While volcanoes and forest fires release mercury, they are relatively small sources compared to the combustion of coal, oil, and other fuels. Mercury is toxic. Microbes turn mercury into a neurotoxin called methylmercury. They also turn the neurotoxin back into inorganic mercury. To predict the levels of inorganic mercury and methylmercury in the environment, scientists need to know how fast microbes act. Using new experiments and re-analyses of previous experiments, scientists developed a model that describes the production of methylmercury over time. The new model takes into account competing processes and results in faster rates of production than previously estimated.