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15.1 Hazardous Materials Identification: The Nine Hazard Classes and UN/NA Numbers
Commercial Driver's License CDL
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1 Chapter 1: CDL Overview, CLP, and Federal Licensing Regulations
1.1 CDL Classes, Endorsements, Restrictions, and Testing Architecture (49 CFR Part 383) 1.2 Entry-Level Driver Training (ELDT) and the Training Provider Registry (49 CFR Part 380) 1.3 Commercial Learner's Permit (CLP) Rules and Driver Disqualifications 1.4 Medical Self-Certification Categories, Other CDL Rules, and the International Registration Plan
2 Chapter 2: Vehicle Inspection and Maintenance
2.1 The Seven-Step Pre-Trip Inspection Method 2.2 Critical Safety Components: Steering, Suspension, Brakes, and Tires 2.3 En Route Inspections, Post-Trip DVIR, and Out-of-Service Criteria
3 Chapter 3: Basic Vehicle Control and Operating Fundamentals
3.1 Instrument Controls, Engine Starting, and Shutdown Procedures 3.2 Shifting Gears: Manual Transmissions, Double-Clutching, and Progressive Shifting 3.3 Backing Fundamentals, GOAL Principle, and Basic Maneuvering 3.4 Putting the Vehicle in Motion: Accelerating, Steering, and Stopping Smoothly
4 Chapter 4: Speed Management and Stopping Distances
4.1 The Three Components of Total Stopping Distance (Perception, Reaction, Braking) 4.2 Speed Management on Curves, Slippery Roads, and Downgrades 4.3 Hydroplaning, Speed vs. Weight, and Reduced Visibility Adjustments
5 Chapter 5: Space Management and Defensive Driving
5.1 Managing Following Distance and Space Ahead (The 1-Second per 10-Foot Rule) 5.2 Space Beside, Behind, and Overhead Clearance Management 5.3 Turning, Off-Tracking, and Managing Space in Intersections 5.4 Space Below and the Space Needed to Cross or Enter Traffic
6 Chapter 6: Visual Search, Hazard Perception, and Communication
6.1 Visual Search: Looking 12-15 Seconds Ahead and Mirror Monitoring (5-8 Seconds) 6.2 Identifying Hazards: Work Zones, Pedestrians, Distracted Drivers, and Slow Vehicles 6.3 Communicating Intent: Signaling, Flasher Use, Horn, and Breakdown Warning Triangles (49 CFR 392.22) 6.4 Distracted Driving and the Federal Mobile Phone and Texting Rules (49 CFR 392.80 and 392.82) 6.5 Aggressive Drivers and Road Rage: Recognition, Prevention, and Response
7 Chapter 7: Driving in Extreme and Adverse Conditions
7.1 Night Driving, Low-Beam/High-Beam Management, and Glare Recovery 7.2 Driving in Fog, Heavy Rain, Snow, and Severe Winter Conditions 7.3 Hot Weather Driving, Tire Blowout Prevention, and Radiator Overheating
8 Chapter 8: Emergency Maneuvers, Skid Control, and Accident Procedures
8.1 Evasive Steering, Off-Road Recovery, and Emergency Braking (Controlled vs. Stab) 8.2 Recognizing and Recovering from Drive-Wheel and Front-Wheel Skids 8.3 Post-Crash Procedures, Fires (Extinguisher Classes), and Emergency Incident Response
9 Chapter 9: Mountain Driving and Downgrade Techniques
9.1 Engine Braking, Auxiliary Retarders, and Gear Selection Before the Grade 9.2 Snub Braking Technique vs. Continuous Braking and Brake Fade 9.3 Runaway Truck Ramps and Emergency Descents
10 Chapter 10: Railroad-Highway Grade Crossings and Special Intersections
10.1 Railroad Crossing Procedures, Warning Systems, and Passive vs. Active Crossings 10.2 Mandatory Stop Requirements (Hazmat, Passenger, School Bus) and Clearance Safety 10.3 Low-Clearance and Low-Ground-Clearance Hanging Hazards at Crossings
11 Chapter 11: Cargo Securement and Weight Distribution
11.1 Federal Weight Limits (GVWR, GCWR, Axle Limits, Bridge Formula) and Center of Gravity 11.2 Cargo Securement Rules, Working Load Limits, Tiedowns, and Covering Requirements 11.3 Cargo Types: Liquids/Baffles, Dry Bulk, Over-Dimensional Loads, and Inspection Intervals (50 mi / 3 hr / 150 mi) 11.4 Blocking, Bracing, Header Boards, Covering Loads, and Sealed or Containerized Cargo
