# ERG2024 Glossary

- **operation:** document
- **citation:** ERG 2024, ERG2024 Glossary
- **title:** ERG2024 Glossary
- **source type:** guidance
- **agency:** U.S. DOT / Transport Canada / SICT
- **status:** guidance
- **official:** true
- **published on:** 2024-04-04
- **effective on:** Not available
- **summary:** Official ERG2024 erg2024 glossary guidance, pages 371-381.
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- **source url:** https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/2024-04/ERG2024-Eng-Web-a.pdf#page=371
**body:**

GLOSSARY
Adsorbed gas A gas which sticks (adsorbs) to the surface of a solid and porous
material (such as activated charcoal) contained within a metal
cylinder. This results in an internal cylinder pressure of less than
101.3 kPa at 20°C (14 psi at 68°F) and less than 300 kPa at 50°C
(43 psi at 122°F). These pressures are much lower than those of
conventional cylinders containing compressed or liquefied gases.
AEGL(s) Acute Exposure Guideline Level(s), AEGLs represent threshold
exposure limits for the general public after a once-in-a-lifetime,
or rare, exposure and are applicable to emergency exposure
periods ranging from 10 minutes to 8 hours. Three levels AEGL-
1, AEGL-2 and AEGL-3 are developed for each of five exposure
periods (10 and 30 minutes, 1 hour, 4 hours, and 8 hours) and
are distinguished by varying degrees of severity of toxic effects;
see AEGL-1, AEGL-2 and AEGL-3.
AEGL-1 AEGL‑1 is the airborne concentration (expressed as parts
per million or milligrams per cubic meter [ppm or mg/m3]) of a
substance above which it is predicted that the general population,
including susceptible individuals, could experience notable
discomfort, irritation, or certain asymptomatic, non‑sensory
effects. However, the effects are not disabling and are transient
and reversible upon cessation of exposure.
AEGL-2 AEGL‑2 is the airborne concentration (expressed as ppm or mg/
m3) of a substance above which it is predicted that the general
population, including susceptible individuals, could experience
irreversible or other serious, long‑lasting adverse health effects
or an impaired ability to escape.
AEGL-3 AEGL‑3 is the airborne concentration (expressed as ppm or mg/
m3) of a substance above which it is predicted that the general
population, including susceptible individuals, could experience
life‑threatening health effects or death.
Air-Reactive See “Pyrophoric”.
Alcohol-resistant foam A foam that is resistant to polar chemicals such as ketones and
esters which may break down other types of foam.
BLEVE Boil over Boiling Liquid Expanding Vapor Explosion
A sudden increase in fire intensity associated with the expulsion
of burning flammable liquid caused by the boiling of water that
has accumulated in the bottom of a tank car.
Page 369

GLOSSARY
Burn Refers to either a chemical or thermal burn, the former may
be caused by corrosive substances and the latter by liquefied
cryogenic gases, hot molten substances, or flames.
Carcinogen A substance or mixture which induces cancer or increases its
incidence.
Category A An infectious substance that poses a high risk to the health of
individuals and/or animals or public health. These substances can
cause serious disease and can lead to death. Effective treatment
and preventative measures may not be available.
Category B An infectious substance that poses a low to moderate risk
to individuals and/or animals and/or public health. These
substances are unlikely to cause serious disease. Effective
treatment and preventative measures are available.
CBRN Chemical, biological, radiological or nuclear agent.
CO2 Carbon dioxide gas.
Cold zone Area where the command post and support functions that
are necessary to control the incident are located. This is also
referred to as the clean zone, green zone or support zone in
other documents. (EPA Standard Operating Safety Guidelines,
OSHA 29 CFR 1910.120, NFPA 472).
Combustible In this guidebook, a solid or liquid capable of burning but
does not catch fire as easily as a flammable liquid. See
“Combustible Liquid”.
Combustible liquid Liquids which have a flash point greater than 60°C (140°F) and
below 93°C (200°F). U.S. regulations permit a flammable liquid
with a flash point between 38°C (100°F) and 60°C (140°F) to be
reclassed as a combustible liquid.
Page 370

