Glossary of Terms and Definitions of Acronyms

ACGIH®: American Conference of Governmental and Industrial Hygienists

ACGIH® STEL: American Conference of Governmental and Industrial Hygienists' threshold limit value short-term exposure limit; a 15-min time-weighted-average exposure which should not be exceeded at any time during a workday. See TLV® Chemical Substances Introduction (available at http://www.acgih.org/tlv-bei-guidelines/tlv-chemical-substances-introduction, 2016).

ACGIH® TLV®: American Conference of Governmental Industrial Hygienists' threshold limit value. The three types of TLV are time-weighted average (TWA), short-term exposure limit (STEL) and ceiling (C). ACGIH® TLVs® are established by committees that review existing published and peer-reviewed literature in various scientific disciplines (e.g., industrial hygiene, toxicology, occupational medicine, and epidemiology). The TLVs® represent conditions under which ACGIH® believes that nearly all workers may be repeatedly exposed without adverse health effects. They are not fine lines between safe and dangerous exposures, nor are they a relative index of toxicology. The TLVs® are not quantitative estimates of risk at different exposure levels or by different routes of exposure. See Statement of Position Regarding the TLVs® and BEIs, adopted by the ACGIH® Board of Directors on March 1, 2002.

AIHA: American Industrial Hygiene Association

AIHA ERPG: American Industrial Hygiene Association's emergency response planning guideline. The ERPG-1 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed up to one hour without experiencing other than mild transient adverse health effects or perceiving a clearly defined objectionable odor. The ERPG-2 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed up to one hour without experiencing or developing irreversible or other serious health effects that could impair their abilities to take protective action. The ERPG–3 is the maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects. The ERPGs are air concentration guidelines for single exposures to agents and are intended for use as tools to assess the adequacy of accident prevention and emergency response plans, including transportation emergency planning, community emergency response plans and incident prevention and mitigation (2015 ERPG/WEEL Handbook of the AIHA Guideline Foundation).

ATSDR: Agency for Toxic Substances and Disease Registry

ATSDR MRL: An estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse noncancer health effects over a specified exposure duration. These substance specific estimates, which are intended to serve as screening levels, are used by ATSDR health assessors and other responders to identify contaminants and potential health effects that may be of concern at hazardous waste sites. MRLs are not intended to define clean up or action levels for ATSDR or other Agencies (http://www.atsdr.cdc.gov/mrls/index.asp).

Benchmark Concentration (BMC): A concentration of a substance that when inhaled produces a predetermined change in the response rate of an adverse effect relative to the background response rate of this effect. This predetermined change is called a “benchmark response” or BMR. See Benchmark Dose Technical Guidance (EPA, 2012).

Benchmark Dose (BMD): A dose or concentration that produces a predetermined change in response rate of an adverse effect (called the benchmark response or BMR) compared to background. See IRIS Glossary. A dose of a substance that when ingested produces a predetermined change in the response rate of an adverse effect relative to the background response rate of this effect. This predetermined change is called a “benchmark response” or BMR. See Benchmark Dose Technical Guidance (EPA, 2012).

Benchmark Response (BMR): A predetermined change in the response rate of an adverse effect relative to the background response rate of this effect. The BMR is the basis for deriving BMDs and BMDLs. See Benchmark Dose Technical Guidance (EPA, 2012).

BMCL: A lower one-sided confidence limit on the benchmark concentration (BMC). See Benchmark Dose Technical Guidance (EPA, 2012).

BMDL: A lower one-sided confidence limit on the benchmark dose (BMD). See Benchmark Dose Technical Guidance (EPA, 2012).

California EPA: California Environmental Protection Agency

California EPA REL:  California EPA’s reference exposure level. The REL is a concentration at or below which no adverse non-cancer health effects anticipated for the specified exposure duration. Exceeding the REL does not automatically indicate an adverse health impact. However, increasing concentrations above the REL value increases the likelihood that the health effect will occur. RELs are developed for acute, 8-hour and chronic exposures. See The Air Toxics Hot Spots Program Guidance Manual for Preparation of Health Risk Assessments (Cal EPA, February 2015).

Dose: The amount of a substance available for interaction with metabolic processes or biologically significant receptors after crossing the outer boundary of an organism. The potential dose is the amount ingested, inhaled, or applied to the skin. The applied dose is the amount of a substance presented to an absorption barrier and available for absorption (although not necessarily having yet crossed the outer boundary of the organism). The absorbed dose is the amount crossing a specific absorption barrier (e.g., the exchange boundaries of skin, lung, and digestive tract) through uptake processes. Internal dose is a more general term denoting the amount absorbed without respect to specific absorption barriers or exchange boundaries. The amount of the chemical available for interaction by any particular organ or cell is termed the deliverable dose for that organ or cell. See Guidelines for Exposure Assessment (EPA, 1992). 
Dose-response Relationship: The relationship between a quantified exposure (dose) and the proportion of subjects demonstrating specific, biological changes (response). See A Review of the Reference Dose and Reference Concentration Processes (EPA, 2002).

Exposure: Contact made between an agent and the outer boundary of an organism. Exposure is quantified as the concentration of the agent in the medium in contact integrated over the time duration of that contact. Exposure concentration is the concentration of the agent in its transport or carrier medium (e.g., air) at the point of contact. See A Review of the Reference Dose and Reference Concentration Processes (EPA, 2002) and Guidelines for Exposure Assessment (EPA, 1992).

High-end Risk Descriptor: A plausible estimate of the individual risk for those persons at the upper end of the risk distribution, conceptually above the 90th percentile but not higher than the individual in the population with the highest risk. Note that persons in the high end of the risk distribution have high risk due to high exposure, high susceptibility, or other reasons, and, therefore, persons in the high end of the exposure or dose distribution are not necessarily the same individuals as those in the high end of the risk distribution. See Guidelines for Exposure Assessment (EPA, 1992).