12 Chapter 12: Air Brake Systems Overview and Operation
12.1 Air Brake System Components: Compressor, Governor, Air Tanks, Safety Valves, and Drain Valves 12.2 Brake Chambers, S-Cam Mechanism, Slack Adjusters, and Spring Parking Brakes 12.3 Dual Air Brake Systems, Low Air Warning (60 psi), and Emergency Spring Brake Pop-Out (20-45 psi) 12.4 Air Brake Pre-Trip Inspection and Step-by-Step Leak Rate Testing 12.5 Antilock Braking Systems (ABS): Required Vehicles, Malfunction Lamps, and Braking With and Without ABS
13 Chapter 13: Combination Vehicles Overview and Coupling
13.1 Combination Vehicle Dynamics: Rollover Risk, Rearward Amplification, and Trailer Jackknifing 13.2 Coupling and Uncoupling Step-by-Step Procedures: Fifth Wheel, Kingpin, Air and Electrical Lines 13.3 Trailer Air Lines (Service vs. Emergency/Supply), Glad Hands, and Trailer Protection Valves 13.4 Inspecting Combination Vehicles: Walk-Around Additions and the Combination Vehicle Air Brake Check
14 Chapter 14: Driver Wellness, Hours of Service, and Drug/Alcohol Rules
14.1 Hours of Service (HOS) Regulations: 11-Hour, 14-Hour, 30-Minute Break, 60/70-Hour Rules & ELD Mandate 14.2 Driver Physical Qualifications, Medical Certification (Part 391), Fatigue, and Sleep Apnea 14.3 Drug and Alcohol Regulations (49 CFR Part 382): 0.04% BAC Limit, Testing Types, and Drug & Alcohol Clearinghouse
15 Chapter 15: Hazardous Materials Awareness for General CDL
15.1 Hazardous Materials Identification: The Nine Hazard Classes and UN/NA Numbers 15.2 Shipping Papers, Emergency Response Guidebook (ERG), and In-Cab Document Rules 15.3 Placarding Thresholds (Table 1 vs. Table 2, 1,001-lb rule), Bulk vs. Non-Bulk, and Hazmat General Rules
15.1 Hazardous Materials Identification: The Nine Hazard Classes and UN/NA Numbers
Key Takeaways Explain This Topic AI
Hazardous Materials Regulations (49 CFR Parts 100–185) establish federal safety standards to protect the public, transport workers, property, and the environment during commercial transit.
The DOT classifies hazardous materials into 9 primary hazard classes based on physical and chemical risks, ranging from Class 1 (Explosives) to Class 9 (Miscellaneous Hazardous Materials), many subdivided into decimal divisions.
UN and NA identification numbers are standardized 4-digit codes (e.g., UN1203 for gasoline, UN1993 for combustible liquids) displayed on diamond hazard placards or orange rectangular panels to instantly identify cargo in emergencies.
Special markings identify unique transportation hazards, including the Marine Pollutant symbol (aquatic toxicity) and the 'HOT' or thermometer marking for Elevated Temperature materials transported at high heat.
Last updated: August 2026
Hazardous Materials Identification: The Nine Hazard Classes and UN/NA Numbers
Hazardous materials (HazMat) are products that pose an unreasonable risk to health, safety, and property during commercial transportation. The federal government regulates the handling, packaging, marking, labeling, placarding, and transportation of these substances under Title 49 of the Code of Federal Regulations, Parts 100 through 185 (49 CFR Parts 100–185), administered by the Pipeline and Hazardous Materials Safety Administration (PHMSA) and the Federal Motor Carrier Safety Administration (FMCSA).
Every commercial driver—even those who do not hold a specialized Hazardous Materials ( H or X ) endorsement—must possess a thorough operational awareness of hazardous materials. Commercial drivers must be able to recognize regulated cargo, verify packaging and labeling integrity, identify emergency identification numbers, and prevent catastrophic transportation incidents.
