* GPM: Gallons Per Minute or how many gallons are being pumped out of a piece of equipment every minute
* GPM method ("gallons per minute"): Calculation of how much water, in GPM, will be necessary to extinguish a given volume of fire, under the circumstances (e.g., fuel class, containment, exposures, etc.).
* Grease fire: A fire involving any manner of cooking oil or other flammable cooking or lubricating materials. Also known as a Class B, F or K fire.
* Goer: An incident with persons reported.
* Hazard: a source of danger of personal injury or property damage; fire hazard refers to conditions that may result in fire or explosion, or may increase spread of an accidental fire, or prevent escape from fire. Under worker safety and health regulations, employers have a general duty to provide a workplace free of hazards. See also fire prevention, and HAZMAT.
* Hard Line: A smaller hose about one inch in diameter used by firefighters to clean apparatus.
* HAZMAT: Hazardous materials, including solids, liquids, or gasses that may cause injury, death, or damage if released or triggered.
* Head pressure:
* High-pressure system: A supplemental pump system used to pressurize the water supply, sometimes used during a large fire, or whenever more than one hydrant is being used.
* High-rise building: Any building taller than three or four stories, depending upon local usage, requiring firefighters to climb stairs or aerial ladders for access to upper floors.
* High-rise pack: A shoulder load of hose with a nozzle and other tools necessary to connect the hose to a standpipe.
* Hotshot crew: An extensively trained group of approximately twenty people which specializes in wildfire suppression with little or no outside logistical support.
* Hot zone: contaminated area of HAZMAT incident that must be isolated; requires suitable protective equipment to enter and decontamination upon exit; minimum hot zone distance from unknown material with unknown release is 330 feet (United Nations Emergency Response Guidebook); surrounded by "warm zone" where decontamination takes place.
* IDLH: Any situation deemed Immediately Dangerous to Life and Health. More narrowly defined by OSHA. See main IDLH article. An area of maximum danger to firefighters.
* Incident Commander: The officer in charge of all activities at an incident. See Incident Command System.
* Incident Safety Officer: The officer in charge of scene safety at an incident. See Incident Command System.
* Indirect attack: Method of firefighting in which water is pumped onto materials above or near the fire so that the splash rains onto the fire, often used where a structure is unsafe to enter.
* Initial attack: First point of attack on a fire where hose lines or fuel separation are used to prevent further extension of the fire.
* Interface zone (also wildland/structural interface or urban/wildland interface): The zone where wildfires threaten structures or structural fires threaten wildlands, such as in residential areas adjacent to forests. This requires both wildland firefighting and structural firefighting in the same location, which involve very different tactics and equipment.
* ISO Rating: (Insurance Services Office Public Protection Classification Rating) This is a rating published by the Insurance Services Office. Insurance companies, in many states, use this number to determine homeowner insurance premiums. Recently some insurance companies, including State Farm, have now adopted a per-zip-code, actual loss, based system in several states and no longer use the ISO (PPC) system.
* Irons: The flathead axe mated with the halligan bar. Firefighters often refer to these as the Crossed Irons, or Married Irons, because the Halligan Bar can fit to the Axe head.