Biomass (e.g., plant biomass, animal biomass, and municipal waste biomass) is processed to produce useful intermediates and products, such as energy, fuels, foods or materials. For example, equipment, systems and methods are described that can be used to treat feedstock materials, such as cellulosic and/or lignocellulosic materials, in a vault in which hazardous gases are removed, destroyed and/or converted. The treatments are efficient and can reduce the recalcitrance of the lignocellulosic material so that it is easier to produce an intermediate or product, e.g., sugars, alcohols, sugar alcohols and energy, from the lignocellulosic material.