1. The Purpose of Hazardous Materials Regulations (49 CFR Parts 100–185)
The federal hazardous materials transportation regulatory system is designed around three core safety objectives :
Contain the Material: Regulate packaging design, manufacturing, inspection, and maintenance to ensure hazardous commodities remain fully contained inside drums, cylinders, totes, and cargo tank walls under normal and adverse transportation conditions.
Communicate the Hazard: Provide immediate, unambiguous visual communication of the specific chemical and physical risks to drivers, dock workers, emergency responders, and law enforcement through shipping papers, package labels, vehicle placards, and markings.
Assure Safe Drivers and Equipment: Establish strict operating rules, vehicle inspection routines, routing restrictions, driver licensing and security background checks, and emergency response training. ┌────────────────────────────────────────────────────────────────────────┐
- │ CORE GOALS OF 49 CFR PARTS 100–185 │
- ├────────────────────┬─────────────────────────────┬─────────────────────┤
- │ 1. CONTAIN CARGO │ 2. COMMUNICATE THE HAZARD │ 3. SAFE DRIVERS/VEH │
- │ Packaging & Tanks │ Papers, Labels, Placards, ID│ Rules, Routes, CDL │
- └────────────────────┴─────────────────────────────┴─────────────────────┘
- 2. The Nine DOT Hazard Classes and Divisions
The Department of Transportation (DOT) classifies all hazardous materials into 9 distinct hazard classes . Several classes are further divided into decimal divisions that describe specific physical or chemical behaviors.
Class 1: Explosives
Class 1 comprises substances or articles that undergo rapid chemical reactions, producing violent gas, shockwaves, or thermal blast effects. Class 1 is divided into six numerical divisions based on the severity of explosive danger:
Division 1.1 — Mass Explosion Hazard: Materials where a detonation affects almost the entire load virtually instantaneously (e.g., dynamite, TNT, bulk artillery shells, black powder).
Division 1.2 — Projection Hazard: Materials with a projection hazard but not a mass explosion hazard (e.g., aerial flares, military rocket motors, fragmentation munitions).
Division 1.3 — Fire and/or Minor Blast/Projection Hazard: Materials that burn vigorously with minor blast or projection effects (e.g., display fireworks, smokeless propellant powder).
Division 1.4 — Minor Explosion Hazard: Materials presenting no significant blast hazard; explosive effects are largely confined to the package (e.g., small arms ammunition, consumer signal flares).
Division 1.5 — Very Insensitive Substances (Blasting Agents): Materials with a mass explosion hazard that are so insensitive that the probability of accidental initiation during normal transport is very low (e.g., ANFO / ammonium nitrate-fuel oil blasting slurries).
Division 1.6 — Extremely Insensitive Articles: Articles containing only extremely insensitive detonating substances with a negligible probability of accidental initiation (e.g., specialized non-mass explosive military munitions).
Class 2: Gases
Class 2 covers compressed, liquefied, dissolved, or cryogenic gases transported in pressurized cylinders or bulk cargo tanks:
Division 2.1 — Flammable Gases: Gases that ignite at 14.7 psi in a mixture of 13% or less by volume with air, or have a flammable range with air of at least 12% (e.g., propane, butane, acetylene, liquefied petroleum gas / LPG).
Division 2.2 — Non-Flammable, Non-Poisonous Compressed Gases: Non-toxic compressed gases, liquefied gases, pressurized cryogenic liquids, and asphyxiant gases (e.g., liquid nitrogen, compressed helium, carbon dioxide, argon, compressed breathing air).
Division 2.3 — Poison/Toxic Gases: Gases known to be toxic or lethal to humans at concentrations of 5,000 ppm or less, or posing an extreme toxic inhalation hazard (e.g., chlorine, anhydrous ammonia, phosgene, hydrogen cyanide).
Class 3: Flammable and Combustible Liquids
Class 3 encompasses liquid commodities that release ignitable vapors:
Flammable Liquid: Any liquid having a flash point of not more than 140°F (60°C) (e.g., gasoline, ethanol, acetone, toluene, methyl ethyl ketone).
Combustible Liquid: Any liquid that does not meet the definition of any other hazard class and has a flash point between 140°F (60°C) and 200°F (93°C) (e.g., diesel fuel, fuel oil, mineral oil, peanut oil derivatives). Under U.S. domestic regulations, combustible liquids in non-bulk packaging are frequently exempt from placarding.
