Hazmat Placards Guide: DOT Classes, Colors, Symbols & Emergency Response

Why Hazmat Placards Matter

When a tanker truck overturns on the highway, the first thing firefighters and hazmat teams look for is the diamond-shaped placard on the vehicle. That placard identifies the hazardous material inside and dictates the entire emergency response — evacuation distance, PPE requirements, suppression agents, and decontamination procedures.

The placarding system is governed by the U.S. Department of Transportation (DOT) under 49 CFR Part 172. Every commercial vehicle transporting hazardous materials must display placards on all four sides of the vehicle or container. Placards must be at least 250 mm (9.84 inches) on each side and clearly visible.

This guide covers all 9 DOT hazard classes, how to read placard information, and how first responders use the Emergency Response Guidebook (ERG) during incidents.

How to Read a Hazmat Placard

Every hazmat placard contains up to four pieces of information:

1. Symbol (top) — A pictogram representing the hazard type (flame, skull, explosion, etc.)
2. Text (middle) — The hazard class name (e.g., "FLAMMABLE," "CORROSIVE")
3. Hazard class number (bottom) — The DOT class number (1 through 9)
4. UN/NA number (center or separate orange panel) — A 4-digit identification number for the specific material

The UN number is critical for first responders. It identifies the exact substance — for example, UN 1203 is gasoline, UN 1005 is anhydrous ammonia. Responders cross-reference this number in the ERG to find specific response procedures.

The 9 DOT Hazard Classes

Class 1 — Explosives

Color: Orange

Symbol: Exploding bomb

What it covers: Any substance or article designed to produce an explosion or pyrotechnic effect. Divided into 6 divisions:

• 1.1 — Mass explosion hazard (dynamite, TNT, detonators)
• 1.2 — Projection hazard but not mass explosion (ammunition, rockets)
• 1.3 — Fire hazard with minor blast/projection (propellant charges, fireworks)
• 1.4 — Minor explosion hazard, confined to package (small arms ammunition, consumer fireworks)
• 1.5 — Very insensitive explosives (blasting agents, ANFO)
• 1.6 — Extremely insensitive detonating articles

First responder note: Class 1 materials require immediate evacuation at distances specified in the ERG. Do not attempt to suppress explosives fires — establish a perimeter and wait for the explosive ordnance disposal (EOD) team.

Class 2 — Gases

What it covers: Compressed, liquefied, dissolved, or refrigerated gases. Three divisions:

• 2.1 — Flammable Gas (red placard, flame symbol): Propane, hydrogen, acetylene, methane. Ignites readily in air
• 2.2 — Non-Flammable/Non-Toxic Gas (green placard, gas cylinder symbol): Nitrogen, helium, carbon dioxide, compressed air. Can displace oxygen in confined spaces
• 2.3 — Toxic Gas (white placard, skull and crossbones): Chlorine, ammonia, phosgene. Inhalation hazard — requires full SCBA and chemical protective clothing

First responder note: Compressed gas cylinders can become projectiles (BLEVE — Boiling Liquid Expanding Vapor Explosion) when exposed to fire. Cool containers from maximum distance with water streams. Always approach from upwind and uphill.

Class 3 — Flammable Liquids

Color: Red

Symbol: Flame

What it covers: Liquids with a flash point below 100°F (37.8°C). Examples: gasoline (UN 1203), diesel fuel (UN 1202), ethanol, acetone, paint thinners, jet fuel.

First responder note: Class 3 is the most commonly transported hazmat class. Vapors are heavier than air and can travel along the ground to ignition sources far from the spill. Use foam (AFFF) for suppression — not water. Shut down ignition sources in the area. Prevent runoff into drains and waterways.

Class 4 — Flammable Solids

What it covers: Three divisions of solid materials that present fire hazards:

• 4.1 — Flammable Solid (red and white striped placard): Matches, sulfur, magnesium ribbons. Ignites easily from friction or heat
• 4.2 — Spontaneously Combustible (red and white placard): White phosphorus, aluminum alkyls. Ignites on contact with air
• 4.3 — Dangerous When Wet (blue placard): Sodium, potassium, calcium carbide. Reacts violently with water, producing flammable or toxic gases

First responder note: Never use water on Class 4.3 materials. Sodium reacts explosively with water. Use dry sand or specialized Class D extinguishing agents.

Class 5 — Oxidizers and Organic Peroxides

• 5.1 — Oxidizer (yellow placard): Ammonium nitrate, potassium permanganate, hydrogen peroxide (>8%). Supplies oxygen to a fire, making it burn hotter and faster. Can cause other materials to ignite spontaneously
• 5.2 — Organic Peroxide (red and yellow placard): Benzoyl peroxide, methyl ethyl ketone peroxide. Thermally unstable — can explode, burn, or react dangerously to friction and contamination

First responder note: Oxidizers intensify fires dramatically. Standard fire suppression may be ineffective because the material generates its own oxygen supply. Flood with water from maximum distance.

