Hazard Classes

All chemicals, including hazardous ones, are commonly described in terms of their physical, chemical, and biological properties. It is necessary to understand the meaning and importance of the various individual properties and also to have some grasp of the significance of the numerical values given to chemical properties in order to fully use this information to recognize and predict behavior so as to avoid potentially dangerous situations.


The Federal Department of Transportation (DOT) groups chemicals into 9 classes, depending on specific properties. Some classes are also broken up into divisions to further clarify groups within each class. Examples and general hazardous properties of different substances are shown in Table DOT.

DOT Classes:

DOT 1: Explosives

DOT 2: Gases

DOT 3: Flammable Liquids

DOT 4: Flammable Solids

DOT 5: Oxidizing Substances

DOT 6: Poisons

DOT 7: Radioactive Materials

DOT 8: Corrosive Materials

DOT 9: Miscellaneous Hazardous Materials

Labels and Placards: The identification system used in transportation consists of labels and placards.

Labels: Labels are symbols (minimum of a 4-inch square diamond) that are attached to the shipping package. Packages classified as hazardous must have appropriate DOT markings or labels, unless otherwise specified.

Additional requirements:

  1. When required, the label(s) must be attached to, or printed on, the surface of the package near the proper shipping name.
  2. Hazardous materials having more than one hazard classification, new DOT regulations have detailed a complex hierarchy for the labeling, packaging and shipping of packages containing these materials.

Placards:Placards are larger symbols (10-3/4” square diamond) that are attached to each side and end of a rail car, freight or portable tank container, or motor vehicle containing hazardous materials (> 640 ft 3).

Highway transportation does not require placards unless more than 1000 lbs of material is being transported. There are exceptions regarding this amount and the type of placard required. One common type of labeling is the NFPA (National Fire Prevention Association) symbol (Fig. NFPA).

Class 1 -- Explosives: Explosives are materials that can react very rapidly, releasing a lot of energy. Flammables can act like explosives, depending on the container. Explosions are extremely difficult to protect against since they are not detectable. TNT (2,4,6-trinitrotoluene) and nitroglycerin are examples of explosives. The explosives hazard class placard is orange with an exploding ball icon.

Class 2.1 -- Flammable Gas: Any material which is a gas at 20¡C (68¡F) or less which is ignitable when in a mixture of 13% or less by volume with air; or has a flammable range with air of at least 12% regardless of lower limit. The placard for flammable gas has a red background with white lettering.

Class 2.2 -- Non-flammable Gas:Non-flammable gases are usually non-toxic, but pose hazards because they can displace oxygen, are under high pressure, and can be extremely cold or cryogenic. Examples of non-flammable gases are nitrogen, argon, and helium. The placard for non-flammable gas has a green background with white lettering.

Class 2.3 -- Inhalation Hazard: A material poisonous by inhalation (see ¤ 171.8 of 49 CFR Part 171, May 6. 1997).

Class 3 -- Flammables and Combustibles: The ease of which a material burns can be categorized as either flammable or combustible. The flashpoint is what determines whether a material is flammable or combustible.

Flammable liquids have a flashpoint equal to or less than 141 o F and are by far the most common hazard class. Most solvents are flammable and can cause irritation and defatting when in contact with skin. Inhalation of vapors can cause dizziness and headache. The DOT placard for flammable materials is a flame on a red background with white lettering.

Combustible liquids do not meet the definition of any other DOT hazard class and have a flashpoint more than 141 o F and up to 200 o F.

Liquids that do not meet the definition of any other DOT hazard class and have a flashpoint of more than 200 o F are classified as non-combustible.

CLASS 4 - Flammable Solids; Substances that Spontaneous Combust or Substances, Which In Contact With Water Emit Flammable Gases

Class 4 deals with substances, other than those classified as explosives, which, under conditions of transport, are readily combustible or may cause or contribute to a fire. Class 4 is subdivided as follows:

Class 4.1 -- Flammable solids:Solids which, under conditions encountered in transport, are readily combustible or may cause or contribute to fire through friction; self-reactive substances (solids and liquids) which are liable to undergo a strongly exothermic reaction; solid desensitized explosives which may explode if not diluted sufficiently. The DOT placard for flammable solids is a red and white vertical striped background with black lettering and a black flame.

Class 4.2 -- Substances liable to spontaneous combustion: Substances (solids and liquids) which are liable to spontaneous heating under normal conditions encountered in transport, or to heating up in contact with air, and being then liable to catch fire. The DOT placard for spontaneously combustible materials is a black flame with a white upper and a red lower background with black lettering.

