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Safety lab practice questions

15 questions from the water and wastewater operator practice test.

Safety lab practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

What this covers

Sample questions

What oxygen range is acceptable for entering a confined space without supplied air?

  1. 19.5% to 23.5%
  2. 16.0% to 19.5%
  3. 20.9% to 25.0%
  4. 12.0% to 21.0%
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A. 19.5% to 23.5%

Normal air is 20.9% oxygen. OSHA treats an atmosphere below 19.5% as oxygen-deficient and one above 23.5% as oxygen-enriched, which is a fire hazard. Entry outside this range requires ventilation or supplied-air breathing apparatus.

Reference: OSHA 29 CFR 1910.146: permit-required confined spaces (hazardous atmosphere)

Why can a worker not rely on the rotten-egg odor to warn of dangerous hydrogen sulfide levels?

  1. The gas has no odor at any concentration
  2. The gas is lighter than air and rises away
  3. The gas quickly deadens the sense of smell
  4. The gas only forms in open, ventilated areas
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C. The gas quickly deadens the sense of smell

Hydrogen sulfide smells of rotten eggs at very low levels, but at higher concentrations it paralyzes the sense of smell within minutes, so the odor seems to vanish just as the gas becomes deadly. It is heavier than air and collects in low spaces; only a gas detector is reliable.

Reference: Hydrogen sulfide hazards: olfactory fatigue; OSHA 29 CFR 1910.146

Chlorine gas is about 2.5 times heavier than air. Where should the exhaust vent intake in a chlorine room be located?

  1. Near the floor
  2. Near the ceiling
  3. At head height
  4. In the doorway
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A. Near the floor

Because leaking chlorine gas sinks and collects at the lowest point, the exhaust must draw from near floor level, with fresh air supplied from high in the room. A ceiling exhaust would leave the gas pooled around the worker's feet and legs.

Reference: Chlorine Institute and Ten States Standards: chlorine room ventilation

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