Publication 52 - Hazardous, Restricted, and Perishable Mail > 3 Hazardous Materials > 33 Hazard 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:
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
Air Transportation
Surface Transportation
1
Explosives
Division 1.1: Mass Explosive Hazard
Prohibited
Division 1.2: Projection Hazard
Division 1.3: Fire and/or Minor Blast/Minor Projection Hazard
Division 1.4: Minor Explosion Hazard
Only with prior HQ approval per 341.2c
Division 1.5: Very Insensitive With Mass Explosion Hazard
Division 1.6: Extremely Insensitive; No Mass Explosion Hazard
2
Gases
Division 2.1: Flammable Gases
Only Limited Quantity Surface material per 342
Division 2.2: Nonflammable Gases
ID8000 material per 342
Division 2.3: Toxic Gases
3
Flammable and Combustible Liquids
All Flammable Liquids
Only Limited Quantity Surface material per 343
All Combustible Liquids
ID8000 material per 343
4
Flammable Solids
Division 4.1: Flammable Solids
Only Limited Quantity Surface material per 344
Division 4.2: Spontaneously Combustible
Division 4.3: Dangerous When Wet
5
Oxidizing Substances, Organic Peroxides
Division 5.1: Oxidizing Substances
Only Limited Quantity Air material per 345
Only Limited Quantity Surface material per 345
Division 5.2: Organic Peroxides
Only Limited Quantity Air material material per 345
6
Toxic Substances and Infectious Substances
Division 6.1: Toxic Substances
ID8000 material per 346; other poisons as permitted in 346.231
Only Limited Quantity Surface material per 346; other poisons as permitted in 346.231
Division 6.2: Infectious Substances
Only as permitted in 346
Only First-Class Package International Service with Registered Mail service per 622
7
Radioactive Material
Only per 347
8
Corrosives (Liquids And Solids)
Only Limited Quantity Air material per 348
Only Limited Quantity Surface material per 348
9
Miscellaneous Hazardous Materials
ID8000 materials, UN3077, UN3082, UN3334, or UN3335 materials
ID8000 material and other materials as permitted in 349
Only Limited Quantity Surface material and other materials as permitted in 349
Prohibited, except for magnetized materials per 349 and 622.4 and lithium batteries per 622.5