The higher the number, the greater the hazard.
Pyrophoric materials can ignite spontaneously with no external source of ignition.
For instance, the Group 1 metals on the periodic table react with water to produce hydrogen and heat. Often this heat is enough to ignite the hydrogen. Long-term exposure to carcinogenic substances can result in cancers of various types. A number of substances have been found to be capable of producing cancer following exposure by inhalation, ingestion, or skin contact. Known carcinogens should not be used by students in our high school lab.