Class 4.3 -- Substances, which in contact with water emit flammable gases: Substances (solids and liquids) which, by interaction with water, are liable to become spontaneously flammable or to give off flammable gases in dangerous quantities. The DOT placard for water sensitive materials is a white flame on a blue background with white lettering.

Class 5 -- Oxidizers:Oxidizing agents are usually recognizable by their structures or names. Oxygen is usually in their structure and often released as a result of thermal decomposition. Oxidizing agents often has "per-" prefixes ( perchlorate, peroxide, and permanganate) and often ends in "-ate."

Strong oxidizers have a larger potential incompatibility than perhaps any other chemical group (with the exception of water reactive substances). It is safe to assume that they shouldn't be stored or mixed with any other material except under carefully controlled conditions.

The placard for the Oxidizer hazard class has a yellow background with black lettering. The icon is a burning " 0" which stands for oxygen. Oxidizers give off oxygen and promote the combustion process in other materials; therefore they are a fire hazard. Oxidizers increase the hazard of a material catching fire. Oxidizers can make flammables extremely flammable, and they make many corrosives act like flammables. Some oxidizers are very reactive and can cause burns similar to corrosives. Oxidizers can also bleach skin and hair.

Class 6.1, Packing Group I or II -- Poisons: A material, other than a gas, which is known to be so toxic to humans as to afford a hazard to health during transportation; or which, in the absence of adequate data on human toxicity, is presumed to be toxic to humans because it falls within any one of the following categories when tested on laboratory animals: oral toxicity, dermal toxicity, or inhalation toxicity. Poisons must enter the body to cause injury or illness and usually only a small amount of material is needed. The extent of injury depends on the route of exposure, the concentration or strength of the chemical, and the length of exposure time. Arsenic and cyanide are examples of poisons. The poison hazard class placard has a white background with black lettering and a skull and cross-bones icon.

Class 6.1, Packing Group I or II -- Inhalation Hazard:A material poisonous by inhalation (see ¤ 171.8 of 49 CFR Part 171, May 6. 1997).

Class 6.1, Packing Group III - Keep Away from Food: Substances that are liable to cause death, serious injury or harm to human health if swallowed, inhaled, or contacted by skin. This poison hazard class placard has a white background with black lettering and an X-ed out ear of corn.

Class 6.2, -- Infectious Substance:A viable microorganism (or its toxin) which causes or may cause disease in humans or animals. Includes those agents listed in the Department of Health and Human Services' Regulations (42 CFR 72.3) and any other agent that has the potential to cause severe, disabling, or fatal disease.

Class 7 - Radiation has two sources: Natural background sources (70%) such as rocks which emit radon gas or contain other radioactive elements, and man-made sources (30%) such as X-ray machines. Man-made or naturally occurring radioactive isotopes are used every day and waste products from these manmade sources fall into two categories: High-level radioactive waste and low-level radioactive waste. There are two common types of radiation.

  1. Ionizing Radiation(concentrated package of energy).
  2. Alpha radiation: Alpha particles are double-charged helium ions and are produced when a radioactive substance such as radium decomposes to produce radon and an alpha particle. Alpha emitters are of concern if ingested or inhaled (internal radiation hazard). Alpha particles can be stopped by a piece of paper and can only travel a few tenths of a centimeter in air.

    Beta radiation: Beta radiation is a fast moving electron that is produced by radioactive decay and it require about 1000 times more mass to stop than does an alpha particle. A one Mev electron can travel 400 cm in air but only 0.5 cm in water. Therefore, water provides a good shield against beta radiation. Do not use shielding materials with high atomic numbers; X-rays will be produced (low atomic number items such as plastic are goodshields).

    Gamma, X-ray: Gamma rays and X-rays are commonly classified as electromagnetic radiation . Both gamma rays and X-rays are similar in properties to UV and visible light. The wavelengths are much shorter and therefore, the energy is greater. More energy means that more damage may occur in the receiving body if most of the energy is deposited. Gamma rays are produced by radioactive nuclei. X-ray production requires an electrical source and high-speed electrons (which are accelerated in a vacuum), which emit X-rays after striking a target. A one Mev gamma ray can travel over 7000 cm in air and 10 cm in water. Lead is frequently used for shielding material.

  3. Non-ionizing Radiation (energy transmitted as a wave).
  4. These types of radiation are heat producing:

    • Microwave radiation
    • Ultra-Violet radiation
    • Infra-red

    Non-ionizing radiation can be created or is utilized for many operations. Radar detectors, microwaves, radio, TV, and cellular phones all use non-ionizing radiation. Electromagnetic radiation is emitted as two fields (magnetic and electrical). The electrical field is produced from the transmitted current and the voltage transmitted produces the magnetic field.

