Well, flammable materials can be described as flammable liquids, hydrocarbon vapors, and other vapors such as hydrogen and carbon disulfide that are readily ignitable when released to atmosphere.

Where as, dangerous materials can be defined as the following:

• Toxic materials such as phenol, hydrogen sulfide, chlorine
• Highly corrosive materials such as acids, caustic and other similar materials
• Boiler feedwater and steam, in systems requiring ASME Class 300 and higher flange ratings
• Oxygen in concentrations greater than 35 percent
• Flammable materials (including light hydrocarbons lighter than 68 degrees API)