Concern over the dangers posed by mercury prompted the United States government to restrict the sale of mercury oxide batteries in 1996. However, the Environmental Protection Agency (EPA) warns that button cells are allowed to contain low levels of added mercury to prevent internal corrosion. Watch batteries also contain several heavy metals and other toxic substances harmful to humans and the environment. For these reasons the EPA advise that watch batteries and other button cells be recycled rather than thrown away. Extracting heavy and precious metals, along with the ease of handling, make recycling watch batteries profitable for some companies.