    Radiation protection measures are based upon limiting or minimizing the absorbed dose. This can be accomplished by reducing the time exposed, increasing the distance from the source, and utilizing shielding (i.e., water, concrete, steel, soil, etc.).

Class 8 -- Corrosivesare defined in terms of pH by the EPA and have a pH < 2 or > 12.5. The DOT defines corrosivity in terms of the substance's ability to cause visible destruction or changes in skin tissue at the site of contact or a liquid that has a severe corrosion rate on steel or aluminum. Strong acids and strong bases are corrosive.

Acids are compounds that yield H+ (hydrogen) ions when dissolved in water. Chemicals ending with "ic" are always acids. Examples of these acids include hydrochlor ic and acet ic acids. The terms "concentrated" and "dilute" refer to the concentration of the acid in solution. Acids are defined as solutions with a pH < 7. The pH is defined as the negative logarithm of the molar H+ concentration (i.e. Ðlog[H+]).

Bases are materials that produce hydroxide ions (OH-) when dissolved in water. The words "alkaline," "basic," and "caustic" are often used interchangeably. Chemicals ending with "ide" are bases 60% of the time and are bases 100% of the time when ending with "hydroxide." Common bases include sodium hydroxide (lye), potassium hydroxide (potash lye), and calcium hydroxide (slaked lime). Strong versus weak bases, and concentrated versus dilute bases are terms that are exactly analogous to those for acids. Strong bases dissociate completely while weak bases such as the amines dissociate only partially. As with acids, bases can be either inorganic or organic.

The halogens such as iodine, fluorine, chlorine and bromine are also very strong corrosives. Although they are too reactive to occur naturally in the environment, they can form dangerous compounds. Extreme caution is necessary when handling halogen compounds because they are also strong oxidizers and readily bond with hydrogen to form acids.

The placard for corrosives consists of white lettering on a split background showing a hand and a piece of material being eaten away. Contacting corrosives causes immediate damage to skin or eyes, and inhalation of vapors will cause irritation and burning of the nose, throat, and lungs. Ingesting corrosives will irritate and bum the mouth, throat and stomach. Corrosives will chemically react and generate heat that can act as an ignition source, potentially causing a fire.

Class 9 -- Miscellaneous: A material which presents a hazard during transport but which is not included in any other hazard class. This class includes any material which has an anesthetic, noxious, or other similar property which could cause extreme annoyance or discomfort, or any material (not included in any other hazard class) which meets the definition of a hazardous substance or hazardous waste. This placard has a white background with black vertical stripping on the upper half.

Table DOT

General Category DOT Classification Examples General Hazardous Properties
Explosives and Blasting  
Class 1.1 Dynamite, Dry TNT, Black Powder Sensitive to heat & shock
Class 1.1-1.3 Propellant Explosives, Rocket Motors, Special Fireworks Contamination could cause explosion
Class 1.4 Common Fireworks, Small Arms Ammunition, Ammonium Nitrate -- Fuel Oil Mixtures Thermal and mechanical impact potential
Class 1.5 Blasting caps Low sensitivity, possible ignition
Gases (Compressed, Liquefied or Dissolved under Pressure)
Class 2.1 Liquefied Petroleum Gas, Acetylene, Hydrogen Explosion potential, Vapor-air flammability hazard
Class 2.2 Carbon Dioxide, Sulfur Dioxide, Anhydrous Ammonia Compressed, some toxic or corrosive
Class 2.2 Cryogenic Nitrogen, Oxygen Liquefied gases -- cold temps -- frostbite, high expansion ratio
Class 2.3 Chlorine Toxic and corrosive
Class 2.3 Carbon Monoxide Poison
Flammable and Combustible Liquids  
Class 3 Acetone, Gasoline, Methanol Flammability hazard (flash-point < 141o F)
Class 4.2 (also some gases here) Aluminum alkyls, Alkyl Boranes, Silane Explosion potential, pyrophoric
Class 3 Fuel oils, Ethylene glycols, Ink Flash point 100o to 200o F; potentially corrosive, toxic, thermally unstable
Flammable Solids  
Class 4.1 Pyroxylin Plastics, Magnesium-Aluminum Powder, Charcoal Readily ignites & burns ex- plosively, some spontaneously
Class 4.2 Phosphorus Pyrophoric or spontaneously combustible
Class 4.3 -- Dangerous When Wet Sodium and Potassium Metals, Calcium Carbide Water reactive potential; Toxic & corrosive potentials
Oxidizers & Organic Peroxides
Class 5.1 Ammonium Nitrate fertilizer, Hydrogen Peroxide Solution Supply O2 to support combustion of normally nonflammable material
Class 5.2 Benzoyl Peroxide, Peractic Acid Soln Explosively sensitive to heat, shock, friction. Potentially toxic
Poisonous and Infectious Substances
Class 6.1 (Poison A) Arsine, Hydrocyanic Acid, Phosgene Extremely harmful by inhalation, ingestion and absorption of tiny amounts
Class 6.1 (Poison B) Aniline, Arsenic, Methyl Bromide, Tear Gas, Xylyl Bromide Flammability potential a moderately harmfull
6.1 pg III Dichloromethane, Copper salts, Caffeine Harmful if ingested or inhaled. Largest toxic category
Class6.2 (Infectious Agents) Anthrax, Botulism, Rabies, Tetanus Biological Agents
Irritant (obsolete in DOT) Tear gas, many solvents & oxidizers Severe -- Lachrymator; Chronic -- Sensitizer; Mild -- Local irritation
Radioactive Substances
Class 7 Plutonium, Cobalt, Uranium Particulate -- alpha & beta
  Uranium Hexafluoride Radiation -- gamma rays (internal & external)
Class 8 Acids -- Hydrochloric Acid, Oleum, Nitric Acid Disintegration of tissues, fuming potential
Other Regulated Materials
Class 9 -- ORM A Dry Ice, Carbon Tetrachloride Toxic
Class9 -- ORM B Quicklime, Metallic Mercury Corrosive
Class 9 -- ORM C Oakum, Bleaching Powder Unique hazards
Class9 -- ORM D Consumer Commodity Limited hazards
Class 9 -- ORM E Hazardous Substances- Pentachlorophenol, Adipic Acid, & Hazardous Wastes Special hazards not otherwise covered

