Coal-fired power plants are the largest aggregate source of the toxic heavy metal mercury: 50 tons per year come from coal power plants out of 150 tons emitted nationally in the US and 5000 tons globally. However, according to the United States Geological Survey, the trace amounts of mercury in coal by-products do not pose a threat to public health. A study in 2013 found that mercury found in the fish in the Pacific Ocean could possibly be linked to coal-fired plants in Asia.