2.1 DOT HazMat Classification & Table 172.101

DOT HazMat Classification & Table 172.101

The safe commercial transport of hazardous materials across public highways, railways, waterways, and air corridors is governed by the Hazardous Materials Regulations (HMR) codified in Title 49 of the Code of Federal Regulations (49 CFR), Parts 100 through 185. Promulgated by the Pipeline and Hazardous Materials Safety Administration (PHMSA)—an operating administration of the U.S. Department of Transportation (DOT)—these regulations establish a strict, cradle-to-destination compliance system. For the Certified Hazardous Materials Manager (CHMM), accurately identifying, classifying, and characterizing hazardous materials according to DOT standards is the foundational legal prerequisite for off-site packaging, marking, labeling, placarding, and documentation.

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1. Regulatory Architecture: 49 CFR Parts 100–185 & PHMSA

Congress enacted the Hazardous Materials Transportation Act (HMTA) of 1975 (amended by the Hazardous Materials Transportation Uniform Safety Act of 1990 and SAFETEA-LU, codified at 49 U.S.C. § 5101 et seq.) to empower the Secretary of Transportation to designate materials as hazardous when their transportation in commerce poses an unreasonable risk to health, safety, property, or the environment.

+-----------------------------------------------------------------------------+
|                   STRUCTURE OF 49 CFR HAZMAT REGULATIONS                    |
|                                                                             |
|   [Part 107] ---> HazMat Program Procedures (Approvals, Exemptions, SPs)    |
|   [Part 171] ---> General Information, Regulations & Definitions            |
|   [Part 172] ---> HazMat Table (§ 172.101), Emergency Response, Training,   |
|                   Marking, Labeling, Placarding & Security Plans            |
|   [Part 173] ---> Shipper Requirements: General Packaging & Hazard Classes  |
|   [Part 174] ---> Carriage by Rail                                          |
|   [Part 175] ---> Carriage by Aircraft                                     |
|   [Part 176] ---> Carriage by Vessel                                        |
|   [Part 177] ---> Carriage by Public Highway                                |
|   [Part 178] ---> Packaging Specifications & Performance-Oriented Standards |
|   [Part 179] ---> Tank Car Specifications                                   |
|   [Part 180] ---> Qualification, Maintenance & Retesting of Packagings      |
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Primary Shipper Responsibilities:

Under 49 CFR § 171.2 and Part 173, the legal burden of compliance rests primarily upon the offeror (shipper). The shipper must:

1. Determine whether the material meets the definition of one or more DOT hazard classes.
2. Assign the authorized Proper Shipping Name (PSN), Identification Number (UN/NA), Hazard Class/Division, and Packing Group (PG) using the Hazardous Materials Table (§ 172.101).
3. Select and assemble DOT-authorized or UN specification packaging.
4. Apply mandatory package markings and hazard labels.
5. Prepare legally compliant shipping papers or hazardous waste manifests.
6. Provide emergency response information and a 24-hour emergency contact telephone number.
7. Provide placards to the motor carrier when required.

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2. The 9 DOT Hazard Classes and Divisions

DOT classifies hazardous materials into nine numerical classes, several of which are subdivided into divisions to delineate specific chemical, physical, or explosive behavior.

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|                        THE 9 DOT HAZARD CLASSES                             |
|                                                                             |
|   [Class 1] Explosives (Divisions 1.1 through 1.6)                          |
|   [Class 2] Gases (2.1 Flammable, 2.2 Non-Flammable/Non-Toxic, 2.3 Toxic)   |
|   [Class 3] Flammable Liquids (Flash Point <= 60°C / 140°F) & Combustibles  |
|   [Class 4] Flammable Solids (4.1 Solid, 4.2 Spontaneous, 4.3 Wet Hazard)  |
|   [Class 5] Oxidizers (5.1) & Organic Peroxides (5.2 Types A through G)     |
|   [Class 6] Toxic Materials (6.1 PG I-III / TIH) & Infectious (6.2 Cat A/B) |
|   [Class 7] Radioactive Materials (White-I, Yellow-II, Yellow-III, TI)      |
|   [Class 8] Corrosive Materials (Skin destruction / Metal corrosion rate)   |
|   [Class 9] Miscellaneous Hazardous Materials (Lithium batteries, Marine)   |
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Class 1: Explosives (49 CFR § 173.50)

An explosive is any substance or article designed to produce a chemical reaction with the rapid release of gas and heat.

