“All things are poison, and nothing is without poison; the dosage alone makes it so a thing is not a poison.”  — Paracelsus

Have you ever noticed that news accounts about incidents in the process industries never talk about “chemicals”? No, the writer always refers to “toxic chemicals”. As though you can’t say one without the other. Perhaps they are taking Paracelsus’ philosophical view, that all substances can be toxic at a high enough dose. Even water.

(Our bodies can only eliminate about 1 liter of water per hour, so drinking water at faster than that rate has the potential to cause water toxemia, i.e. water poisoning. Symptoms can include head pain, cramping, spasms, or weakness, nausea or vomiting, drowsiness and fatigue, and extreme cases, seizures, loss of consciousness, and death.)

But no, reporters use the phrase “toxic chemical” as a lazy shorthand to give their stories an edge, an urgency that they might not otherwise have. That said, however, we should always keep in mind that we work with substances that are toxic.

Measuring Toxicity

The toxicity of substances is measured and reported in terms of concentrations that can lead to a harmful exposure, or in some cases, can avoid a harmful exposure. Two concentrations that are often reported as safe limits are TLVs and PELs.

Threshold Limit Values—TLVs—are established by the American Conference of Governmental Industrial Hygienists (ACGIH), a private not-for-profit organization founded in 1938. The TLV is the concentration that the ACGIH believes a worker could be exposed to for the entirety of their working lifetime and not experience harmful effects. The ACGIH updates their TLVs as new data comes available. Typically, the ACGIH report TLVs in terms of ppmv in air.

Permissible Exposure Levels—PELs—are regulatory limits set by the Occupational Safety and Health Administration (OSHA). They are enforceable levels and exceeding them can result in citations and penalties. OSHA often sets them equal to the TLVs established by ACGIH, but doesn’t update them very often. They are also typically reported in terms of ppmv in air.

Unsafe Limits

At the other end of the spectrum are the limits above which harm can be expected. Three types of values are often reported as unsafe limits: LC50, IDLH, and ERPG levels.

LC50 is the concentration at which half of the exposed population would die. If you are an optimist, it is the concentration at which half the exposed population would survive. It is a measurement from a study, and the tests are usually done on mice or rats, or other lab animals. We don’t do LC50 studies on humans—that would never make it past the bioethics committees—and so infer LC50 for humans from other animal studies.

IDLH is the concentration that is Immediately Dangerous to Life or Health. IDLH is established by the National Institute for Occupational Safety and Health (NIOSH). NIOSH is part of the U.S. Centers for Disease Control and Prevention (CDC), which is in the U.S. Department of Health and Human Services. NIOSH is a federal agency, but it has no enforcement authority. NIOSH is not part of OSHA, but OSHA relies on NIOSH for much of OSHA’s rulemaking.

NIOSH develops IDLH levels to define acute exposures that can overwhelm workers, impairing their ability to escape from unsafe conditions, which in turn can lead to death. Working in levels above the IDLH should only be done while wearing highly reliable breathing apparatus. IDLH levels do not address chronic exposures.

Emergency Response Planning Guidelines

There are three Emergency Response Planning Guideline (ERPG) levels: ERPG-1, ERPG-2, and ERPG-3. They are set by the American Industrial Hygiene Association (AIHA), a private not-for-profit organization founded in 1939. The AIHA makes a point of stating that ERPG levels “cannot be overly conservative: no emergency responder wants to evacuate a downtown area to protect a population from a very mild health effect. This is quite a different approach from other groups setting population guidelines, for example, drinking water, residue tolerances in food, air quality guidelines, etc. ERPG values need to be exactly on target to support emergency responders to predict the frequency and severity of health effects that may result from the emergency exposure.”

ERPGs are prepared for emergency responders. The three levels establish different levels of response.

  • ERPG-1 The maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing more than mild, transient adverse health effects or without perceiving a clearly defined objectionable odor.”
  • ERPG-2 The maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual’s ability to take protective action.
  • ERPG-3 The maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects.

Interestingly, the endpoint concentration for toxic substances that the Environmental Protection Agency (EPA) uses in its Risk Management Planning rule (RMP), in 40 CFR 68, is typically the ERPG-2 level. This is the concentration to which people could be exposed for an hour without experiencing irreversible or other serious effects.  It is not a lethal concentration.  Contrary to what some advocates will say, the toxic endpoint distance in RMP does not establish a “circle of death”. Instead, it establishes a “circle of serious effects after exposure for an hour.” But whoever got a crowd worked up with that?


OSHA’s Process Safety Management (PSM) standard, 29 CFR 1910.119, and the EPA’s RMP Rule both apply to toxic chemicals as well as flammable chemicals. Neither use a toxicity level to establish coverage, however. Instead, they rely on lists.  The PSM list in Appendix A of the standard has 137 entries. Six of them are synonyms, however, so PSM only addresses 131 toxic chemicals. The RMP list of toxics in 40 CFR 68.130 has 77 entries.

These chemicals are not the most toxic chemicals known to man. They are not even the most toxic chemicals routinely used in commerce. They are a list that two different federal agencies came up with. In fact, there are only 46 chemicals that managed to make both lists. So, don’t conclude that a chemical is not a toxic hazard just because it is not regulated under PSM or RMP.

About Those “Toxic Chemicals”

While the substances we work with may not be as toxic as news reports would suggest, everything has some level of toxicity. It is important that we understand the toxicity of the chemicals we work with, and that we understand the measures of that toxicity. We can manage them, as long as we know what we’re doing.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.