Rainwater was once the purest form of water available but now is often contaminated by pollutants in the air. Acid rain is caused when industrial emissions mix with atmospheric moisture. Pollutants may be carried in clouds for long distances before falling, which means that forests and lakes far away from factories may be damaged by acid rain. In the near vicinity of the factories, additional damage is caused by the deposition of larger pollutant particles falling to the ground in dry form. Air pollution has been increasing since the Industrial Revolution but only recently have side effects such as acid rain become severe and widespread enough to evoke international concern.