Wood pellets are a type of wood fuel, generally made from compacted sawdust. They are usually produced as a bye-product of sawmilling and other wood transformation activities. The pellets are extremely dense and can be produced with a low humidity content (below 10%) that allows them to be burned with a very high combustion efficiency. Pellet heating systems provide a low net CO2 solution, because the quantity of CO2 emitted during combustion is equal to the CO2 absorbed by the tree during its growth. Using the high efficiency burners developed in recent years, other emissions (such as NOx) are also very low, making this one of the cleanest heating options available.