Most spent catalysts are at least flammable, but some are also self-heating (pyrophoric, exothermic or spontaneously combustible) when exposed to air and therefore must be stored and transported under specific careful conditions. The management of such hazardous wastes is governed by the international waste regulations. As well as the transportation hazards there are also hazards for the recycling of the spent catalysts, such as the contaminants like sulfur, phosphorus and mercury, which may affect the purity of the end product if they exceed a certain level as well as pose a threat to the environment if they are not handled correctly.