There is no specific number that defines either safe or unsafe mold exposure. Some experts have proposed airborne mold guidelines, however none of these have been adopted by regulatory agencies. Mold concentrations outdoors vary greatly with respect to time, species, and amount. Exposure to airborne mold outdoors, where levels often exceed thousands of spores per cubic meter, is considered safe for the general public. Except in buildings with extensive mold growth, the amount of mold found in indoor air is usually much less than what is found outdoors. For people with allergies to mold however, there may be no practical level of exposure, either indoors or outdoors, that would not create discomfort or harm. It is therefore wise to remove and prevent indoor mold growth.