A study by scientists of USDA Forest Service and Dornsife School of Public Health, Drexel University has discovered that Moss growing on urban trees is a useful bio-indicator of Cadmium Air Pollution. The scientists used moss to map atmospheric cadmium in Portland, Oregon. They used moss to track down previously unknown pollution sources in a complex urban environment with many possible sources.The new finding could provide a low-cost source of information to complement existing monitoring.