Hazard identification and dose-response assessment information for acute exposure was obtained from various sources, and is presented in Table 2. However, in contrast to the presentation of chronic information in Table 1, no prioritization scheme has been applied and no oral values have been included. That is, Table 2 presents acute inhalation values from multiple sources. We included multiple sources because the various assessments used methods that were different enough that, in our judgment, many are not directly comparable. Therefore, we judged that Table 2 would better serve risk assessors by simply providing this array of assessments. Generally two types of assessments are presented in Table 2 drawing on the following sources.

Assessments estimating exposure levels likely to be without appreciable risk of adverse noncancer effects or for which noncancer effects are not anticipated:

• US Agency for Toxic Substances and Disease Registry (ATSDR). In addition to its chronic minimum risk levels (MRLs), ATSDR also develops MRLs for acute inhalation exposure, which ATSDR defines as 1-14 days. As with ATSDR's chronic MRLs, acute MRLs are estimates of human exposure to a substance that is likely to be without an appreciable risk of adverse effects (other than cancer). Acute inhalation MRLs are published as part of pollutant-specific toxicological profile documents, and also in a table of "comparison values" that ATSDR regularly updates and distributes (available on-line).
• California Environmental Protection Agency (CalEPA). CalEPA has developed acute dose-response assessments for many substances, expressing the results as acute inhalation reference exposure levels (RELs). As with its chronic RELs, CalEPA defines the acute REL as a concentration level at (or below) which no health effects are anticipated. CalEPA's acute RELs are available on-line .

Assessments estimating exposure levels that may be associated with mild (or more severe) effects:

• National Advisory Committee for Acute Exposure Guideline Levels (NAC). EPA's Office of Prevention, Pesticides and Toxic Substances established the NAC in 1995 to develop Acute Exposure Guideline Levels (AEGLs) and supplementary information on hazardous substances for federal, state, and local agencies and organizations in the private sector concerned with emergency planning, prevention, and response. The NAC is a discretionary Federal advisory committee that combines the efforts of stakeholders from the public and private sectors to promote efficiency and utilize sound science. The NAC began  AEGL development with an initial priority list of 85 chemicals in May 1997. The many AEGLs produced to date are available on EPA's website. The AEGLs for a substance take the form of a matrix, with separate levels for mild (AEGL-1), moderate (AEGL-2), and severe (AEGL-3) effects. Each of the effect levels are provided for as many as five different exposure periods, typically 10 and 30 minutes and 1, 4, and 8 hours. Table 2 provides the 1-and 8-hour concentrations for the AEGL-1 and -2, with a superscript that identifies whether the value is final, interim, or proposed.
• American Industrial Hygiene Association (AIHA). AIHA has developed emergency response planning guidelines (ERPGs) for acute exposures at three different levels of severity. These guidelines (available on-line Exit)  represent concentrations for exposure of the general population for up to 1 hour associated with effects expected to be mild or transient (ERPG-1), irreversible or serious (ERPG-2), and potentially life-threatening (ERPG-3). Table 2 provides the ERPG-1 and -2 values.
• National Institute for Occupational Safety and Health (NIOSH). As part of its mission to study and protect worker health, NIOSH determines concentrations of substances that are immediately dangerous to life or health (IDLHs). IDLHs were originally determined for 387 substances in the mid-1970's as part of the Standards Completion Program (SCP), a joint project by NIOSH and the Occupational Safety and Health Administration (OSHA), for use in assigning respiratory protection equipment. NIOSH is currently evaluating the scientific adequacy of the criteria and procedures used during the SCP for establishing IDLHs. In the interim, the IDLHs have been reviewed and revised. NIOSH maintains an on-line database of IDLHs, including the basis and references for both the current and original IDLH values (as paraphrased from the SCP draft technical standards). For substances that lack AEGL and ERPG values, Table 2 provides IDLH values divided by 10 to more closely match the mild-effect levels developed by other sources, consistent with methodology used to develop levels of concern under Title III of the Superfund Amendments and Reauthorization Act, and their use in the accidental release prevention requirements under section 112(r) of the Clean Air Act.
• U.S. Department of Energy (DOE). DOE has defined Temporary Emergency Exposure Limits (TEELs), which are temporary levels of concern (LOCs) derived according to a tiered, formula-like methodology. DOE has developed TEELs with the intention of providing a reference when no other LOC is available. DOE describes TEELs as "approximations of potential values" and "subject to change." The EPA's emergency planning program (section 112(r)) does not generally rely on them, and they are provided in Table 2 purely to inform situations in which no other acute values are available. For example, a finding of an acute exposure near a TEEL may indicate the need for a more in-depth investigation into the health effects literature. TEELs are not recommended as the basis of regulatory decision-making. Like ERPGs, TEELs are multiple-tiered, representing concentrations associated with no effects (TEEL-0), mild, transient effects (TEEL-1), irreversible or serious effects (TEEL-2), and potentially life-threatening (TEEL-3). Consistent with DOE's intent, Table 2 provides the TEEL-1 concentrations for substances that lack acute values from other sources.