• Division 1.1: Mass explosion hazard affecting almost the entire load instantaneously (e.g., TNT, dynamite, black powder).
• Division 1.2: Projection hazard without mass explosion (e.g., military flares, projectile shells).
• Division 1.3: Fire hazard and minor blast or projection hazard without mass explosion (e.g., commercial display fireworks, solid rocket motors).
• Division 1.4: Minor explosion hazard largely confined to the package with no projection of fragments of appreciable size (e.g., consumer fireworks 1.4G, small arms ammunition 1.4S).
• Division 1.5: Very insensitive substances with a mass explosion hazard (e.g., Ammonium Nitrate-Fuel Oil [ANFO] blasting agents).
• Division 1.6: Extremely insensitive articles without a mass explosion hazard.
• Compatibility Groups: Denoted by letters A through S (excluding I, M, O, P, Q, R) to ensure safe segregation in storage and transport.

Class 2: Gases (49 CFR § 173.115)

• Division 2.1 Flammable Gas: A gas that at $20^\circ\text{C}$ ($68^\circ\text{F}$) and $101.3\text{ kPa}$ ($14.7\text{ psi}$) is ignitable in a mixture of $\le 13%$ by volume with air, or has a flammable range with air of at least $12%$ regardless of the lower limit (e.g., propane, methane, hydrogen, acetylene).
• Division 2.2 Non-Flammable, Non-Toxic Gas: Compressed, liquefied, or cryogenic gases exerting an absolute pressure of $\ge 280\text{ kPa}$ ($40.6\text{ psia}$) at $20^\circ\text{C}$ (e.g., nitrogen, carbon dioxide, argon, helium).
• Division 2.3 Toxic Gas (Gas Poisonous by Inhalation): Known to be toxic or lethal to humans, or having an inhalation $LC_{50} \le 5,000\text{ ppm}$ in albino rats. Division 2.3 gases are assigned to Hazard Zones A through D based on lethal concentration:
  • Hazard Zone A: $LC_{50} \le 200\text{ ppm}$
  • Hazard Zone B: $200\text{ ppm} < LC_{50} \le 1,000\text{ ppm}$
  • Hazard Zone C: $1,000\text{ ppm} < LC_{50} \le 3,000\text{ ppm}$
  • Hazard Zone D: $3,000\text{ ppm} < LC_{50} \le 5,000\text{ ppm}$ (Examples: Chlorine UN1017, Phosgene UN1076, Arsine UN2188). Anhydrous ammonia is regulated as 2.3 internationally and 2.2 with an Inhalation Hazard marking domestically.

Class 3: Flammable Liquids & Combustible Liquids (49 CFR § 173.120)

• Flammable Liquid: Any liquid having a flash point of $\le 60^\circ\text{C}$ ($140^\circ\text{F}$), or any material in liquid phase with a flash point $\ge 37.8^\circ\text{C}$ ($100^\circ\text{F}$) intentionally heated and offered for transport at or above its flash point.
• Combustible Liquid: A domestic U.S. provision (49 CFR § 173.120(b)) defining any liquid that does not meet the definition of any other hazard class and has a flash point $> 60^\circ\text{C}$ ($140^\circ\text{F}$) and $< 93^\circ\text{C}$ ($200^\circ\text{F}$).
• Domestic Exception (§ 173.150(f)): A flammable liquid with a flash point at or above $38^\circ\text{C}$ ($100^\circ\text{F}$) may be reclassified as a combustible liquid for domestic highway/rail transport, and non-bulk packages of combustible liquids are exempt from HMR requirements unless shipped by air, vessel, or as a hazardous waste/substance.

Class 4: Flammable Solids (49 CFR § 173.124–125)

• Division 4.1 Flammable Solids: Readily combustible solids, wetted explosives (desensitized), self-reactive materials, and solid polymerizing substances.
• Division 4.2 Spontaneously Combustible Materials: Includes Pyrophoric liquids/solids (ignite within 5 minutes of air contact without external energy) and Self-Heating materials (generate internal heat via oxidation).
• Division 4.3 Dangerous When Wet: Substances that, upon contact with water, emit flammable or toxic gases at a rate $> 1\text{ L/kg/hr}$ (e.g., sodium metal, calcium carbide, magnesium powder).

