Long range transport refers to the fact that winds—especially high-altitude winds—can carry pollutants over incredibly long distances. People once believed that pollutants from sources such as smoke stacks might travel a few miles before settling onto the ground and water supplies. Now it is known that these particulates and toxic gases can make their way into the upper atmosphere. Scientists first started to become aware of this issue in the mid-twentieth century, when nuclear bomb tests resulted in radioactive clouds that would circumnavigate the planet. Acid-rain-causing chemicals can easily cross the entire United States, and, likewise, pesticides and herbicides travel long distances. Natural air pollutants, such as volcanic ash and organic material like fungi, spores, and pollen, can similarly range over long distances.