Different types of fire extinguishers employ any of a variety of chemicals. These chemicals vary in how effective they are for certain kinds of fires, such as electrical or grease fires. Some extinguishers are not only ineffective against specific types of fires, but they can actually exacerbate conditions and cause the fire to worsen. The label on a fire extinguisher should clearly indicate what kinds of fires it is safe to use that device on.
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Some fire extinguishers simply spray water to control and eliminate fires. However, water fire extinguishers are useful only in situations when basic combustibles — paper, wood, plastics, cloth and objects made of related substances — are burning. Fire extinguishers that work with water are common in households, but they are not suited for electrical or grease fires. A fire extinguisher's principal purpose is to eliminate the oxygen molecules that drive the burning process. In cases where electrical devices are involved, water raises the danger of electrocution and can damage expensive electronics that are possibly salvageable. In the case of an oil-burning fire, the fire quickly becomes so large and out of control that water only serves to lend more oxygen and thus exacerbates the problem.
Fluorocarbons are perhaps the most effective fire extinguisher chemicals, but they are also the most dangerous in the long run. They are both effective and dangerous because of the chlorine and bromine atoms they contain, which pick up hydrogen molecules and eliminate chain reactions that facilitate burning but also cause irreparable damage to the ozone layer as they gradually make their way up into the atmosphere. Bromochlorodifluoromethane was once the most common fluorocarbon found in fire extinguishers, but it is now banned precisely because of the dangers it poses to the ozone. Bromotrifluoromethane poses less of a risk because it is not as toxic in nature as other fluorocarbons. Fluorocarbon fire extinguishers were once common in large computer facilities because they limit damage to electronics. While fluorocarbons are still used in some fire extinguishers, today it is rare for individual homeowners or large companies or businesses to use fire extinguishers with fluorocarbons.
Scientists have been evaluating other options because fire extinguishers featuring fluorocarbons are not feasible environmentally. Hydrofluorocarbons are the most promising chemicals they have found. These compounds are extremely effective in disrupting the chain reactions involved in burning associated with oxygen and hydrogen, although not to the degree of fluorocarbons. The selling point is that they pose no threat to the ozone, as chlorine and bromine atoms are not involved. Some examples of hydrofluorocarbons employed in fire extinguisher functioning include fluoroform and pentafluoroethane.
If a fire is electrical or oil-related in origin, a fire extinguisher using carbon dioxide is the best to use. The low temperature of carbon dioxide reduces the heat and intensity of the fire. Carbon dioxide also displaces the oxygen that keeps the fire going. In an office, company or school, carbon dioxide also provides the advantage of not posing a threat to computers and other electronic equipment as water would. Fire extinguishers operating on carbon dioxide are most frequently used in offices and schools.
Fire extinguishers that employ dry chemicals function by separating the fire from oxygen that is necessary to keep it going. It does this by sweeping a coat of chemical dust across the fire. Typically, the active agent in this dry dust is monoammonium phosphate. Some fire extinguishers with dry chemicals will work for all types of fires — electrical, oil-related and general combustibles — but others may work only for electrical and oil-related fires. The fire extinguisher label should clearly indicate what kinds of fires it is safe to use on. Fire extinguishers operating on dry chemicals are most often utilized in settings where a fire stands a good chance of growing rapidly and dramatically, such as in gas stations or other places with large oil reserves.