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Chemical incompatibility can manifest in many ways, with combinations resulting in fires, explosions, extreme heat, evolution of toxic gases, and polymerization. Because of the great number of chemicals and subsequent multiple numbers of potential reactions, it is impractical to list all potential reactions; five of the more common incompatibility reactions are shown below. Different hazard classes should never be stored together because violent reactions occur when the following hazard classes are mixed together:

  1. Corrosives + Flammables -- Explosion/Fire
  2. Corrosives + Poisons -- Poison Gas
  3. Flammables + Oxidizers -- Explosion/Fire
  4. Acids + Bases -- Salts/Heat
  5. Water Reactives + Water -- Toxic/Flammable gas


NFPA Labeling System

The diamond shaped diagram gives a general idea of the inherent hazards and the severity of these hazards under emergency conditions. The Health, Flammability and Reactivity are designated with 0 (or blank) indicating no hazard and 4 indicating an extreme or severe hazard; Special Hazards contain symbols that designate other hazards (if present). Table NFPA contains the full explanation of the numbering system for Health, Flammability and Reactivity. The symbols seen in Special Hazards include:

Oxidizer OXY
Alkali ALK
Corrosive COR
Radiation Hazard  
Health Hazard Flammability Hazard Reactivity Hazard
Health hazards are noted in the blue quadrant and are rated from 0 to 4 with 4 as the most dangerous level Flammability hazards are noted in the red quadrant and are rated from 0 to 4 with 4 as the most dangerous level Reactivity HazardReactivity hazards are noted in the yellow quadrant and are rated from 0 to 4 with 4 as the most dangerous level.
Type of Possible Injury Susceptibility of Materials to Burn Susceptibility to Release of Energy
4: Extremely Hazardous (deadly) -- very short exposure can cause death or major long-term injury. 4: Extremely Flammable (below 73o F) -- turns into a gas rapidly under normal conditions and burns easily. 4: Extremely Unstable (may detonate) -- under normal conditions, this chemical may explode or react violently.
3: Highly Hazardous (extreme danger) -- short exposure can cause serious temporary or possible long-term injury. 3: Highly Flammable (below 1000 F) -- liquid or solid can be ignited at almost any ordinary temperature. 3: Unstable (shock or heat may detonate or explode) -- may react with water, or may need heating or another strong initiating source.
2: Moderately Hazardous (hazardous) -- intense or continued exposure can cause temporary or possible long-term injury. 2: Moderately Combustible (between 100o F and 200o F) -- must be heated somewhat or be in a very hot place before ignition can occur. 2: Unstable (violent chemical changes) -- may react violently with water, or undergo violent chemical changes without exploding.
1: Slightly Hazardous (slightly hazardous) -- exposure can cause irritation, but only minor injury. 1: Slightly Combustible (above 2000 F) -- must be heated before ignition can occur. 1: Unstable if Heated -- normally stable, but can become unstable when hot or under pressure. Reactions with water are not violent.
0: No Health Hazard (normal) -- exposure under fire conditions would offer no hazard beyond that of ordinary combustibles. 0: Nonflammable or Noncombustible -- will not burn. 0: Stable -- normally stable, even in a fire. Does not react with water.