Pollution is the introduction of contaminants into the natural environment that can cause harm. Pollution can take various forms including chemical substances and energies like noise, heat, and light. Pollutants are either foreign substances or naturally occurring contaminants that make up pollution. In 2015, pollution caused 9 million deaths worldwide. During the Industrial Revolution, untreated chemicals and waste were dumped into water supplies. Air pollution has been present since ancient civilizations, as evidenced by soot found on ceilings in prehistoric caves.