In what order should atmospheric tests be conducted after a fire?

The correct order for conducting atmospheric tests after a fire is to check for oxygen levels first, then test for explosive gases, and finally assess toxic gases.

Starting with oxygen levels is crucial because a fire consumes oxygen, and ensuring that the atmosphere has sufficient oxygen (between 19.5% and 23.5%) is vital for the safety of personnel entering the area. Before any other tests are performed, it is essential to ascertain that there is enough breathable air, as low oxygen levels can be immediately life-threatening.

Next, testing for explosive gases is important because fires can produce flammable vapors or gases that create an explosive atmosphere. This step ensures that, if there are any risks of ignition or explosion, those hazards are identified before anyone enters the area.

Finally, testing for toxic gases completes the safety protocol. Fires can release various harmful chemicals and combustion products that can pose serious health risks. Assessing toxic gas levels after confirming safe oxygen and explosive concentrations helps ensure a comprehensive understanding of the atmospheric conditions without immediately exposing personnel to potential risks.

In contrast, beginning with toxic gas or explosive gas tests without first ensuring adequate oxygen levels could leave workers vulnerable to dangerous conditions, making the order in option D the safest and most logical approach.