Toxic Levels of Concern

When you are modeling a toxic chemical release in ALOHA, you must choose one or more toxic Levels of Concern (LOCs) to assess the threat. A toxic LOC tells you what level (threshold concentration) of exposure to a chemical could hurt you or other people if you breathe it in for a defined length of time (exposure duration). Generally, the lower the toxic LOC value for a substance, the more toxic the substance is by inhalation. Toxic LOCs also may be referred to as exposure limits, exposure guidelines, or toxic endpoints.

This page discusses the following topics:

• What are common toxic LOCs?
  
• What are ALOHA's default toxic LOCs?
  
• What other toxic LOCs can I use?

What are common toxic Levels of Concern?

The most commonly-used LOCs for air modeling are public exposure guidelines such as:

• AEGLs (Acute Exposure Guideline Levels)
  
• ERPGs (Emergency Response Planning Guidelines)
  
• TEELs (Temporary Emergency Exposure Limits)

Each of these guidelines has three tiers of exposure values (e.g., AEGL-1, AEGL-2, and AEGL-3) for each covered chemical. There are some key differences between the exposure guidelines; however, at a very general level, the tiers are similar: the first tier (e.g., ERPG-1) is a mild effects threshold, the second tier is an escape impairment threshold, and the third tier is a life-threatening effects threshold. Public exposure guidelines like these are intended to predict how members of the general public would be affected if exposed to a particular hazardous chemical.

Most common chemicals will be defined under one or more of these 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. These are usually defined for healthy adult workers, and typically incorporate safety factors to ensure that workers won't be overexposed to hazardous chemicals in the workplace.

However, if you're considering this option, be aware that:

• These limits were designed to protect workers, not for use in assessing hazard to people exposed during an accidental release.
  
• 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 those who are old, young, or sick).

Common workplace exposure limits include:

• IDLHs (Immediately Dangerous to Life and Health limits)
  
• TLVs (Threshold Limit Values)
  
• RELs (Recommended Exposure Limits)
  
• PELs (Permissable Exposure Limits)

What are ALOHA's default toxic Levels of Concern?

ALOHA is used primarily for emergency response or planning situations, where the goal is to assess the threat posed to the general public by a chemical release. For toxic releases, ALOHA uses public exposure guidelines preferentially for the default toxic LOCs, because these guidelines are specifically designed to predict how the general public will respond to a short-term release.

ALOHA determines its default toxic LOCs based on the following hierarchy:

1. AEGLs are considered the best public exposure guidelines to date, because they undergo a rigorous review process, have multiple exposure durations, and are designed as guidelines for nearly all members of the general public--including sensitive individuals. As part of the review process, interim AEGL values are established and then evaluated thoroughly (sometimes over several years) before the final AEGL values are approved. (ALOHA only includes the final AEGLs with a 60-minute exposure duration.) However, because the AEGL development process is so exhaustive, relatively few final AEGLs have been defined.

2. ERPGs are based on experimental data, but unlike AEGLs they are only available for a 1-hour exposure duration and they are not designed as guidelines for sensitive individuals.

3. TEELs are derived using existing LOCs and by manipulating current data. This process is less intensive than the AEGL or ERPG process, and many more chemicals are defined under the TEEL system. By definition, TEELs are temporary limits designed to be used as exposure limits for chemicals for which AEGLs or ERPGs have not yet been defined.

4. IDLH may be used when no public exposure guidelines are defined for a given chemical. IDLH is a workplace exposure limit that is used primarily for making decisions regarding respirator use. In the 1980s, before public exposure guidelines were available for most common chemicals, the IDLH limit was used in public exposure situations. For example, the 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. However, unlike the three-tiered public exposure guidelines, only a single IDLH value is defined for applicable chemicals.

If you model a toxic vapor cloud scenario in ALOHA using the default public exposure guidelines as your toxic LOCs, your threat zone plot will look similar to the one pictured below. The red, orange, and yellow threat zones indicate the areas where the ground-level pollutant concentration is predicted to exceed the corresponding LOC at some time after the release begins.

What other toxic Levels of Concern can I use?

If you prefer, you can enter up to three toxic values of your own (rather than use the default values). On the Toxic Level of Concern dialog box, simply choose User Specified from the pulldown list of LOC values and type in your own LOCs. (Note: The red threat zone LOC should be the highest value and the yellow threat zone LOC should be the lowest value.)

You can also add your own default toxic LOC for any pure chemical in ALOHA's chemical library. For more information on setting your own LOCs, read the Ask Dr. ALOHA article on choosing toxic LOCs (see the list of links below).

For More Information

Ask Dr. ALOHA: Choosing Toxic Levels of Concern This article contains an overview of toxic LOCs (including AEGLs, ERPGs, and TEELs), a discussion on how to choose toxic LOCs in ALOHA, and some suggestions about when you should use LOCs other than ALOHA's standard values.

Ask Dr. ALOHA: Working with Toxic and Odor Thresholds This article describes a scenario where a responder needs to assess a chemical release using both toxic and odor thresholds as LOCs in order to evaluate the threat posed to the public.

Ask Dr. ALOHA: Working with the Concentration Graph This article discusses exposure duration and how to use the concentration graph. The concentration graph shows indoor and outdoor concentrations of a chemical in the air at a specific location for one hour after the start of a chemical release.

ALOHA Overview An overview of CAMEO's hazard modeling program.

ALOHA Tools A collection of tools for ALOHA users.