Workplace Exposure Limits

Workplace exposures limits are intended to protect workers from excessive exposure to toxic chemicals in the workplace. These limits typically incorporate safety margins to ensure that workers won't be overexposed to hazardous chemicals. Generally, employers must ensure that these limits are not exceeded.

The most common workplace limits are:

• IDLHs (Immediately Dangerous to Life and Health limits)
  
• TLVs (Threshold Limit Values)
  
• RELs (Recommended Exposure Limits) -- developed by the National Institute for Occupational Safety and Health (NIOSH)
  
• PELs (Permissable Exposure Limits) -- developed by the Occupational Safety and Health Administration (OSHA)

How does ALOHA use workplace exposure limits?

Most people use ALOHA for emergency response and planning purposes, where the goal is to assess the severity of a hazard to the general public when a short-term, hazardous chemical release has occurred.

We do not recommend using workplace exposure limits in ALOHA to assess a toxic chemical release where the public is (or may be) exposed because:

• Workplace limits are designed to protect healthy, adult workers. Because age, health, and exertion influence how susceptible people will be to a pollutant, it's possible that some workplace limits may underestimate risk to the sensitive portions of the general population (such as old, sick, or young people).
• Some workplace limits are designed to protect workers from long-term, repeated exposure to chemicals over the course of a working lifetime--which is very different than protecting people from a single exposure during an emergency situation.

Instead, we recommend using public exposure guidelines (such as AEGLs, ERPGs, and TEELs) as your toxic Levels of Concern (LOCs) when modeling these types of releases in ALOHA. (A toxic LOC is the value above which the toxic gas concentration might be high enough to harm people.) Today, most common toxic chemicals are defined under at least one of the public exposure guidelines.

However, you may occasionally be faced with a chemical for which no public guideline exists. No clear rules describe what you should do if you're in this situation, and your goal is to assess the hazard to the general public posed by a release of that chemical. One option is to use a workplace exposure limit (such as an IDLH limit). In fact, before public guidelines were commonly available, the IDLH limit was used in public exposure situations. For example, Technical Guidance for Hazards Analysis, which was developed in 1987 by the Environmental Protection Agency and other federal agencies to provide guidance for hazard planning, primarily used 1/10th of the IDLH limit.

When modeling a toxic chemical release, ALOHA will provide you with default toxic LOC options based on a hierarchy of well-known exposure values: AEGLs are used preferentially, then ERPGs, then TEELs, and then the IDLH value. ALOHA allows you to select up to three toxic LOCs. ALOHA uses those LOCs and other scenario information to generate a threat zone plot where red, orange, and yellow zones indicate areas where the LOCs were exceeded at some time after the chemical release began. The red zone typically indicates the most hazardous LOC chosen.

Note: You could use TLVs, RELs, or PELs as your toxic LOCs, if they are appropriate for your scenario. However, none of these values are included in ALOHA, and you would have to specify the values yourself. (For more information on setting your own LOCs, read the Ask Dr. ALOHA article on choosing toxic LOCs.)