Pollution is the introduction of contaminants into the natural environment that can cause harm. Pollution can take the form of chemical substances or energies such as noise, heat, or light from both natural and human-made sources. In 2015, air and water pollution killed over 9 million people worldwide. During the Industrial Revolution, factories released untreated chemicals and waste into waterways, worsening pollution problems. Air pollution has been an issue since ancient times when early humans built fires without proper ventilation, leaving soot deposits as evidence.