The 9 DOT hazard classes and their divisions, with examples
The US DOT sorts hazardous materials into 9 hazard classes, from Class 1 explosives to Class 9 miscellaneous. See each division and how a class is set.
The 9 DOT hazard classes and divisions
Fully regulated hazardous materials fall into nine hazard classes under the US Hazardous Materials Regulations (HMR), and several classes split into divisions. The official list of DOT classes is Table 1 in 49 CFR 173.2. For a listed material, column 3 of the Hazardous Materials Table in 49 CFR 172.101 shows the hazmat class or division.
A material's class can be shown by its class number, division number or name (173.2).
The nine hazmat classes chart
| Class | Divisions and name | Definition in 49 CFR |
|---|---|---|
| 1 Explosives | 1.1 mass explosion hazard; 1.2 projection hazard; 1.3 predominately a fire hazard; 1.4 no significant blast hazard; 1.5 very insensitive explosives, blasting agents; 1.6 extremely insensitive detonating substances | 173.50 |
| 2 Gases | 2.1 flammable gas; 2.2 non-flammable compressed gas; 2.3 poisonous gas | 173.115 |
| 3 Flammable and combustible liquid | No numbered divisions | 173.120 |
| 4 Flammable solids, etc. | 4.1 flammable solid; 4.2 spontaneously combustible material; 4.3 dangerous when wet material | 173.124 |
| 5 Oxidizer and organic peroxide | 5.1 oxidizer; 5.2 organic peroxide | 173.127, 173.128 |
| 6 Poisonous and infectious | 6.1 poisonous materials; 6.2 infectious substance (etiologic agent) | 173.132, 173.134 |
| 7 Radioactive | Radioactive material | 173.403 |
| 8 Corrosive | Corrosive material | 173.136 |
| 9 Miscellaneous | Miscellaneous hazardous material | 173.140 |
Names and divisions are from 49 CFR 173.2, Table 1. That table also lists forbidden materials and forbidden explosives, with no class number. Neither is a tenth class. ORM-D is no longer in that list or the Hazardous Materials Table.
What each class covers, with examples
Fireworks (UN0336) are 1.4G. Class 2 hazmat covers flammable, non-flammable compressed and poisonous gases (173.115). Propane (UN1978) is Division 2.1. Chlorine (UN1017) is 2.3 and also carries 5.1 and 8 labels (172.101).
Hazard class 3 is the flammable liquids class, and combustible liquid sits in it too. A flammable liquid has a flash point of not more than 60 °C (140 °F) (173.120). A combustible liquid has a flash point above 60 °C and below 93 °C (200 °F) and meets no other hazard-class definition, but it is not a separate class. A Class 3 table entry is read as combustible liquid in that flash-point range. Gasoline (UN1203) is Class 3, packing group II (172.101).
Division 4.1 covers desensitized explosives, self-reactive materials and readily combustible solids. Division 4.2 covers pyrophoric and self-heating materials. Division 4.3 covers a material that, on contact with water, gives off flammable or toxic gas at more than 1 L per kg per hour (173.124).
An oxidizer (5.1) can cause or enhance combustion of other materials, generally by yielding oxygen (173.127). Hydrogen peroxide over 60%, stabilized (UN2015) is 5.1, packing group I (172.101). Division 5.2 covers organic peroxides under the criteria in 173.128.
A class 6.1 hazardous material is a poison other than a gas. It is known to be toxic to humans in transport, or presumed toxic at an oral LD50 of 300 mg/kg or less, a dermal LD50 of 1,000 mg/kg or less, or a dust or mist inhalation LC50 of 4 mg/L or less (173.132). Arsenic trioxide (UN1561) is 6.1, packing group II (172.101). Division 6.2 covers material known or reasonably expected to contain a pathogen, assigned UN2814, UN2900, UN3291, UN3373 or UN3549 (173.134).
Class 7 is radioactive material for which both the activity concentration and the total consignment activity exceed the values in 173.436 (173.403). Class 8 covers liquids or solids that cause irreversible skin damage within a specified period, and liquids or solids that may become liquid in transport and severely corrode steel or aluminum (173.136). Sulfuric acid with more than 51% acid (UN1830) is Class 8, packing group II (172.101).
Class 9 covers a transport hazard that meets no other class definition. It includes elevated temperature materials, hazardous substances, hazardous wastes and marine pollutants (173.140). Lithium ion batteries (UN3480) and dry ice (UN1845) are both Class 9 (172.101).
Packing groups inside a class
Packing groups rank danger inside a class: PG I is great danger, PG II medium and PG III minor (171.8). Class 2, Class 7 and Division 6.2 have no packing groups (172.101).
How a material's hazard class is set
Hazardous materials classification is the shipper's job. Under 173.22, the person who offers a hazardous material for transport must class and describe it in accordance with Parts 172 and 173.
- Look the material up by name in the 172.101 table. Column 3 gives the class or division, or "Forbidden". A "+" in column 1 fixes the proper shipping name, hazard class and packing group without regard to whether the material meets that class definition.
- If it is not listed by name, class it against the Part 173 definitions. It may meet more than one class but is assigned only one (171.8). If it is not specifically listed and meets more than one definition, use 173.2a precedence, not the lowest class number, then the generic or n.o.s. entry for that class. On that precedence table, the most stringent packing group of any hazard applies. A material that is not forbidden and meets no class definition is not a hazardous material.
"Flammable liquids, corrosive, n.o.s." (UN2924) is Class 3 with labels 3 and 8. "Corrosive liquids, flammable, n.o.s." (UN2920) is Class 8 with labels 8 and 3 (172.101). The 173.2a precedence table decides which entry fits.
DOT hazard classes vs OSHA hazard classes
If you came from a safety data sheet, OSHA uses "hazard class" differently. Its Hazard Communication Standard means the nature of a physical or health hazard, such as flammable solid, carcinogen or oral acute toxicity, and splits each class into categories (29 CFR 1910.1200). Those are workplace classes. The nine DOT classes cover transport.
To look up a specific substance's class, packing group and labels, ask RegAffairs AI how a material's DOT hazard class is decided and open the cited table entries.