Many passengers are unaware of what consist of hazardous material (hazmat) & how dangerous it could be. Most passenger airlines don't carry hazardous material, but still required to train their flight crew & ramp personnel about hazardous material. In a nutshell, hazardous material could be any substance of liquid, gasses, vapor, or even dry ice that can fuel a fire ,or cause disruption of the systems of the aircraft. Hazardous material can be dangerous as it was for one flight a month ago that crash in United Arab Emirates. This flight was a UPS cargo carrier that was carrying lithium batteries. Lithium batteries must be carried in special cases. They do have risk of fire if not handled properly. In this case that was exactly what happen on this UPS flight. The last passenger flight that crash due to carrying hazardous materials was in the mid-90s when a Valujet (AirTran) flight when down in Florida when a contractor shipped hazmat without properly disclosing the contents to the airline. The  shipment of oxygen cylinders erupted & cause a fire on a DC-9 causing it to crash. Since that accident, better safety measure are in place to ensure hazmat doesn't make it on passenger airlines. The airlines are only allowed to carry company material. They are not allowed to carry hazardous material. Only cargo airlines like UPS ,or FexEx carry hazardous material under strict guidelines. Today, it is difficult for hazmat to make it on a passenger airline because there are so many layers of safety. All airline personnel are trained annually on the hazards of hazmat & how to spot it. Ramp personal & pilots both preflight the aircraft to make sure it's in compliance. As the FAA's mottto quotes, "Safety is not an accident, It must be practice" .