Ash produced as a consequence of explosive volcanic eruptions can cause multiple hazards both close to the volcano and at great distances. Explosive volcano eruptions often release volcanic plumes into the atmosphere, which consist of tephra (submillimeter-sized rock particles), water vapor and other gases such as carbon dioxide (CO2), sulfur dioxide (SO2) and hydrogen sulfide (H2S). Particles from volcano eruptions are transported by wind to thousands of kilometers away, or even over 10,000 km from their source for some fine particles. This book discusses the environmental impact and health risks volcanic ash poses as well as its chemical composition.