GLOSSARY
Compatibility Group Control temperature Letters identify explosives that are deemed to be compatible.
The definition of these Compatibility Groups in this Glossary are
intended to be descriptive. Please consult the transportation of
hazardous materials/dangerous goods or explosives regulations
of your jurisdiction for the exact wording of the definitions. Class
1 materials are considered to be “compatible” if they can be
transported together without significantly increasing either the
probability of an incident or, for a given quantity, the magnitude
of the effects of such an incident.
A Substances which are expected to mass detonate very
soon after fire reaches them.
B Articles which are expected to mass detonate very soon
after fire reaches them.
C Substances or articles which may be readily ignited and
burn violently without necessarily exploding.
D Substances or articles which may mass detonate (with
blast and/or fragment hazard) when exposed to fire.
E & F Articles which may mass detonate in a fire.
G Substances and articles which may mass explode and
give off smoke or toxic gases.
H Articles which in a fire may eject hazardous projectiles
and dense white smoke.
J Articles which may mass explode.
K Articles which in a fire may eject hazardous projectiles
and toxic gases.
L Substances and articles which present a special risk and
could be activated by exposure to air or water.
N Articles which contain only extremely insensitive
detonating substances and demonstrate a negligible
probability of accidental ignition or propagation.
S Packaged substances or articles which, if accidentally
initiated, produce effects that are usually confined to the
immediate vicinity.
The maximum temperature at which a temperature-controlled
substance can be safely transported. Above this temperature,
self-accelerating decomposition or polymerization may occur.
Page 371

Control zones Criticality safety
index (CSI)
Cryogenic liquid Decomposition
products
Decontamination Dry chemical Edema ERPG(s) ERPG-1 ERPG-2 Page 372
GLOSSARY
Designated areas at hazardous materials/dangerous goods
incidents, based on safety and the degree of hazard. Many terms
are used to describe control zones; however, in this guidebook,
these zones are defined as the hot/exclusion/red/restricted zone,
warm/contamination reduction/yellow/limited access zone, and
cold/support/green/clean zone. (EPA Standard Operating Safety
Guidelines, OSHA 29 CFR 1910.120, NFPA 472).
A number value assigned to packages and overpacks containing
fissile materials that limits the number of packages containing fissile
materials during transportation.
A refrigerated, liquefied gas that has a boiling point colder than
-90°C (-130°F) at atmospheric pressure or is handled or transported
at a temperature equal to or less than -100°C (-148°F).
Products of a chemical or thermal break-down of a substance.
The removal of hazardous materials/dangerous goods from
personnel and equipment to the extent necessary to prevent
potential adverse health effects. For more information, consult the
“Decontamination” section.
A preparation designed for fighting fires involving flammable
liquids, pyrophoric substances and electrical equipment.
Common types contain sodium bicarbonate or potassium
bicarbonate.
The accumulation of an excessive amount of watery fluid in
cells and tissues. Pulmonary edema is an excessive buildup of
water in the lungs, for instance, after inhalation of a gas that is
corrosive to lung tissue.
Emergency Response Planning Guideline(s). Values intended
to provide estimates of concentration ranges above which one
could reasonably anticipate observing adverse health effects;
see ERPG-1, ERPG-2 and ERPG-3.
The maximum airborne concentration below which it is believed
nearly all individuals could be exposed for up to 1 hour without
experiencing more than mild, transient adverse health effects or
without perceiving a clearly defined objectionable odor.
The maximum airborne concentration below which it is believed
nearly all individuals could be exposed for up to 1 hour without
experiencing or developing irreversible or other serious health
effects or symptoms that could impair an individual’s ability to
take protective action.