Class 4: Flammable Solids
Class 4 covers solid materials capable of causing fires through friction, moisture absorption, or spontaneous chemical changes:
Division 4.1 — Flammable Solids: Readily combustible solids, self-reactive substances, and solid desensitized explosives (e.g., safety matches, sulfur, nitrocellulose flakes, magnesium ribbon).
Division 4.2 — Spontaneously Combustible Materials: Pyrophoric liquids or solids that ignite within five minutes of air exposure without an ignition source, and self-heating materials that heat up when exposed to air (e.g., white phosphorus, activated charcoal, oily cotton waste rags).
Division 4.3 — Dangerous When Wet (Water-Reactive): Substances that emit flammable or toxic gases when brought into contact with water (e.g., pure metallic sodium, potassium, calcium carbide, aluminum phosphide).
Class 5: Oxidizers and Organic Peroxides
Class 5 materials readily release oxygen, accelerating and intensifying combustion in surrounding materials:
Division 5.1 — Oxidizers: Substances that yield oxygen readily to stimulate or increase the combustion of other matter (e.g., ammonium nitrate fertilizer, hydrogen peroxide solutions >20%, calcium hypochlorite swimming pool shock).
Division 5.2 — Organic Peroxides: Thermally unstable organic compounds containing the bivalent -O-O- structure; highly susceptible to heat, friction, shock, or self-accelerating decomposition (e.g., benzoyl peroxide paste, methyl ethyl ketone peroxide / MEKP, peracetic acid).
Class 6: Toxic and Infectious Substances
Class 6 covers biological and chemical hazards posing acute poisoning or disease risks:
Division 6.1 — Poisonous / Toxic Materials: Materials (other than gases) capable of causing death, serious injury, or severe harm to human health through ingestion, inhalation, or dermal absorption (e.g., potassium cyanide, arsenic trioxide, parathion pesticides, nicotine concentrates).
Division 6.2 — Infectious Substances (Biohazards): Materials known or reasonably expected to contain viable pathogens (bacteria, viruses, parasites, fungi) causing disease in humans or animals (e.g., regulated medical waste, clinical viral cultures, anthrax spore samples).
Class 7: Radioactive Materials
Class 7 encompasses materials that spontaneously emit ionizing radiation exceeding 0.002 microcuries per gram (e.g., enriched uranium, cobalt-60 medical radiotherapy sources, industrial radiography isotopes, yellowcake). Packaged radiation levels are categorized into three warning tiers:
Radioactive White-I: Lowest surface radiation levels (<0.5 mrem/hr on contact; Transport Index = 0).
Radioactive Yellow-II: Moderate radiation levels (0.5 to 50 mrem/hr on contact; Transport Index ≤ 1.0).
Radioactive Yellow-III: High surface radiation levels (50 to 200 mrem/hr on contact, or Transport Index > 1.0; vehicle placarding mandatory).
Class 8: Corrosives
Class 8 includes liquids or solids that cause full-thickness destruction of human skin tissue at the site of contact within a specified observation period, or exhibit a severe corrosion rate on steel or aluminum surfaces (e.g., sulfuric acid / battery electrolyte, hydrochloric acid, nitric acid, sodium hydroxide / caustic soda lye, potassium hydroxide).
Class 9: Miscellaneous Hazardous Materials
Class 9 covers materials presenting a hazard during transportation that do not meet the definition of Classes 1 through 8, but require regulatory oversight due to environmental, thermal, or anesthetic properties (e.g., lithium-ion batteries, dry ice / solid carbon dioxide, asbestos, hazardous waste mixtures not otherwise specified, genetically modified microorganisms).
3. Comprehensive DOT Hazard Class Reference Table
Class # Division Class / Division Title Placard Color & Symbol Representative Cargo Examples
1 1.1–1.6 Explosives Orange diamond; exploding bomb icon with division number. Dynamite, blasting caps, fireworks, ammunition, rocket motors.
2 2.1 Flammable Gas Red diamond; flame icon with number '2'. Propane, acetylene, methane, lighter fluid gas.
2 2.2 Non-Flammable Compressed Gas Green diamond; gas cylinder icon with number '2'. Helium, compressed nitrogen, argon, liquid carbon dioxide.
2 2.3 Poison (Toxic) Gas White diamond; skull and crossbones icon with number '2'. Chlorine gas, anhydrous ammonia, sulfur dioxide.
