and grounding power lines to guard against electrical feedback, and using appropriate personal protective equipment. Proper respiratory protection can protect against hazardous substances. Proper ventilation of an area is an engineering control that can be used to avoid or minimize exposure to hazardous substances. When ventilation is insufficient or dust cannot be avoided, personal protective equipment such as N95 respirators can be used. Reconnaissance and "reading" the fire The first step in a firefighting operation is reconnaissance to search for the origin of the fire (which may not be obvious for an indoor fire, especially if there are no witnesses), to identify any specific risks, and to detect possible casualties. An outdoor fire may not require reconnaissance, but a fire in a cellar or an underground car park with only a few centimeters of visibility may require long reconnaissance to identify the source of the fire. The "reading" of a fire is the analysis by firefighters of indications of thermal events such as flashover, backdraft or smoke explosion. It is performed during reconnaissance and fire suppression maneuvers. The main signs are: Hot zones, which can be detected with a gloved hand, for example by touching a door before opening it; Soot on windows, which usually means that combustion is incomplete, and thus, a lack of air in the room; Smoke pulsing in and out around a door frame, as if the fire were breathing, which usually also means a lack of air to support combustion. Spraying water on the ceiling in short pulses of a diffused spray (e.g., a cone with an opening angle of 60°) can be undertaken to test the heat of smoke: If the temperature is moderate, the water falls down in drops with a sound like rain; if the temperature is high, the water vaporizes with a hiss—the sign of a potentially extremely dangerous impending flashover. Ideally, part of reconnaissance is consulting a plan for the building that provides information about structures, firefighter hazards, and in some cases the most appropriate strategies and tactics for fighting a fire in that context. Science of extinguishment There are four elements needed to start and sustain a fire and/or flame: temperature, a fuel, an oxidizing agent (oxygen), and a chemical reaction. A fire can be extinguished by taking away any of the four components. The fuel is the substance being oxidized or burned in the combustion process. The most