The location of the tank and the temperature of the storage area both influence the storage life of fuel. As diesel fuel is stored, it slowly reacts with the oxygen in the air to form gums and varnishes, which are harmful to the environment.  As the temperature increases, the rate of the reaction rises up as well. The addition of contaminants such as water or rust speeds up this procedure. Generally speaking, fuel should not be stored for more than six months at a time.  The tank should be located underground or inside to reduce temperature extremes, and it should be equipped with proper ventilation caps if it is to be used for long-term storage. Testing is recommended if the storage period exceeds one year.