Chemicals can be Corrosive, Explosive, Toxic, Flammable, http://en.wikipedia.org/wiki/Flammable Combustible or Oxidizing. A corrosive chemical can burn the skin or deeper tissue. A toxic chemical is a poison. The poison may enter the body through the skin and not simply by the obvious routes of ingestion or breathing. Effects may not be immediate; a chemical may have a chronic effect, rather than an immediate acute effect.
A chemical is said to be flammable if it will burn or explode at temperatures lower than 140 degrees fahrenheit. Some substances can reach this phase below room temperature.
A chemical is said to be combustible if it will burn or explode at temperatures above 140 degrees fahrenheit.
A chemical is said to be oxidizing if it reacts with oxygen and other substances, resulting in a release of energy in the form of a fire or an explosion.