When firefighters are in an atmosphere that is IDLH, at least how many fully trained and equipped firefighters must be standing outside?

In an immediately dangerous to life or health (IDLH) atmosphere, it is crucial to ensure the safety of firefighters who are performing interior operations. The National Fire Protection Association (NFPA) guidelines mandate that there must be at least two fully trained and equipped firefighters standing by outside the hazard zone for every two firefighters who enter the IDLH atmosphere. This is known as the "two-in, two-out" rule.

The rationale behind requiring two firefighters outside is to ensure that there is sufficient personnel readily available to assist in a rescue or provide support if the firefighters inside encounter any difficulties. This mutual accountability helps to maintain the safety of all personnel involved, ensuring that help is on hand in case of an emergency.

While having only one firefighter outside may not provide enough support for a rescue operation, and three or four firefighters standing by may be seen as unnecessary under certain circumstances, having at least two provides a solid safety protocol. This standard ensures that the firefighting team can operate effectively while also prioritizing safety and preparedness for emergencies.