Fume hoods are installed in (chemical) laboratories, to protect laboratory personnel by preventing hazardous contaminants such as chemical vapors, dusts, mists and fumes from escaping into the laboratory environment. Laboratory fume hoods also provide a lab personnel a physical barrier to chemicals and their reactions.

Two types of fume hoods are common; bench top fume hoods and walk-in fume hoods. Bench top fume hoods are used for small scale experiments (laboratory distillation/reactions in beakers, test-tubes, Erlenmeyers). Walk-in fume hoods are used for larger scale experiments, and sometimes the storage of hazardous chemicals. In general, fume hoods consist of an enclosed work space, that is accessible from the front, and an air vent on in the ceiling. Air is removed continuously, generating a small negative pressure inside the fume hood, and suction of fresh air from the frontal access. The removed air is either diluted and vented to the environment, or sent to a cleaning unit (filter, gas scrubber).

Fume hoods need to be used with care; the sash covering the frontal access needs to be lowered to the proper height to ensure a proper face velocity. This also allows for a physical protection against splashes or explosions. The airflow alarm (warning for low face velocity needs to be working, and the exhaust system needs to be in proper condition. Therefore, fume hoods need to be inspected regularly. Don't rely on a false sense of security when conducting hazardous experiments.

Some (potentially) dangerous situations I have heard of (or encountered personally) are:

  • Cluttered fume hood: This is a VERY COMMON hazard in any chemical lab. Fume hood space is usually limited. Laboratory workers sometimes store their chemicals in fume hoods alongside their experiments in (never do this). I saw one case where a reaction went out of control but because the fume hood was so cluttered, the worker could not reach the proper valves to secure the experiment.
  • Leaking exhaust system: A worker was doing experiments with hydrogen sulfide in an old fume hood, and then left the laboratory. Upon return she noticed a distinct sulphur smell. The exhaust pipe was damaged, and thus venting the very toxic gas into the lab. Fortunately it wasn't the odorless carbon monoxide.
  • Improper exhaust/ventilation system: The main shutoff valve on a gas cylinder (again hydrogen sulfide) failed, causing it to release all of its contents. The fumes were contained inside the fume hood, and vented out of the building. However, all the labs in the building were equipped with a negative pressure system to contain hazardous fumes inside each lab. Therefore, the released hydrogen sulfide was sucked back into all the laboratories (through the window air conditioners). The entire building had to be evacuated.
  • Gas scrubber explosion: This accident happened at a company a few years before I worked there, so I don't know the full details. Two workers were conducting separate experiments in adjacent laboratory halls. The exhausts gases were supposed to be sent to two separate gas scrubbers. Due to an erroneous valve setting, both reactive exhaust gases were sent to the same gas scrubber, causing an explosion. The glass shards from the broken gas scrubber killed one worker.
  • Power failure: A power failure caused the ventilation system of a gas hood to shut down. Fortunately, the worker (yours truly this time) realized that his computer screen and the surroundings were rather dark. Gas cylinders were closed before gas emission to the lab could occur.
Fume hoods are also excellent to capture the results of flatulency. Just hover around the fume hood area, and your co-workers will be grateful.