The selfheating of coal is influenced by many factors, but mainly it involves exothermic reactions between reactive sites in the coal and oxygen from the air. Moisture affects the nature of the radical sites where oxidation occurs, and affects the tendency of coal to combust spontaneously. Adiabatic tests show the coal selfheating rate increases as the moisture content increases. At approximately 40% to 50% of the moisture holding capacity of the coal, the selfheating rate becomes measurable. Above this critical level of moisture content, the heat produced by oxidation is dissipated by moisture evaporation and coal selfheating is significantly delayed.