in the air ("penciling"). Modern methods for extinguishing an urban fire dictate the use of a massive initial water flow, e.g. 500 L/min for each fire hose. The aim is to absorb as much heat as possible at the beginning to stop the expansion of the fire and to reduce the smoke. If the flow is too low, the cooling is insufficient, and the steam that is produced can burn firefighters (the drop of pressure is too small and the vapor is pushed back in their direction). Although it may seem paradoxical, the use of a strong flow with an efficient fire hose and an efficient strategy (diffuse spray, small droplets) requires a smaller amount of water. This is because once the temperature is lowered, only a limited amount of water is necessary to suppress the fire seat with a straight spray. For a living room of , the required amount of water is estimated as 60 L (15 gal). French firefighters used an alternative method in the 1970s: spraying water on the hot walls to create a water vapor atmosphere and asphyxiate the fire. This method is no longer used because it turned out to be risky; the pressure created pushed the hot gases and vapor towards the firefighters, causing severe burns, and pushed the hot gases into other rooms where they could start other fires. Asphyxiating a fire In some cases, the use of water is undesirable. This is because some chemical products react with water to produce poisonous gases, or they may even burn when they come into contact with water (e.g., sodium), see water-reactive substances. Another problem is that some products float on water, such as hydrocarbons (gasoline, oil, and alcohol, etc.); a burning layer can then be spread by the fire. If a pressurized fuel tank is endangered by fire it is necessary to avoid heat shocks that may damage the tank if it is sprayed with cooling water; the resulting decompression might produce a BLEVE (boiling liquid expanding vapor explosion). Electrical fires cannot be extinguished with water since the water could act as a conductor. In such cases, it is necessary to asphyxiate the fire. This can be done in a variety of ways. Chemical products which react with the fuel can be used to stop the combustion. A layer of water-based fire retardant foam can be applied by the fire hose in order