basis for most major tactics, such as controlled burns. The fire stops because a lower concentration of fuel vapor in the flame leads to a decrease in energy release and a lower temperature. Removing the fuel thereby decreases the heat. Without sufficient oxygen, a fire cannot begin, and it cannot continue. With a decreased oxygen concentration, the combustion process slows. Oxygen can be denied to a fire using a carbon dioxide fire extinguisher, a fire blanket or water. Role of water in fire-fighting The water can have two different roles. In the case of a solid combustible, the solid fuel produces pyrolyzing products under the influence of heat, commonly radiation. This process is halted by the application of water, since water is more easily evaporated than the fuel is pyrolyzed. Thereby energy is removed from the fuel surface and it is cooled and the pyrolysis is stopped, removing the fuel supply to the flames. In fire fighting, this is referred to as surface cooling. In the gas phase, i.e. in the flames or in the smoke, the combustible can not be separated from the oxidizer, and the only possible action consists of cooling down. In this case, water droplets are evaporated in the gas phase, thereby lowering the temperature and adding water vapour making the gas mixture non combustible. This requires droplets of a size less than about 0.2 mm. In fire fighting, this is referred to as gas cooling or smoke cooling. Cases also exist where the ignition factor is not the activation energy. For example, a smoke explosion is a very violent combustion of unburned gases contained in the smoke created by a sudden fresh air input (oxidizer input). The interval in which an air/gas mix can burn is limited by the explosive limits of the air. This interval can be very small (kerosene) or large (acetylene). Water cannot be used on certain type of fires: Fires where live electricity is present – as water conducts electricity it presents an electrocution hazard. Hydrocarbon fires – as it will only spread the fire because of the difference in density/hydrophobicity. For example, adding water to a fire with an oil source will cause the oil to spread, since oil and water do not mix. Metal fires – as these fires produce huge amounts of energy (up to 7.550 calories/kg for aluminium) and water can also create violent chemical reactions with