33 Hazard Classes

331 Nine Classes

Every hazardous material is assigned to one of nine hazard classes as defined in 49 CFR 172.101 and 173. The nine hazard classes are as follows:

Class 1: Explosives.
Class 2: Gases.
Class 3: Flammable and Combustible Liquids.
Class 4: Flammable Solids.
Class 5: Oxidizing Substances, Organic Peroxides.
Class 6: Toxic Substances and Infectious Substances.
Class 7: Radioactive Materials.
Class 8: Corrosives.
Class 9: Miscellaneous Hazardous Materials.

Some of the nine hazard classes are further separated into divisions based on their physical or chemical properties. For postal purposes, Exhibit 331 summarizes the mailability of hazardous materials by hazard class.

Exhibit 331

DOT Hazard Classes and Postal Mailability
Class	Name of Hazard Class (and Division when applicable)	Domestic Mail		International Mail and APO/FPO/DPO Mail
Class	Name of Hazard Class (and Division when applicable)	Air Transportation	Surface Transportation	International Mail and APO/FPO/DPO Mail
1	Explosives
	Division 1.1: Mass Explosive Hazard	Prohibited	Prohibited	Prohibited
	Division 1.2: Projection Hazard	Prohibited	Prohibited	Prohibited
	Division 1.3: Fire and/or Minor Blast/Minor Projection Hazard	Prohibited	Prohibited	Prohibited
	Division 1.4: Minor Explosion Hazard	Prohibited	Only with prior HQ approval per 341.2c	Prohibited
	Division 1.5: Very Insensitive With Mass Explosion Hazard	Prohibited	Prohibited	Prohibited
	Division 1.6: Extremely Insensitive; No Mass Explosion Hazard	Prohibited	Prohibited	Prohibited
2	Gases
	Division 2.1: Flammable Gases	Prohibited	Only ORM–D material per 342	Prohibited
	Division 2.2: Nonflammable Gases	Only Consumer Commodity material per 342	Only ORM–D material per 342	Prohibited
	Division 2.3: Toxic Gases	Prohibited	Prohibited	Prohibited
3	Flammable and Combustible Liquids
	All Flammable Liquids	Prohibited	Only ORM–D material per 343	Prohibited
	All Combustible Liquids	Only Consumer Commodity material per 343	Only ORM–D material per 343	Prohibited
4	Flammable Solids
	Division 4.1: Flammable Solids	Prohibited	Only ORM–D material per 344	Prohibited
	Division 4.2: Spontaneously Combustible	Prohibited	Only ORM–D material per 344	Prohibited
	Division 4.3: Dangerous When Wet	Prohibited	Only ORM–D material per 344	Prohibited
5	Oxidizing Substances, Organic Peroxides
	Division 5.1: Oxidizing Substances	Only Mailable Limited Quantity material per 345	Only ORM–D material per 345	Prohibited
	Division 5.2: Organic Peroxides	Only Mailable Limited Quantity material per 345	Only ORM–D material per 345	Prohibited
6	Toxic Substances and Infectious Substances
	Division 6.1: Toxic Substances	Consumer Commodity material per 346; other poisons as permitted in 346.231	ORM–D material per 346; other poisons as permitted in 346.231	Prohibited
	Division 6.2: Infectious Substances	Only as permitted in 346	Only as permitted in 346	Only First-Class Package International Service with Registered Mail service per 622
7	Radioactive Material	Prohibited	Only per 347	Only First-Class Package International Service with Registered Mail service per 622
8	Corrosives (Liquids And Solids)	Only Mailable Limited Quantity material per 348	Only ORM–D material per 348	Prohibited
9	Miscellaneous Hazardous Materials ID8000 materials, UN3077, UN3082, UN3334, or UN3335 materials	Consumer Commodity material and other materials as permitted in 349	ORM–D material and other materials as permitted in 349	Prohibited, except for magnetized materials per 349 and 622.4