Hazardous materials are identified through a layered system of labels, symbols, color codes, and documents that tell you what a substance is and how dangerous it is. Whether you’re looking at a shipping container, a workplace chemical, or a product under your kitchen sink, the identification method follows the same logic: a visual warning gets your attention, and standardized codes or paperwork give you the details.
The Nine UN Hazard Classes
Every hazardous material in the world falls into one of nine classes established by the United Nations. These classes form the backbone of identification across industries and borders.
- Class 1: Explosives, further divided into six divisions based on blast and projection risk
- Class 2: Gases, split into flammable gases, non-flammable gases, and toxic gases
- Class 3: Flammable and combustible liquids, such as gasoline and certain solvents
- Class 4: Flammable solids, including materials that ignite spontaneously or react dangerously with water
- Class 5: Oxidizers and organic peroxides, which can intensify fires or react violently
- Class 6: Toxic and poison inhalation hazards
- Class 7: Radioactive materials
- Class 8: Corrosives, which destroy living tissue or metal on contact
- Class 9: Miscellaneous dangerous goods, a catch-all for hazards that don’t fit neatly into the other eight
When you see a four-digit number on a placard, container, or shipping document, that’s the UN identification number tied to a specific substance within one of these classes. Knowing the class tells you the general type of danger. The four-digit ID number tells you the exact material.
GHS Labels and Pictograms
The Globally Harmonized System (GHS) is the international standard for labeling chemicals in workplaces and consumer products. It uses nine diamond-shaped pictograms with red borders and white backgrounds, each representing a specific type of hazard. A single product can carry more than one.
The pictograms are designed to be understood at a glance. An exploding bomb means explosives. A flame means flammable materials. A flame over a circle means oxidizers. A gas cylinder means compressed gases. A skull and crossbones signals acute toxicity, meaning the substance can cause serious harm or death from a single exposure. A corrosion symbol (liquid eating through a surface and a hand) means the material destroys skin or metals. An exclamation mark indicates a lower-level irritant. A silhouette of a person with a starburst on the chest warns of serious long-term health effects like cancer or reproductive harm. And a dead tree over water signals environmental toxicity.
Every GHS label also carries one of two signal words. “Danger” is reserved for the more severe hazards. “Warning” is used for less severe ones. If a chemical poses multiple hazards and one warrants “Danger” while another warrants “Warning,” only “Danger” appears on the label. You will never see both signal words on the same container.
The NFPA 704 Diamond
If you’ve seen a color-coded diamond shape on the side of a building, storage tank, or industrial container, that’s the NFPA 704 system. It’s designed for emergency responders to quickly assess risk at a fixed location, but it’s useful for anyone who encounters it.
The diamond has four quadrants, each color-coded. Blue sits at the nine o’clock position and represents health hazards. Red is at the top (twelve o’clock) and represents flammability. Yellow is at three o’clock and represents instability or reactivity. White is at the bottom (six o’clock) and is reserved for special hazards, though it isn’t always filled in.
Each colored section contains a number from 0 to 4. Zero means minimal hazard. Four means severe hazard. So a chemical with a red 4 is extremely flammable, while one with a blue 1 poses only a slight health risk. You can read the diamond in seconds: check each quadrant’s number to understand the overall risk profile of whatever is stored inside.
DOT Placards on Vehicles and Containers
During transportation, hazardous materials are identified by diamond-shaped placards displayed on all four sides of a truck, railcar, or shipping container. These placards use a combination of color, symbols, and numbers to communicate the hazard class.
Orange placards indicate explosives. Red signals flammable materials. Green means non-flammable compressed gas. Yellow is for oxidizers. White with a skull and crossbones marks toxic or poisonous substances. White with a trefoil symbol means radioactive. A half-white, half-black placard indicates corrosives. The four-digit UN identification number often appears in the center of the placard or on a separate orange panel nearby.
If you can see a placard but can’t get close enough to read the number, the color and symbol alone narrow down the hazard class. This is intentional. The system is built so that someone 50 feet away from an overturned truck can still get useful information.
Shipping Papers and the ISHP Sequence
Every hazardous material shipment is accompanied by paperwork that spells out exactly what’s being transported. These shipping papers follow a standardized format with four key data points, remembered by the acronym ISHP: Identification number, Shipping name, Hazard class, and Packing group.
The identification number is the UN four-digit code. The proper shipping name is the official standardized name for the material (not a brand name or abbreviation). The hazard class corresponds to one of the nine UN classes. The packing group (I, II, or III) indicates how dangerous the material is within its class, with Group I being the most dangerous. The paperwork also lists the quantity being shipped and the type of packaging used. In a truck, these documents are typically kept in the cab, within reach of the driver.
Safety Data Sheets for Workplace Chemicals
In any workplace that handles chemicals, Safety Data Sheets (SDS) are the most detailed identification resource available. Every hazardous chemical must have one, and employers are required to keep them accessible to workers.
Section 2 of an SDS is the most immediately useful for identification. It lists the hazard classification, the signal word (Danger or Warning), all relevant pictograms, and both hazard statements and precautionary statements that explain the specific risks. If a chemical carries hazards not covered by standard classifications, those are described here too. For mixtures containing ingredients with unknown toxicity, the SDS must state what percentage of the mixture is made up of those unknowns.
Section 3 breaks down exactly what’s in the product. For a single substance, this includes the chemical name, common names, and a unique identifier called a CAS number. For mixtures, it lists every hazardous ingredient along with its concentration. Manufacturers can withhold specific chemical identities as trade secrets, but the SDS must explicitly state that a trade secret claim has been made.
The Emergency Response Guidebook
The Emergency Response Guidebook (ERG) is a pocket-sized reference published by the U.S. Department of Transportation and used by first responders, but it’s freely available as a PDF and useful for anyone who works around hazardous materials. It connects the identification information you find on placards and shipping papers to specific safety guidance.
The process works in steps. If you know the four-digit UN/NA identification number, you look it up in the yellow-bordered pages, which are organized numerically. If you only know the material’s name, you search alphabetically in the blue-bordered pages. Either route gives you a three-digit guide number. You then turn to the orange-bordered pages to find that guide number, which provides specific information about potential hazards, recommended safe distances, and immediate actions. Materials highlighted in green in the yellow or blue pages have additional isolation distance recommendations listed in a separate green-bordered table.
Identifying Hazardous Products at Home
Consumer products use the same GHS pictogram system found in workplaces, though the symbols may be smaller and easier to overlook. The four most common on household items are the flame (found on aerosol sprays, lighter fluid, and some cleaning products), the corrosion symbol (drain cleaners, oven cleaners, and certain pool chemicals), the skull and crossbones (pesticides, antifreeze, and some rodent poisons), and the exclamation mark (milder irritants like some laundry detergents or adhesives).
Beyond the pictograms, household products carry the same “Danger” and “Warning” signal words as industrial chemicals. A bottle marked “Danger” poses a significantly greater risk than one marked “Warning” in the same product category. The back label also lists precautionary statements, first aid measures, and sometimes a phone number for poison control. If you’re trying to figure out whether something in your garage or under your sink qualifies as hazardous waste for disposal purposes, the presence of any GHS pictogram is a reliable starting indicator.