In the workplace workers encounter several types of hazards; such as chemical hazards (liquids solids dusts vapours and gases) physical hazards (noise vibration unsafe machinery inadequate lighting radiation extreme temperatures) biological hazards (bacteria viruses parasites or infectious waste) and psychological hazards (stress and strain). Since the body's own defence systems cannot always win the war against these hazards it is important to learn what types of hazards exist in the workplace and what steps can be taken to prevent exposure to them. Knowledge of hazards helps workers work with their union and employer to eliminate them.