To understand acid deposition's causes and effects, and to track changes in the environment, scientists from EPA, state governments, and academia study acidification processes. They collect air and water samples and measure them for various characteristics such as pH and chemical composition, and research the effects of acid deposition on human-made materials such as marble and bronze. Finally, scientists work to understand the effects of sulfur dioxide (SO2) and nitrogen oxides (NOx)—the pollutants that cause acid deposition and contribute to particulate matter —on human health. See the acid rain effects section for more information.