HazMat Quick Reference
The hazmat knowledge test draws on rules scattered across five parts of 49 CFR. This page collects the ones that come up most and that a single practice question can't show on its own — what the nine hazard classes actually are, when a load needs placards at all, and what the segregation table's X and O actually mean.
The 9 hazard classes
| Class | Covers | Example |
|---|---|---|
| 1 | Explosives (Divisions 1.1–1.6) | Dynamite, fireworks, ammunition |
| 2 | Gases — flammable (2.1), non-flammable (2.2), poisonous/toxic (2.3) | Propane, oxygen, chlorine |
| 3 | Flammable liquids | Gasoline, diesel, paint thinner |
| 4 | Flammable solids (4.1); spontaneously combustible (4.2); dangerous when wet (4.3) | Matches, white phosphorus, calcium carbide |
| 5 | Oxidizers (5.1); organic peroxides (5.2) | Ammonium nitrate, bleach concentrate |
| 6 | Poison/toxic materials (6.1); infectious substances (6.2) | Pesticides, medical waste |
| 7 | Radioactive materials | Medical isotopes, enriched uranium |
| 8 | Corrosive materials | Battery acid, drain cleaner |
| 9 | Miscellaneous hazardous materials | Lithium batteries, dry ice |
When a load actually needs placards
Two different rules apply, and mixing them up is a common test trap:
- Table 1 materials placard in any quantity. Explosives (1.1–1.3), poison gas (2.3), dangerous-when-wet materials (4.3), certain organic peroxides, some poison-inhalation-hazard materials, and radioactive materials all need placards no matter how small the shipment.
- Table 2 materials placard only above 1,001 pounds (454 kg) aggregate — the combined weight of every Table 2 material on the vehicle, not per class. Below that, no placard is required for those materials.
- The 1,001-pound exemption never applies to bulk packaging. A cargo tank or other bulk container of a Table 2 material is placarded regardless of weight.
Segregation table: what X and O mean
The segregation table in 49 CFR 177.848 tells you which hazard classes can share a vehicle:
- X — never together. Those two material types may not be loaded, transported, or stored together in the same vehicle, however they're packaged.
- O — together only if separated. They may share a vehicle only if arranged so a leak from one, under normal shipping conditions, could not reach or mix with the other.
- Blank — no table-based restriction between that particular pair of classes.
Bulk vs. non-bulk packaging
"Bulk" is a defined size threshold, not a judgment call:
| Material | Bulk packaging starts at |
|---|---|
| Liquid | Capacity over 119 gallons (450 liters) |
| Solid | Net mass over 882 lb (400 kg) and capacity over 119 gallons (450 liters) |
| Gas | Water capacity over 1,000 lb (454 kg) |
Security plans and attended vehicles
A written security plan is required — regardless of quantity — for Division 1.1–1.3 explosives and any poison-inhalation-hazard material, and it must cover three things: personnel security, unauthorized access, and en route security. Large bulk shipments (generally over 3,000 kg or liters) of certain other classes trigger it too.
Separately, a parked placarded vehicle counts as "attended" only when the responsible person is awake, not in the sleeper berth, and either on the vehicle or within 100 feet with it in clear, unobstructed view.
Take the free HazMat practice test
More CDL guides
See every CDL knowledge test and guide →
Frequently asked questions
Do all hazardous materials need placards in any quantity?
No. Only Table 1 materials — explosives, poison gas, dangerous-when-wet materials among them — placard at any quantity. Table 2 materials only need placards once their combined weight reaches 1,001 pounds, and that exemption never applies to bulk packaging.
What does an X mean on the hazmat segregation table?
An absolute prohibition — those two material types may never be loaded, transported, or stored together in the same vehicle. An O means they may travel together only if separated so a leak from one could not reach the other.
What makes a hazmat package 'bulk packaging'?
A defined size threshold: over 119 gallons for a liquid, over 882 pounds and 119 gallons for a solid, or over 1,000 pounds of water capacity for a gas. Below that, it's non-bulk.
Does every hazmat shipment need a written security plan?
No. It's required regardless of quantity for Division 1.1–1.3 explosives and poison-inhalation-hazard materials, and for large bulk shipments of some other classes — not for every hazmat load.
