Workplace exposure 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) are developed by the National Institute for Occupational Safety and Health.
• TLVs (Threshold Limit Values) are developed by the American Conference of Governmental Industrial Hygienists.
• RELs (Recommended Exposure Limits) are developed by the National Institute for Occupational Safety and Health.
• PELs (Permissible Exposure Limits) are developed by the Occupational Safety and Health Administration.

ALOHA does not use workplace exposure limits very much, because 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, one-time, hazardous chemical release has occurred. Using workplace exposure limits in ALOHA to assess a chemical release where the public is (or may be) exposed is not recommended 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, you should use public exposure guidelines (such as AEGLs, ERPGs, and PACs) as your toxic Levels of Concern (LOCs) when modeling these types of releases in ALOHA. Most common chemicals will be defined under one or more 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, which is why ALOHA defaults to the IDLH value if AEGLs, ERPGs, and PACs are not defined for a chosen chemical. Note: You could also set your toxic LOCs to TLVs, RELs, or PELs, 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.)