Methylmercury is a biocaccumulative environmental toxicant. This means that it builds up in the tissues of living things and an organism higher in the food chain, can accumulate more due to consuming other organisms in the food chain below it. When fossil fuels like coal are burned, this adds inorganic mercury to the environment. Boilers that burn fossil fuels in industry add some as well. Natural sources are volcanos, forest fires and weathering of rocks that contain the element mercury. In an aquatic environment, anerobes will typically convert inorganic mercury to methylmercury. This can occur naturally, but, when industries dump inorganic mercury into a body of water, this will occur on a larger scale and can cause poisoning of the environment. It is not easy to remove methylmercury from the environment as it naturally forms in aquatic environments and is magnified through the food chain to the largest organisms--larger fish, mammals like mammals and even humans. Limits on mercury emissions add an economic cost on coal-fired boilers but that is a way to reduce mercury in the environment. Trying to use alternative "clean energy technology" that doesn't rely on the burning of fossil fuels is another method to reduce mercury emissions.