A hazardous material is any substance or material capable of posing an unreasonable risk to health, safety, or property, particularly during transport, storage, or disposal. The U.S. Department of Transportation formally designates these materials under federal law, but several agencies play a role in defining what counts as hazardous depending on the context: shipping, workplace safety, environmental cleanup, or household disposal.
The Federal Definition
Under federal regulation (49 CFR 171.8), the DOT defines a hazardous material as any substance the Secretary of Transportation has determined poses an unreasonable risk when transported in commerce. This broad definition covers hazardous substances, hazardous wastes, marine pollutants, elevated temperature materials, and anything listed in the official Hazardous Materials Table or meeting the criteria for one of the nine hazard classes.
The Pipeline and Hazardous Materials Safety Administration (PHMSA), a branch of the DOT, oversees these classifications and updates the rules to stay aligned with international standards. The EPA handles a parallel system for hazardous waste, focusing on materials after they’ve been discarded rather than while they’re in transit.
The Nine Hazard Classes
The DOT sorts all hazardous materials into nine classes based on the primary danger they present. These classes apply to commercial shipping by road, rail, air, and sea.
- Class 1: Explosives. Materials that can detonate or deflagrate, ranging from mass-explosion hazards (Division 1.1) down to extremely insensitive articles (Division 1.6).
- Class 2: Gases. Includes flammable gases like propane (Division 2.1), non-flammable compressed gases like nitrogen (Division 2.2), and toxic gases like chlorine (Division 2.3).
- Class 3: Flammable and combustible liquids. A flammable liquid has a flash point at or below 60°C (140°F). Combustible liquids fall between 60°C and 93°C (140°F to 200°F). Gasoline, acetone, and certain adhesives fall here.
- Class 4: Flammable solids. Covers solids that ignite easily (4.1), materials that can spontaneously combust (4.2), and substances that become dangerous when wet (4.3).
- Class 5: Oxidizers and organic peroxides. These materials intensify fire by supplying oxygen or can react violently on their own.
- Class 6: Toxic and infectious substances. Poisons that can cause serious harm through ingestion, skin contact, or inhalation. A liquid or solid is classified as toxic if a lethal dose for oral exposure falls at or below 300 mg/kg of body weight in animal testing.
- Class 7: Radioactive materials. Anything emitting ionizing radiation above regulatory thresholds, from medical isotopes to nuclear fuel.
- Class 8: Corrosives. Substances that destroy living tissue or degrade metals on contact, such as sulfuric acid or sodium hydroxide.
- Class 9: Miscellaneous dangerous goods. A catch-all for hazards that don’t fit neatly into other classes. Lithium batteries, dry ice, and certain environmentally hazardous substances fall here.
How the EPA Classifies Hazardous Waste
The EPA uses a separate but overlapping framework. When a material becomes waste, it’s considered hazardous if it exhibits any one of four characteristics: ignitability, corrosivity, reactivity, or toxicity. A waste that catches fire easily, corrodes metal or skin, is unstable enough to explode, or leaches toxic chemicals qualifies under this system.
The EPA also maintains lists of specific hazardous wastes from particular industrial processes and chemical products. If a substance appears on these lists, it’s hazardous waste by definition regardless of its measurable properties. When any of these substances is released into the environment in a quantity that meets or exceeds its assigned Reportable Quantity, the release must be reported to federal authorities. The default Reportable Quantity is one pound, though the EPA has adjusted this threshold for many substances. Unlisted hazardous wastes that exhibit toxicity carry a Reportable Quantity of 100 pounds.
Common Household Hazardous Materials
You don’t need to work in a chemical plant to have hazardous materials in your home. The EPA considers leftover household products hazardous if they can catch fire, react or explode, corrode surfaces, or release toxic chemicals. Everyday examples include paints, solvents, cleaners, motor oil, car batteries, pesticides, herbicides, pool chemicals, and aerosol cans.
These products are safe when used as directed, but they require careful disposal. Pouring them down the drain, dumping them on the ground, or tossing them in regular trash can contaminate water supplies and soil. Many communities run collection programs for household hazardous waste, either at permanent drop-off sites or during designated collection days. Some local businesses, like auto shops, will accept used motor oil for recycling.
A few practical rules: keep products in their original labeled containers, never mix different chemicals together, and treat even empty containers with care since residual chemicals inside can still be dangerous. If a container is visibly corroding, contact your local hazardous materials office or fire department before handling it.
Lithium Batteries as Hazardous Materials
Lithium batteries are one of the most commonly shipped hazardous materials today, found in phones, laptops, power tools, and electric vehicles. They’re regulated because they can overheat, catch fire, or explode if damaged or improperly packaged. Federal regulations assign four UN identification numbers depending on the battery type and how it’s shipped: UN3480 for lithium-ion batteries alone, UN3481 for lithium-ion batteries packed with or inside equipment, UN3090 for lithium metal batteries alone, and UN3091 for lithium metal batteries in or with equipment.
Since May 2024, every lithium-ion battery must display its watt-hour rating on the outside case. Packages containing lithium batteries generally need a lithium battery handling mark, and certain shipments are forbidden on passenger aircraft entirely. Batteries shipped by ground or rail with quantities exceeding standard limits must be marked as forbidden for both aircraft and vessel transport. These rules exist because lithium battery fires in cargo holds have caused fatal aviation incidents.
Placards, Labels, and Shipping Marks
When hazardous materials travel by truck or rail, the vehicles carrying them must display diamond-shaped placards that identify the hazard class. Each placard must be clearly visible from the direction it faces, securely attached, and kept in readable condition. Placards have to sit at least 3 inches away from any advertising or other markings, be placed on a contrasting background, and display any text or identification numbers horizontally. They can’t be hidden behind ladders, pipes, tarps, or doors, and carriers are responsible for keeping them clean and legible throughout transit.
These placards are color-coded by hazard type: red for flammable, yellow for oxidizers, white for toxic or infectious substances, and so on. The system is designed so that emergency responders arriving at an accident scene can immediately identify what they’re dealing with before approaching the vehicle.
How Classifications Are Kept Current
Hazardous material regulations are not static. PHMSA periodically updates U.S. rules to align with international standards, including the International Civil Aviation Organization’s Technical Instructions, the International Maritime Dangerous Goods Code, and the United Nations Model Regulations for transport of dangerous goods. The most recent proposed harmonization round incorporates updates that took effect internationally on January 1, 2025.
Among the notable changes under consideration are new shipping classifications for sodium-ion batteries (an emerging alternative to lithium-ion), a requirement that certain lithium and sodium-ion batteries shipped by air be charged to no more than 30 percent of capacity, and the addition of several new marine pollutants to regulated lists. These updates reflect the reality that the types of materials being manufactured and shipped evolve constantly, and the regulatory framework has to keep pace.