GLOSSARY
ERPG-3 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.
Evacuate Evacuate aims to protect as many people as possible by
removing persons from inside a zone safely. If removal is too
risky, sheltering-in-place can also be considered in this zone.
Flammable liquid Flash point A liquid that has a flash point of 60°C (140°F) or lower.
Lowest temperature at which a liquid or solid gives off vapor in
such a concentration that, when the vapor combines with air near
the surface of the liquid or solid, a flammable mixture is formed.
Hence, the lower the flash point, the more flammable the material.
Flooding quantities Hazard zones
(Inhalation Hazard
Zones)
Minimum of 1900 L/min (500 US gal/min) of water.
HAZARD
Gases: LC50 of less than or equal to 200 ppm,
ZONE A:
Liquids: V equal to or greater than 500 LC50 and
LC50 less than or equal to 200 ppm.
HAZARD
ZONE B:
Gases: LC50 greater than 200 ppm and less
than or equal to 1000 ppm, Liquids: V equal to
or greater than 10 LC50; LC50 less than or equal
to 1000 ppm and criteria for Hazard Zone A are
not met.
HAZARD
LC50 greater than 1000 ppm and less than or
ZONE C:
equal to 3000 ppm.
HAZARD
LC50 greater than 3000 ppm and less than or
ZONE D:
equal to 5000 ppm.
Please note: even though the term “zone” is used, hazard zones
are not an actual area or distance. How zones are assigned is
strictly a function of the lethal concentration 50 (LC50) of the
product. For example, TIH Zone A is more toxic than Zone D.
High expansion foam Foams that have a high expansion ratio (over 1:200) with a low
water content.
Hot zone Area immediately surrounding a hazardous materials/dangerous
goods incident which extends far enough to prevent adverse
effects from the released product to personnel outside the zone.
This zone is also referred to as exclusion zone, red zone or
restricted zone in other documents. (EPA Standard Operating
Safety Guidelines, OSHA 29 CFR 1910.120, NFPA 472).
IED See “Improvised Explosive Device”.
Immiscible In this guidebook, means that a material does not mix readily
with water.
Page 373

Improvised
Explosive Device
Isolate Large spill LC50 Mass explosion MAWP mg/m3 Miscible mL/m3 Mutagen Narcotic n.o.s. Noxious Page 374
GLOSSARY
A bomb that is manufactured from commercial, military or
homemade explosives.
Isolate indicates a zone of no entry that applies to the public and
first responders who are not equipped, trained, and prepared to
mitigate the incident.
A spill that involves quantities that are greater than 208 liters (55
US gallons). This usually involves a spill from a large package,
or multiple spills from many small packages.
Lethal concentration 50. The concentration of a material
administered by inhalation that is expected to cause the death
of 50% of an experimental animal population within a specified
time. (Concentration is reported in either ppm or mg/m3).
Explosion which affects almost the entire load virtually
instantaneously.
Maximum Allowable Working Pressure: The maximum allowable
internal pressure that the tank may experience during normal
operations.
Milligrams of a material per cubic meter of air.
In this guidebook, means that a material mixes readily with water.
Milliliters of a material per cubic meter of air. (1 mL/m3 equals
1 ppm).
An agent giving rise to an increased occurrence of mutations
in populations of cells and/or organisms. Mutation means a
permanent change in the amount or structure of the genetic
material in a cell.
A substance which acts as a central nervous system depressor
producing effects such as drowsiness, narcosis, reduced
alertness, loss of reflexes, lack of coordination, and vertigo.
These effects can also be manifested as severe headache or
nausea, and can lead to reduced judgment, dizziness, irritability,
fatigue, impaired memory function, deficit in perception and
coordination, reaction time, or sleepiness.
These letters refer to “not otherwise specified”. The entries
which use this description are generic names such as “Corrosive
liquid, n.o.s.” This means that the actual chemical name for that
corrosive liquid is not listed in the regulations; therefore, a generic
name must be used to describe it on shipping papers.
In this guidebook, means that a material may be harmful or
injurious to health or physical well-being.

