Levels of Concern: What Are They and Why Are They Important?

The term “level of concern” is often used in the hazmat response world, however as a hazmat responder have you ever really thought about what the term truly means and how we use the concept in real world responses? 


In simple terms, a level of concern is the concentration of a substance either established by regulation or that is derived through an educated decision process that if exceeded will detrimentally affect human life and well-being. In the hazmat response arena, we use levels of concern to determine the proper personal protective equipment — such as respiratory protection — for responders to use and to guide us in deciding the appropriate measures for protection of the public during hazmat incidents.

Levels of concern are usually expressed in units of parts per million (ppm). One part per million can be visually thought of as if you divided a room into one million equally sized boxes and one of those boxes was one hundred percent full of a substance. As can be seen, one part per million is a very small concentration. If we do the math, we can derive that it takes ten thousand parts per million to equal one percent concentration. Levels of concern are also sometimes expressed in units of milligrams per cubic meter (mg/m3). We can covert mg/m3 to ppm by multiplying the concentration in mg/m3 by 24.45 and then dividing the product by the molecular weight of the substance.

We would be remiss in our discussion if we did not address a few fundamental concepts relating to levels of concern prior to delving deeper into the subject. Levels of concern are often expressed as a time weighted average (TWA) in which the concentration is averaged over a specified period of time (such as an eight-hour TWA). A short-term exposure limit (STEL) is stipulated as the maximum exposure over a short term — usually 15-minute time frame. Hazmat responders may also note levels of concern stated as a ceiling (C) that should never be exceeded for even a split second.

The first level of concern that we will discuss is the Recommended Exposure Limit (REL) that is defined by the National Institute for Occupational Safety and Health (NIOSH). The REL is — just as the name implies — a recommended level of concern that is not legally binding unless formally adopted. I personally remember the concept of the REL as personnel should not be exposed to concentrations in excess of the stated amount. An example of a published REL is that of ammonia at 25 ppm for a 10-hour TWA or 35 ppm STEL. Another level of concern that is not legally binding unless formally adopted but is a consensus recommendation is the Threshold Limit Value (TLV) published by the American Conference of Governmental Industrial Hygienists (ACGIH). The ACGIH is the representative body for industrial hygienists — professionals who monitor hazards in the workplace such as chemical exposures and noise. The TLV for ammonia is 25 ppm for an eight-hour TWA or 35 ppm STEL.

One level of concern that is legally binding is the Permissible Exposure Limit (PEL) stated by the Occupational Safety and Health Administration (OSHA). The PEL of a substance is the maximum concentration a worker can be exposed to and can be thought of as the concentration that shall not be exceeded. Whereas the TLVs published by the ACGIH are regularly updated, OSHA’s PELs have not been updated for quite some time. The PELs can be found in Subpart Z of 29 CFR 1910 (OSHA’s General Industry Standards) in Tables Z-1, Z-2, and Z-3 (collectively known as the “Z Tables”). Table Z-1 lists the PELs for air contaminants, Table Z-2 lists substances with published ceiling values that also have established “peaks” or maximum allowable concentration excursions above the ceiling values with corresponding maximum exposure times; and Table Z-3 lists the PEL’s for mineral dusts. Although OSHA states that the use of personal protective equipment should be the last method selected for controlling exposures — with the elimination or substitution of the hazard; engineering controls, and administrative controls utilized in that order to alleviate the hazard prior to the use of PPE), in the hazmat response world the use of personal protective equipment is usually the only viable option. The PEL can also be practically thought of as the concentration above which respiratory protection shall be worn. By comparison, the PEL of ammonia is 50 ppm for an eight-hour TWA.

NIOSH has also developed a level of concern titled Immediately Dangerous to Life or Health (IDLH), which is the maximum concentration a worker can be exposed to without incurring escape impairing symptoms or irreversible health effects. Prior to 1994 the IDLH value allowed for a 30-minute exposure time at the specified concentration to allow for an escape from the environment, however since that time the IDLH concentration has no associated exposure time and therefore applies to any length of exposure. Practically the IDLH concentration can be thought of as the concentration above which supplied air respiratory protection (such as an SCBA or Supplied Air Respirator [SAR]–also known as an airline respirator) shall be worn. OSHA does define what is termed the Maximum Use Concentration for each type of respiratory protection and in some situations a supplied air respirator is required to be utilized even below the IDLH concentration, however we will not muddy the waters with that discussion at this time. For the purposes of comparison, the IDLH concentration for ammonia is 300 ppm.

The levels of concern that we discussed above generally apply to workers — or in our case hazmat response personnel since we fall under OSHA’s General Industry Standards. When we encounter an incident in which the public is impacted by exposure to hazardous materials we should look at other levels of concern that are more appropriate. These levels of concern include Acute Exposure Guideline Levels (AEGLs) developed by the Environmental Protection Agency that represent concentrations of airborne chemicals above which health effects will occur. The AEGLs are designed to protect the elderly, children, and other civilians susceptible to the effects of hazardous chemicals.

Another group of similar levels of concern are the Emergency Response Planning Guidelines (ERPG’s) developed by the American Industrial Hygiene Association, which estimate the concentrations at which average members of the public will begin to experience health effects if exposed for one hour. A third group of such levels of concern are the Temporary Emergency Exposure Limits (TEEL’s) developed by the U.S. Department of Energy Subcommittee on Consequence Assessment and Protective Actions, which are similar to the ERPGs in that they estimate the concentrations at which most people will begin to experience health effects if they are exposed to a hazardous airborne chemical for a given duration.

The AEGLs, ERPGs, and TEELs all have three levels of categorization, with Level 1 (e.g. AEGL-1) corresponding to the concentration at which mild health effects will occur, Level 2 corresponding to the concentration at which irreversible or other serious health effects leading to the inability to take protective action will occur, and Level 3 corresponding to the concentration at which life-threatening health effects will occur. The accepted level of hierarchy for the use of the three aforementioned levels of concern is the use of AEGLs first, ERPGs second, and TEELs third.

The question that then begs to be answered is “Which level of concern is used for emergency responders and which is used for the public in terms of exposure to hazardous chemicals?” The reality is that there is no one “magic answer” for all situations and each level of concern is but one tool in your hazmat toolbox. We should use our training, education, and experience along with the published data and situational details to determine the correct level of concern to use for any given hazmat situation. As hazmat responders we need to be thoroughly familiar with the concepts discussed above to ensure that we can provide the highest level of safety and protection for ourselves and our citizens.

As always, stay safe out there and be sure to visit the North Carolina Association of Hazardous Materials Responders website at www.nchazmat.com.

Glenn Clapp is a past president of the North Carolina Association of Hazardous Materials Responders and has over 23 years of fire service and emergency management experience. He is currently an Improvement Specialist with the Industry Expansion Solutions Division of North Carolina State University and is a volunteer firefighter with the Fairview Fire Department. He is also a Technician-Level Hazmat Instructor, an Executive Fire Officer, a Certified Hazardous Materials Manager and a Certified Fire Protection Specialist.

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