The most significant symbols used in the Laboratory are warning labels. Storage closets and chemical containers display the NFPA (National Fire Protection Association) fire diamond. The four quadrants of the fire diamond are color coded. Red in the top quadrant denotes the fire hazard, blue in the left quadrant denotes the health hazard, and yellow in the right quadrant denotes the chemical reactivity hazard. The degree of severity of the fire, health, and chemical reactivity hazards is expressed by the numbers 0 through 4. O denotes no hazard and 4 denotes the maximum hazard. The bottom quadrant of the fire diamond is reserved for special symbols such as the radiation hazard symbol (propeller), the letter W with a line drawn through its center to caution against applying water, and the letters OXY to warn that the chemical is an oxidant.
Hazard warning signs at the entrance to a Laboratory and individual containers may also display the radiation hazard symbol, the biohazard symbol, and the capital letter C. The biohazard symbol is three overlapping symmetric circles that form a triple Venn diagram. The biohazard symbol warns on a blazing orange background warns that potentially infectious materials are present. Red hard sided containers are used to dispose infectious contaminated sharp objects such as syringes and needles. Red bags are used to dispose of materials contaminated with body fluids such as gauze and bandages. C means that carcinogens are used in the Laboratory or that the substance in a container is a carcinogen. A carcinogen is known to cause cancer and requires special training and precautions when in use to protect the worker and co-workers from developing cancer. Poisons will be labeled with a skull and cross bones. Some flammable solvents will display a flame symbol.
The MSDS (Material Safety Data Sheet) which is required to describe the known hazards of working with a product may also contain some common symbols. The LD50 is the lethal dose or when 50 percent of those exposed at that level will die from the exposure. The LC50 is the lethal concentration when 50 percent who are exposed in the specified parts per million by volume will die. The TLV is the threshold limit value. The TLV is the upper limit of a toxicant to which an average healthy person may be repeatedly exposed without suffering any adverse effects. Pictogram symbols may also be used on an MSDS to indicate what type of PPE (personal protective equipment) should be worn. Common pictograms are goggles, respirator, apron, and gloves.