in the air may have a severe asthmatic reaction when exposed to it while the air temperature is below zero. The cold triggers the attack; the presence of the particulate matter makes it worse.

How BEIs Are Related to TLVs

Biological Exposure Indices are the actual (tested) measurement of the uptake of chemicals and physical agents by a worker, based on a laboratory analysis of samples. They apply to eight-hour exposures, five days per week. They are a comparator to the TLV.

Testing and Sampling

When an exposure is suspected or possible, testing must be done to determine the level of “uptake.”

Air monitoring and urine testing are common test methods. All testing must be conducted using a quality assurance methodology and, according to the ACIGH, you should provide the testing laboratory with a blind challenge specimen to help assess the laboratory’s ability to process, analyze, and report the results properly.

Your decision-making process should not be influenced by a single test sample, but by multiple samples and repeat analysis of a repeat specimen.

Which raises the question: if you have identified a chemical or physical agent in the workplace, should you sample before or after an adverse effect is noted?

Sadly, most sampling is done after the fact, once a health effect has already been noted. But if you want my personal opinion, if you have a potential chemical or physical agent in your workplace, test before, during, and after its use.

Not sure when, what, or how to sample? Talk to an industrial hygienist. They’ll give you the guidance you need.

Not sure of what to do with the results of your testing? Again, you need additional knowledge about how our bodies react to the chemical or physical agent. That means understanding metabolism, distribution, accumulation, excretion, and effects

And yes, BEIs and their values (adopted biological exposure determinants), just like TLVs, are only guidelines. They do not draw a rigid line between safe and dangerous concentrations, nor do they speak to toxicity.

TLV Standards, Regulations, and Guides

OSHA has CFR 1910-1200, Hazard Communication, which details the right of a worker to know about toxic and hazardous substances, as well as the employer’s responsibility to communicate that information.

It’s important to note that OSHA, NIOSH, and the ACGIH all use different terminology in their publications. Truly understanding each definition and how they relate to each other will require a good knowledge of the terms PEL, OEL, and REL, along with TLV and BEI.

When it comes to these measures, the absolute best guide is compiled and updated by the American Conference of Governmental Industrial Hygienists. NIOSH also publishes The Pocket Guide to Chemical Hazards, an excellent resource for the safety professional and the industrial hygienist alike.

Two additional sources of important legislation, administered by the EPA, should be reviewed: the Toxic Substances Control Act of 1976 (TSCA) and the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA).


There are thousands of chemicals and physical agents present in our workplaces. Determining the impacts on worker health is an ongoing and permanent process, one that is literally undertaken worldwide.

As safety professionals, we know that a great deal of the work regarding chemical substances and physical agents is ongoing. Our task is to identify the hazards and risks and implement controls that protect workers.

Threshold limit values are one of the guidelines we can use to ensure our workers are safe, but it is critical to always remember that these guidelines represent averages. While they are scientifically sound, there is a built-in limitation to these values: human variability. Simply put, there’s no way to predict how sensitive each individual is to the chemical substances in your workplace. This means that airborne concentrations of a hazardous substance that fall below the TLV still have the potential to harm some of your workers and may require control measures.

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