Class 5: Oxidizers and Organic Peroxides (49 CFR § 173.127–128)

• Division 5.1 Oxidizers: Substances that yield oxygen readily to stimulate or accelerate the combustion of organic matter.
• Division 5.2 Organic Peroxides: Organic compounds containing the bivalent $-O-O-$ peroxy bond structure (considered derivatives of hydrogen peroxide). Assigned to Types A through G based on thermal instability and detonability:
  • Type A: Cannot transport in package (detonates/deflagrates rapidly).
  • Types B through F: Authorized in specific packages, many requiring Self-Accelerating Decomposition Temperature (SADT) control.
  • Type G: Non-reactive, desensitized peroxide exempt from Division 5.2 regulations.

Class 6: Toxic Materials & Infectious Substances (49 CFR § 173.132–134)

• Division 6.1 Toxic (Poisonous) Materials: Other than gases, substances known to be toxic to humans or meeting oral ($LD_{50} \le 300\text{ mg/kg}$), dermal ($LD_{50} \le 1,000\text{ mg/kg}$), or inhalation dust/mist ($LC_{50} \le 4.0\text{ mg/L}$) thresholds, as well as Toxic Inhalation Hazard (TIH) liquids assigned to Inhalation Hazard Zones A and B.
• Division 6.2 Infectious Substances: Microorganisms (bacteria, viruses, parasites, fungi) or prions capable of causing disease in humans or animals:
  • Category A: Transported in a form capable of causing permanent disability, life-threatening, or fatal disease (UN 2814 infectious substance affecting humans; UN 2900 affecting animals).
  • Category B: An infectious substance not meeting Category A criteria (UN 3373, Biological Substance, Category B).
  • Regulated Medical Waste: UN 3291.

Class 7: Radioactive Materials (49 CFR § 173.403)

Materials containing radionuclides where both the activity concentration and total activity exceed specified threshold values ($> 70\text{ Bq/g}$ or $0.002\ \mu\text{Ci/g}$). Packages are labeled according to maximum surface radiation levels and the Transport Index (TI) (dose rate in mrem/hr measured at 1 meter from package surface):

Class 8: Corrosive Materials (49 CFR § 173.136–137)

Liquids or solids that cause full-thickness destruction of intact human skin tissue within specified observation periods, OR exhibit a corrosion rate on steel or aluminum surfaces exceeding $6.25\text{ mm/year}$ ($0.25\text{ in/year}$) at a test temperature of $55^\circ\text{C}$ ($130^\circ\text{F}$).

Class 9: Miscellaneous Hazardous Materials (49 CFR § 173.140)

Materials presenting a hazard during transportation that do not meet the definition of any other class. Includes:

• Lithium batteries (UN 3480 Lithium ion, UN 3481 Lithium ion in equipment, UN 3090 Lithium metal, UN 3091 Lithium metal in equipment).
• Environmentally hazardous substances (UN 3077 solid, UN 3082 liquid) meeting CERCLA Reportable Quantities (RQ) or marine pollutant criteria.
• Elevated temperature materials (liquids $\ge 100^\circ\text{C} / 212^\circ\text{F}$, solids $\ge 240^\circ\text{C} / 464^\circ\text{F}$, or liquids with flash points $\ge 37.8^\circ\text{C}$ intentionally heated).

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3. Packing Groups (PG I, II, III)

Packing groups reflect the relative degree of danger presented by a hazardous material and dictate packaging performance ratings:

• Packing Group I (PG I): Great danger (high hazard).
• Packing Group II (PG II): Medium danger (medium hazard).
• Packing Group III (PG III): Minor danger (low hazard).

[!NOTE] Classes Without Packing Groups: Packing Groups are not assigned to Class 2 (gases), Class 7 (radioactive materials), Division 6.2 (infectious substances), Division 4.1 self-reactives, Division 5.2 organic peroxides, and certain Class 9 articles (e.g., lithium batteries).

Class 3 Packing Group Determination Matrix:

Class 8 Packing Group Determination (Skin Corrosion Testing):

• PG I: Full-thickness skin destruction after exposure time $\le 3\text{ min}$ (observation period $\le 60\text{ min}$).
• PG II: Skin destruction after exposure time $> 3\text{ min}$ and $\le 60\text{ min}$ (observation period $\le 14\text{ days}$).
• PG III: Skin destruction after exposure time $> 60\text{ min}$ and $\le 4\text{ hours}$ (observation period $\le 14\text{ days}$), OR metal corrosion rate $> 6.25\text{ mm/year}$ on steel/aluminum at $55^\circ\text{C}$.

