The biohazard symbol was designed in 1966, by Charles Baldwin, an engineer at the Dow Chemical Company.

Below, a set of hazard warning pictograms.

GHS The Globally Harmonized System is a universal set of symbols from the United Nations.

Nuclear waste
How do we warn people of buried toxic waste without words? And stop them from digging it up. We don’t know if mankind, thousands of years in the future, will understand any radiation hazard symbols, or a written warning (in any current language). This clever visual solution, by Nick Shelton, was for a School of Visual Arts infographics project.

A detail.

Mosquito sign
 Maleria is a huge problem in some countries.

Flight ban
The items that you should not take to the airport. (From an American Airlines check-in page.)

Designed at the University of California in 1946. It symbolizes activity radiating out from an atom.

We know to proceed with caution if we see this.