Sulfur or pollen? Chemical, biological, and toxicological basis for the Correct risk communication of urban yellow dust deposition

Urban yellow dust deposition is a common phenomenon in many parts of the world, which is sometimes called “sulfur shower,” “sulfur rain,” or “pollen storm.” Most people, especially those living in the vicinity of industrial facilities, wrongly perceive the yellow dust as sulfur when in fact it is pollen. The misunderstanding increases risk perception as people believe the “yellow powder” is a serious threat to their health. Based on simple observations, it is virtually impossible to differentiate sulfur from pollen, so risk communication should consider the chemical, biological, and toxicological aspects of these agents. In this review, we clarify that industrial emissions of sulfur are under the form of sulfides, oxides, and other volatile compounds which are gaseous and noncolored, and we explain that it is chemically impossible for gaseous sulfur to become solid yellow sulfur under normal environmental conditions. We also describe pollen and its release from trees, shrubs, and herbs a process influenced by atmospheric conditions. We suggest take-home messages that risk communicators may use when explaining the phenomenon to their communities.