If hydrogen sulfide is detected in a tank environment, which sequence of actions should be followed?

• A. Wait for a fresh air survey after ventilation.
• B. Evacuate, ventilate, re-test, use approved respirators, and re-evaluate before re-entry.
• C. Enter with a basic dust mask and proceed.
• D. Ventilate only and wait for the space to clear.

Answer: (B)

Hydrogen sulfide is a highly toxic gas that can incapacitate or kill quickly, and in confined spaces like tanks it can accumulate at the bottom where workers might enter. The safest approach is to follow a controlled sequence that first protects people, then clears the atmosphere, and finally verifies safety before entry. Evacuation removes all personnel from the space to prevent exposure. Ventilation helps dilute and remove the gas, bringing in fresh air. After ventilation, re-testing the atmosphere is essential to know the actual levels of hydrogen sulfide and to check that oxygen is at safe levels. If the test shows hazardous concentrations or an unsafe oxygen level, entry must not be attempted. When conditions are acceptable, entering with approved respirators provides the necessary protection, and re-evaluating the space before re-entry ensures that conditions remain safe during the work. Relying on a basic mask or waiting only for fresh air to clear without testing can leave workers exposed to lethal concentrations, and ventilation alone may not achieve a safe atmosphere immediately.