Air pollutants come from natural and human sources. Natural sources include dust blown by wind (Figure 7-1, p. 140), pollutants from wildfires and volcanic eruptions, and volatile organic chemicals released by some plants. Most natural air pollutants are spread out over the globe or removed by chemical cycles, precipitation, and gravity. However, chemicals emitted from volcanic eruptions and some natural forest fires can temporarily reach harmful levels in areas where they occur.