| 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. | 