Class 6 — Toxic and Infectious Substances

• 6.1 — Toxic Substances (white placard, skull and crossbones): Pesticides, cyanide compounds, arsenic. Harmful or fatal if inhaled, ingested, or absorbed through skin
• 6.2 — Infectious Substances (white placard, biohazard symbol): Medical waste, diagnostic specimens, biological agents. Requires full BSI (body substance isolation) and decontamination protocols

First responder note: Full PPE including SCBA for any Class 6.1 release. Establish hot/warm/cold zones. For 6.2, follow bloodborne pathogen protocols.

Class 7 — Radioactive Materials

Color: Yellow and white

Symbol: Trefoil (radioactive symbol)

What it covers: Materials emitting ionizing radiation. Three categories based on radiation level at the package surface (I, II, III). Examples: medical isotopes, nuclear fuel, industrial radiography sources.

First responder note: Time, distance, and shielding are your three protections. Minimize time near the source, maximize distance, and put mass (vehicles, walls, earth) between you and the source. Contact your regional radiological response team immediately. Do not touch or move containers.

Class 8 — Corrosive Substances

Color: Black and white

Symbol: Liquid dripping onto hand and metal surface

What it covers: Materials that destroy or irreversibly damage living tissue or metal on contact. Examples: sulfuric acid (UN 1830), sodium hydroxide (lye), hydrochloric acid, battery acid.

First responder note: Corrosives cause severe chemical burns on contact. Full chemical-resistant PPE required. Flush affected skin with large amounts of water for at least 20 minutes. Contain runoff — corrosives can contaminate water sources.

Class 9 — Miscellaneous Dangerous Goods

Color: Black and white stripes

What it covers: Hazardous materials that don't fit into Classes 1-8 but still present a danger during transport. Examples: lithium batteries (UN 3481), dry ice (UN 1845), asbestos, magnetized materials, elevated-temperature materials.

First responder note: Lithium batteries (increasingly common due to EV transport) can undergo thermal runaway — producing intense heat, toxic gases, and re-ignition even after apparent suppression. See our guide on EV and lithium battery fires for detailed response procedures.

Quick Reference Table

Class	Hazard	Placard Color	Symbol	Common Examples
1	Explosives	Orange	Exploding bomb	Dynamite, fireworks, ammo
2.1	Flammable Gas	Red	Flame	Propane, hydrogen, acetylene
2.2	Non-Flammable Gas	Green	Gas cylinder	Nitrogen, helium, CO2
2.3	Toxic Gas	White	Skull	Chlorine, ammonia
3	Flammable Liquid	Red	Flame	Gasoline, diesel, ethanol
4.1	Flammable Solid	Red/white stripes	Flame	Matches, sulfur
4.2	Spontaneously Combustible	Red/white	Flame	White phosphorus
4.3	Dangerous When Wet	Blue	Flame	Sodium, potassium
5.1	Oxidizer	Yellow	Flame over circle	Ammonium nitrate, H2O2
5.2	Organic Peroxide	Red/yellow	Flame	Benzoyl peroxide
6.1	Toxic	White	Skull	Pesticides, cyanide
6.2	Infectious	White	Biohazard	Medical waste
7	Radioactive	Yellow/white	Trefoil	Medical isotopes
8	Corrosive	Black/white	Corrosion	Sulfuric acid, lye
9	Miscellaneous	Black/white stripes	—	Lithium batteries, dry ice

The Emergency Response Guidebook (ERG)

The ERG is a pocket-sized reference published by the DOT/PHMSA, updated every 4 years (current edition: 2024). Every fire apparatus in the United States carries a copy. It's designed for the first 15-30 minutes of a hazmat incident — before specialized hazmat teams arrive.

How to use the ERG:

1. Identify the material by its UN/NA number (on the placard or shipping papers) or by name
2. Look up the Guide number in the yellow-bordered pages (by UN number) or blue-bordered pages (by material name)
3. Turn to the Guide page (orange-bordered) for specific response information: potential hazards, public safety actions, evacuation distances, and fire/spill/leak response
4. Check the green-bordered pages for Table of Initial Isolation and Protective Action Distances for materials that produce toxic gases

Placarding Thresholds: When Placards Are Required

Not every hazmat shipment requires placards. The DOT uses a two-table system:

• Table 1 materials: Require placards in any quantity. These are the most dangerous — poison inhalation hazards, explosives (1.1-1.3), radioactive Class III. Even a single container requires full placarding
• Table 2 materials: Require placards only when shipping 1,001 pounds (454 kg) or more. This includes most Class 3, 4, 5, 6.1, 8, and 9 materials

Important for first responders: The absence of a placard does not guarantee the vehicle is carrying nothing hazardous. Shipments under Table 2 thresholds may still contain dangerous quantities. Always check shipping papers in the cab.

Key Takeaways

• Hazmat placards use 9 classes with color-coded, diamond-shaped signs on all 4 sides of a vehicle
• The UN/NA number identifies the exact substance — cross-reference it in the ERG for response guidance
• Always approach hazmat scenes from upwind and uphill
• Never use water on Class 4.3 (Dangerous When Wet) materials
• The ERG covers the first 15-30 minutes — it's a bridge until hazmat specialists arrive
• Test your hazmat knowledge with our firefighter exam practice quizzes