Human Equivalent Concentration (HEC): The human concentration (for inhalation exposure) of an agent that is believed to induce the same magnitude of toxic effect as the experimental animal species concentration. This adjustment may incorporate toxicokinetic information on the particular agent, if available, or use a default procedure. See A Review of the Reference Dose and Reference Concentration Processes (EPA, 2002).

Inhalation Unit Risk Estimate: The upper-bound excess lifetime cancer risk estimated to result from continuous exposure (usually lifetime) to an agent at a concentration of 1 µg/m³ in air. See IRIS Glossary (2016).

LC₅₀ (Lethal Concentration₅₀): A calculated concentration of a chemical to which exposure for a specific length of time is expected to cause death in 50% of a defined experimental animal population. See Casarett & Doull’s Toxicology, The Basic Science of Poisons (Klaassen, 1996).

LD₅₀ (Lethal Dose₅₀): A calculated dose of a chemical expected to cause death in 50% of a defined experimental animal population. See Casarett & Doull’s Toxicology, The Basic Science of Poisons (Klaassen, 1996).

LOAEL (lowest-observed-adverse-effect level): The lowest exposure (or dose) level of a chemical at which there are statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control group. See A Review of the Reference Dose and Reference Concentration Processes (EPA, 2002).

NIOSH: National Institute of Occupational Safety and Health

NIOSH Ceiling REL: A NIOSH-recommended REL that should not be exceeded at any time. See NIOSH Pocket Guide to Chemical Hazards.

NIOSH IDLH: NIOSH's immediately dangerous to life and health. The purpose for establishing an IDLH value in the NIOSH Standards Completion Program was to determine the airborne concentration from which a worker could escape without injury or irreversible health effects from an IDLH exposure in the event of the failure of respiratory protection equipment.  The IDLH was considered a maximum concentration above which only a highly reliable breathing apparatus providing maximum worker protection should be permitted.  In determining IDLH values, NIOSH considered the ability of a worker to escape without loss of life or irreversible health effects along with certain transient effects, such as severe eye or respiratory irritation, disorientation, and incoordination, which could prevent escape.  See NIOSH Pocket Guide to Chemical Hazards.

NIOSH REL: NIOSH's recommended exposure limit. A NIOSH REL may be a time-weighted average (TWA) concentration for up to a 10-hour workday during a 40-hour week time-weighted-average exposure and/or a ceiling, which, unless otherwise noted should not be exceeded at any time. A short-term exposure limit (STEL), unless otherwise noted, is a 15-minute TWA exposure that should not be exceeded at any time during the workday. See NIOSH Pocket Guide to Chemical Hazards.

NIOSH STEL: NIOSH's recommended short-term exposure limit; a 15-minute TWA exposure which should not be exceeded at any time during a workday. See NIOSH Pocket Guide to Chemical Hazards.

NOAEL (no-observed-adverse-effect level): Th highest exposure (or dose) level of a chemical at which there are no statistically or biologically significant increases in frequency or severity of adverse effects seen between the exposed population and its appropriate control; some effects may be produced at this level, but they are not considered to be adverse (or precursors to adverse effects, when used in deriving reference values). See A Review of the Reference Dose and Reference Concentration Processes (EPA, 2002).

Oral Cancer Slope Factor: An upper bound, approximating a 95% confidence limit, on the increased cancer risk from a lifetime exposure to an agent. This estimate, usually expressed in units of proportion (of a population) affected per mg/kg-day, is generally reserved for use in the low-dose region of the dose-response relationship, that is, for exposures corresponding to risks less than 1 in 100. See IRIS Glossary (2016).

OSHA: Occupational Safety and Health Administration

OSHA PEL: Occupational Safety and Health Administration's permissible exposure limit expressed as a time-weighted average. The PEL is enforced by OSHA; it is the concentration of a substance which must not be exceeded during any 8-hour workshift of a 40-hour workweek. A STEL is measured over a 15-minute period unless noted otherwise, and OSHA ceiling concentrations must not be exceeded during any part of the workday; if instantaneous monitoring is not feasible, the ceiling must be assessed as a 15-minute TWA exposure.  In addition, there are a number of substances (e.g., beryllium, ethylene dibromide) that have PEL ceiling values that must not be exceeded except for specified excursions.  For example, a "5-minute maximum peak in any 2 hours" means that a 5-minute exposure above the ceiling value, but never above the maximum peak, is allowed in any 2 hours during an 8-hour workday. See NIOSH Pocket Guide to Chemical Hazards.

RfC (inhalation reference concentration): An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure of a chemical to the human population through inhalation (including sensitive subpopulations), that is likely to be without an appreciable risk of deleterious effects during a lifetime. Generally used in EPA's noncancer health assessments. See IRIS Glossary (2016).

RfD (oral reference dose): An estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure of a chemical to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. Generally used in EPA's noncancer health assessments. See IRIS Glossary (2016).

Statistical significance: An inference that the probability is low that the observed difference in quantities being measured could be due to variability in the data rather than an actual difference in the quantities themselves. The inference that an observed difference is statistically significant is typically based on a test to reject one hypothesis and accept another. By convention, a difference between two groups is usually considered statistically significant if chance could explain it only 5% of the time or less. Study design considerations may influence the a priori choice of a different level of statistical significance. See Guidelines for Exposure Assessment (EPA, 1992) and IRIS Glossary (2016).

TWA (time-weighted average): A concentration averaged over a specified time period (e.g., an 8-hour workday or a 40-hour workweek).