What should be the first action when a fire is detected in a building?

Activating the fire alarm system is the first crucial action when a fire is detected in a building because it ensures that all occupants are alerted to the danger promptly. The fire alarm system is designed to notify everyone in the building of the fire, which allows for a swift and organized evacuation. Prompt activation of the alarm can significantly reduce the potential for injury and loss of life, and it may also help to control the situation by ensuring that emergency services are notified quickly.

While calling for backup, evacuating the premises, or assessing the fire's size are also important actions, they should occur after the alarm has been activated. Calling for backup or assessing the size of the fire can take valuable time that could jeopardize occupant safety. Similarly, evacuation should only commence once everyone in the building has been notified to do so, ensuring that everyone is aware of the fire and can exit in an orderly manner. Therefore, activating the fire alarm system is the most immediate and critical action to initiate a safe response to a fire.