GLOSSARY
Organic peroxide An organic (carbon-containing) compound having two oxygen
atoms joined together. Organic peroxides are thermally
unstable chemicals. They may have one or more of the following
properties: be liable to explosive decomposition; burn rapidly;
be sensitive to impact or friction; react dangerously with other
substances.
Oxidizer A chemical which supplies its own oxygen and which helps other
combustible material burn more readily.
P See “Polymerization”.
Packing Group The Packing Group (PG) is assigned based on the degree of
danger presented by the hazardous material/dangerous good:
PG I : Great danger
PG II : Medium danger
PG III : Minor danger
PG See “Packing Group”.
pH pH is a value that represents the acidity or alkalinity of a water
solution. Pure water has a pH of 7. A pH value below 7 indicates
an acid solution (a pH of 1 is extremely acidic). A pH above 7
indicates an alkaline solution (a pH of 14 is extremely alkaline).
Acids and alkalies (bases) are commonly referred to as corrosive
materials.
PIH Polar Poison Inhalation Hazard. See “TIH”.
A molecule in which one side of the molecule has a partial
positive charge while another side has a partial negative charge.
Examples includes alcohols and ketones.
Polymerization A chemical reaction that often produces heat and pressure.
Once initiated, the reaction is accelerated by the heat that it
produces. The uncontrolled buildup of heat and pressure can
cause a fire or an explosion, or can rupture closed containers.
The letter (P) following a guide number in the yellow and blue
sections identifies a material that may polymerize violently
under high temperature conditions or contamination with other
products during a transportation incident. It is also used to identify
materials that have a strong potential for polymerization in the
absence of an inhibitor due to depletion of this inhibitor caused
by accident conditions.
ppm Parts per million. (1 ppm equals 1 mL/m3).
Page 375

Protective clothing Pyrophoric Radiation Authority Radioactivity Refrigerated liquid Refrigerated liquefied
gas
Respiratory sensitizer Right-of-way Self-heating material Page 376
GLOSSARY
In this guidebook, protective clothing includes both respiratory
and physical protection. One cannot assign a level of protection
to clothing or respiratory devices separately. These levels were
accepted and defined by response organizations such as U.S.
Coast Guard, NIOSH, and U.S. EPA.
Level A: SCBA plus totally encapsulating chemical resistant
clothing (permeation resistant).
Level B: SCBA plus hooded chemical resistant clothing (splash
suit).
Level C: Full or half-face respirator plus hooded chemical
resistant clothing (splash suit).
Level D: Coverall, including structural firefighters’ protective
clothing (SFPC), with no respiratory protection.
SCBA: Self-contained breathing apparatus.
For more information, consult the “Protective Clothing” section.
A material which ignites spontaneously upon exposure to air
(or oxygen).
As referred to in GUIDES 161 through 166 for radioactive
materials, the Radiation Authority is either a Federal, state/
provincial agency or state/province designated official. The
responsibilities of this authority include evaluating radiological
hazard conditions during normal operations and during
emergencies. If the identity and telephone number of the authority
are not known by emergency responders, or included in the local
response plan, the information can be obtained from the agencies
listed on the inside back cover. They maintain a periodically
updated list of radiation authorities.
The property of some substances to emit invisible and potentially
harmful radiation.
See “Refrigerated liquefied gas”.
A gas which when packaged for transport is made partially liquid
because of its low temperature. See “Cryogenic liquid”.
A substance that induces hypersensitivity of the airways following
inhalation of the substance.
A defined area on a property containing one or more high-
pressure natural gas pipelines.
Material that may spontaneously ignite or generate heat on
contact with oxygen (in air) after long periods of time (hours or
days).

Self-reactive material Shelter-in-place Skin corrosion Skin irritation Skin sensitizer Small spill Specific gravity Spontaneously
combustible material
GLOSSARY
Material that is thermally unstable and produces heat upon
decomposition, even without participation of air.
People should seek shelter inside a building and remain inside
until the danger passes. Sheltering-in-place is used when
evacuating the public would cause greater risk than staying
where they are, or when an evacuation cannot be performed.
Direct the people inside to close all doors and windows and to
shut off all ventilating, heating and cooling systems. In-place
protection (shelter-in-place) may not be the best option if (a) the
vapors are flammable; (b) if it will take a long time for the gas to
clear the area; or (c) if buildings cannot be closed tightly. Vehicles
can offer some protection for a short period if the windows are
closed and the ventilating systems are shut off. Vehicles are not
nearly as effective as buildings for in-place protection.
The production of irreversible damage to the skin following the
application of a test substance for up to 4 hours.
The production of reversible damage to the skin following the
application of a test substance for up to 4 hours.
A substance that will induce an allergic response following skin
contact.
A spill that involves quantities that are 208 liters (55 US gallons)
or less. This generally corresponds to a spill from a single small
package (for example, a drum), a small cylinder, or a small leak
from a large package.
Weight of a substance compared to the weight of an equal volume
of water at a given temperature. Specific gravity less than 1
indicates a substance is lighter than water; specific gravity greater
than 1 indicates a substance is heavier than water.
In this guidebook, a spontaneously combustible material means
a pyrophoric (air-reactive) material or self-heating material. Refer
to each term in the glossary.
Page 377

