SAN FRANCISCO (KRON) — Anyone who habitually follows the news is likely to regularly come across stories referring to a “one-alarm,” “two-alarm,” or even “five-alarm” fire, as was the case with the recent San Jose Home Depot fire. But while most of us generally understand the number of alarms to correspond with the severity of a fire, how many of us know what the number of alarms designated for a fire actually mean?

For some clarification, we spoke with Chase Lambert, a Northern California firefighter who founded the website, firefighterinsider.com. Lambert confirmed that the number of alarms assigned to a fire can be a good indicator of how serious the incident is.

“Fire alarm assignments refer to the number of resources — fire engines, trucks, chiefs, specialty units, and personnel — needed to manage an emergency incident,” Lambert explained. “These can vary by area, but the number of alarms correlates to how severe and complex a fire or other incident is to mitigate.”

While all fires are potentially serious, as you’d expect, a one-alarm fire would typically be regarded as “less serious” than a two- or three-alarm fire. But as Lambert explained, even a one-alarm fire typically results in a significant deployment of resources.

“A first alarm usually will dispatch two to three fire engines (pumpers), one to two fire trucks (aerial ladders), one chief officer, and one light/air refill unit,” Lambert said. “Each additional alarm will send one to two additional engines and one additional truck. These are general alarm assignments, but they vary by department.”

Most fires are one to two alarms, according to Lambert.

“A typical house fire will be one to two alarms, but can be bigger depending on the size of the house, how far involved the fire is and if the fire has potential to spread to other buildings,” he explained.

For a typical house fire, just one fire station is involved in the response.

“Most fire stations in this area have just one apparatus (truck or engine) per fire house, though some will have two or more per station (called a dual house). A five-alarm fire could have as many as six trucks and 11 engines. That could mean up to 17 different stations sending resources to a large fire,” Lambert said. “They can even be sent from different fire departments through the mutual aid system, where neighboring departments assist each other when managing large incidents.”

Although five-alarm fires don’t occur that frequently Lambert said.

“Not very common to have a five-alarm fire, though there was one last week in San Jose when the Home Depot burned,” he said.

The exact numbers of resources deployed to a fire based on the number of alarms can vary from region to region. Lambert emphasizes that the information he provided is based on his experience working in the South Bay.

“All this information about fire alarm assignments is based on this area and my experience, the exact numbers and procedures will vary in other areas,” he said.

For more information on firefighting and fire safety tips, visit firefighterinsider. com.