3 — Flammable / Combustible Liquid Red diamond; flame icon with number '3'. Gasoline, ethanol, acetone, kerosene, diesel fuel.
4 4.1 Flammable Solid White/Red vertical stripes diamond; flame icon with '4'. Safety matches, sulfur powder, magnesium ribbon.
4 4.2 Spontaneously Combustible Top half white, bottom half red diamond; flame icon with '4'. White phosphorus, charcoal briquettes, oily rags.
4 4.3 Dangerous When Wet Blue diamond; flame icon with text 'DANGEROUS WHEN WET' & '4'. Metallic sodium, calcium carbide, potassium metal.
5 5.1 Oxidizer Yellow diamond; flaming 'O' icon with number '5.1'. Ammonium nitrate, hydrogen peroxide, pool bleach.
5 5.2 Organic Peroxide Top half red, bottom half yellow diamond; flame icon with '5.2'. Benzoyl peroxide, MEKP curing catalysts.
6 6.1 Poison (Toxic Material) White diamond; skull and crossbones icon with number '6'. Potassium cyanide, arsenic, concentrated pesticides.
6 6.2 Infectious Substance White diamond; biohazard three-crescent symbol with '6'. Biohazard medical waste, diagnostic blood specimens.
7 — Radioactive Material Top half yellow, bottom half white; trefoil radiation symbol with '7'. Uranium hexafluoride, medical radiation isotopes.
8 — Corrosive Material Top half white, bottom half black; test tubes pouring acid onto hand and steel bar with '8'. Sulfuric acid, battery fluid, hydrochloric acid, liquid lye.
9 — Miscellaneous HazMat Top half vertical black/white stripes, bottom half solid white with '9'. Lithium-ion batteries, dry ice, asbestos insulation.
- 4. UN/NA Identification Numbers and Special Markings
- The 4-Digit UN/NA Number System
The United Nations (UN) and North American (NA) identification numbering system assigns a four-digit number to specific chemicals or generic families of hazardous commodities:
UN Numbers: Internationally recognized 4-digit codes assigned by the United Nations Committee of Experts (e.g., UN1203 for Gasoline, UN1993 for Flammable Liquid, N.O.S., UN1005 for Anhydrous Ammonia).
NA Numbers: North American codes recognized only within the United States and Canada for substances not assigned an international UN number (e.g., NA1993 for Combustible Liquid, N.O.S.).
Display Mechanisms: Placards vs. Orange Panels
Under 49 CFR § 172.301 and § 172.332, the 4-digit identification number must be clearly displayed on cargo tanks, portable tanks, and bulk packaging using one of two approved formats:
Centered on the Diamond Placard: The 4-digit number is printed in large black numbers across the horizontal center of the hazard diamond in place of the hazard class name.
Orange Rectangular Panel: The 4-digit number is printed in black on an orange rectangular panel (measuring at least 160 mm high by 400 mm wide with a black border) mounted immediately adjacent to the hazard placard. ┌────────────────────────────────┐ ┌────────────────────────────────┐
- │ 4-DIGIT ON PLACARD │ │ 4-DIGIT ON ORANGE PANEL │
- │ │ │ │
- │ /\ │ │ ┌────────────────────┐ │
- │ / \ │ │ │ UN 1203 │ │
- │ / 🔥 \ │ │ └────────────────────┘ │
- │ / 1203 \ │ │ + │
- │ \ / │ │ /\ │
- │ \ / │ │ /🔥 \ │
- │ \ / │ │ /FLAMM\ │
- │ \/ 3 │ │ \ 3 / │
- └────────────────────────────────┘ └────────────────────────────────┘
- Special Markings: Marine Pollutants and Elevated Temperature ('HOT')
Marine Pollutant Mark: Features a diamond border containing a dead tree and a dead fish symbol. Applied to packages containing materials toxic to aquatic life, warning responders against washing spills into storm drains, creeks, or waterways.
Elevated Temperature Material Marking: Applied to bulk packages carrying materials transported at high temperatures:
Liquids transported at or above 212°F (100°C) (e.g., liquid asphalt, molten sulfur).
Liquids with a flash point at or above 100°F (37.8°C) intentionally heated and transported at or above their flash point.
Solids transported at or above 464°F (240°C) .
Display: Marked with the word 'HOT' on the bulk packaging or displaying the international white triangular elevated temperature symbol with a red thermometer icon.