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4. The 10-Column Hazardous Materials Table (49 CFR § 172.101)

The § 172.101 Table is the operational master key for all DOT compliance:

+-----------------------------------------------------------------------------------------------------------------------------+
|                                             THE 10 COLUMNS OF 49 CFR § 172.101                                              |
|                                                                                                                             |
| [Col 1] Symbols: Symbols (+, A, D, G, I, W) modifying shipping name applicability                                          |
| [Col 2] Hazardous Materials Descriptions and Proper Shipping Names (PSN)                                                    |
| [Col 3] Hazard Class or Division (e.g., 3, 6.1, 8, 9)                                                                       |
| [Col 4] Identification Numbers: UN (international) or NA (domestic North America)                                          |
| [Col 5] Packing Group: Assigned Roman numerals (I, II, III)                                                                 |
| [Col 6] Label Codes: Primary and subsidiary hazard labels required                                                          |
| [Col 7] Special Provisions: Codes indicating packaging, mode, or handling rules                                            |
| [Col 8] Packaging Authorizations (49 CFR Part 173):                                                                         |
|         (8A) Exceptions | (8B) Non-Bulk Packaging | (8C) Bulk Packaging                                                      |
| [Col 9] Quantity Limitations: (9A) Passenger aircraft/rail | (9B) Cargo aircraft only                                      |
| [Col 10] Vessel Stowage Requirements: (10A) Stowage location (A-E) | (10B) Other stowage provisions                        |
+-----------------------------------------------------------------------------------------------------------------------------+

Explanation of Column 1 Symbols:

• + (Plus sign): Fixes the proper shipping name, hazard class, and packing group; the material must be shipped under this description regardless of whether it meets the technical hazard definition.
• A: Material is subject to HMR only when offered or transported by aircraft.
• D: Proper shipping name is authorized only for domestic transportation (NA numbers or domestic entries).
• G: Identifies generic or n.o.s. (not otherwise specified) shipping descriptions. The shipper must supplement the shipping name with the recognized technical chemical name in parentheses on shipping papers and markings (e.g., "Flammable liquids, n.o.s. (contains Acetone, Xylene)").
• I: Identifies descriptions preferred for international transportation.
• W: Material is subject to HMR only when transported by vessel / water.

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5. Precedence of Hazards Table (49 CFR § 173.2a)

When a hazardous material, chemical mixture, or solution possesses characteristics falling into more than one DOT hazard class or division, the primary hazard class must be determined using the Precedence of Hazards hierarchy in 49 CFR § 173.2a.

Tier 1: Highest Precedence Hazards (Never Subordinate)

The following hazard categories always take precedence over all other hazard classes and must be assigned as the Primary Hazard Class:

1. Class 7 Radioactive Materials (except limited quantities).
2. Division 2.3 Toxic Gases (Inhalation Hazard).
3. Division 2.1 Flammable Gases and Division 2.2 Non-Flammable Gases.
4. Division 6.1 materials that are Poison Inhalation Hazards (PIH / TIH, PG I, Inhalation Hazard Zone A).
5. Division 4.2 Pyrophoric materials.
6. Division 4.3 Dangerous When Wet materials.
7. Division 5.2 Organic Peroxides.
8. Division 4.1 Self-reactive materials and desensitized explosives.

Tier 2: Secondary Evaluation Matrix

For materials exhibiting combinations of Class 3 (Flammable Liquids), Class 8 (Corrosives), Division 6.1 (Poison liquids/solids, oral/dermal or Zone B inhalation), Division 4.1, and Division 5.1, the shipper must consult the Precedence of Hazards Table in § 173.2a(b):

• Special Rule for Class 8 vs Division 6.1: If a material exhibits both Class 8 corrosive properties and Division 6.1 toxicity:
  • If the toxicity is due to inhalation of vapors in PG I (Zone A or B), Division 6.1 takes precedence over Class 8.
  • If the toxicity is solely due to oral ($LD_{50}$) or dermal ($LD_{50}$) toxicity and the material falls into Class 8 PG I for skin corrosion, Class 8 takes precedence as the primary hazard, and Division 6.1 becomes a subsidiary hazard.