How long after exposure to very high concentrations can unconsciousness occur?

• A. Within 5 seconds
• B. Within 30 seconds
• C. Within 60 seconds
• D. Within 15 seconds

Answer: (D)

Very high concentrations of toxic gas can cause unconsciousness in a matter of seconds. Hydrogen sulfide acts as a rapid CNS depressant and respiratory inhibitor: it enters the bloodstream through the lungs and swiftly disrupts cellular respiration, leading to quick brain hypoxia. When exposure is extreme, the time to loss of consciousness is extremely short, typically around 15 seconds. This is why emergency procedures stress immediate evacuation and use of proper breathing protection, since a person may lose consciousness before any rescue is possible. Also note that at high levels the familiar warning smell can be gone, so you cannot rely on odor to judge danger.