Fly ash is produced when pulverised fuels are burnt. Like other types of dust, fly ash may affect vegetation by both physical and chemical processes. Physically, particles may cover the leaf surface and reduce the amount of light available for photosynthesis, or may occlude stomata. Occlusion may lead to increased resistance to gas exchange, or may prevent full stomatal closure, leading to water stress. Increased transpiration is a common response to dust exposure. Deposition rates to tall vegetation are greater than to short vegetation.