GLOSSARY
Straight or solid streams Method used to apply or distribute water from the end of a
hose. The water is delivered under pressure for penetration.
In an efficient stream, approximately 90% of the water passes
through an imaginary circle 38 cm (15 inches) in diameter at
the breaking point. Hose solid streams are typically created by
passing water through a non-adjustable bore tip. On the other
hand, hose straight streams originate from fog nozzles that are
dialed into a straight stream. Hose (solid or straight) streams are
frequently used to cool tanks and other equipment exposed to
flammable liquid fires, or for washing burning spills away from
danger points. However, these streams will cause a spill fire to
spread if improperly used or when directed into open containers
of flammable and combustible liquids.
Thermal runaway A chain reaction that leads to a violent release of stored energy
and flammable gas. This reaction can spread to other batteries
or combustible materials that are nearby, which could lead to a
large-scale thermal event with severe consequences.
TIH Toxic Inhalation Hazard. Term used to describe gases and
volatile liquids that are toxic when inhaled (same as PIH). These
materials pose a known hazard to human health during transport
or is presumed to be toxic to humans because of animal-based
studies.
V Saturated vapor concentration in air of a material in mL/m3 (ppm)
at 20°C and standard atmospheric pressure.
Vapor density Weight of a volume of pure vapor or gas (with no air present)
compared to the weight of an equal volume of dry air at the same
temperature and pressure. A vapor density less than 1 (one)
indicates that the vapor is lighter than air and will tend to rise.
A vapor density greater than 1 (one) indicates that the vapor is
heavier than air and may travel along the ground.
Vapor pressure Pressure at which a liquid and its vapor are in equilibrium at a
given temperature. Liquids with high vapor pressures evaporate
rapidly.
Viscosity Measure of a liquid’s internal resistance to flow. This property is
important because it indicates how fast a material will leak out
through holes in containers or tanks.
Page 378

GLOSSARY
Warm zone Water insoluble Water-Reactive Material Water-sensitive Water soluble Water spray (fog) Area between Hot and Cold zones where personnel and
equipment decontamination and hot zone support take place. It
includes control points for the access corridor and thus assists
in reducing the spread of contamination. Also referred to as
the contamination reduction corridor (CRC), contamination
reduction zone (CRZ), yellow zone or limited access zone in
other documents. (EPA Standard Operating Safety Guidelines,
OSHA 29 CFR 1910.120, NFPA 472).
A substance that does not easily dissolve in water.
In this guidebook, a material which produces a large amount of
toxic gas when it comes in contact with water.
Substances which may produce flammable, toxic and/or corrosive
decomposition products upon contact with water.
A substance that easily dissolves in water. Polar substances are
generally soluble in water.
Method or way to apply or distribute water. The water is finely
divided to provide for high heat absorption. Water spray patterns
can range from about 10 to 90 degrees. Water spray streams can
be used to extinguish or control the burning of a fire or to provide
exposure protection for personnel, equipment, buildings, etc. This
method can be used to absorb vapors, knock-down vapors
or disperse vapors. Direct a water spray (fog), rather than a
straight or solid stream, into the vapor cloud to accomplish
any of the above.
Water spray is particularly effective on fires of flammable liquids
and volatile solids having flash points above 37.8°C (100°F).
Regardless of the above, water spray can be used successfully
on flammable liquids with low flash points. The effectiveness
depends particularly on the method of application. With proper
nozzles, even gasoline spill fires of some types have been
extinguished when coordinated hose lines were used to sweep
the flames off the surface of the liquid. Furthermore, water
spray carefully applied has frequently been used with success
in extinguishing fires involving flammable liquids with high flash
points (or any viscous liquids) by causing frothing to occur only
on the surface, and this foaming action blankets and extinguishes
the fire.
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