What is the practical meaning of this velocity? It is important for us to appreciate its speed in relative terms, because this is the airflow we depend upon to protect us from exposure to harmful chemical fumes, gases and vapours.

100 feet per minute is about one and one eighth miles, or 1.8km, per hour.

A gentle walking speed is 4 miles (6km) per hour. When we walk, our bodies push the air in front to make a positive pressure wave, and the air collapses into the space behind our backs, creating a negative pressure wave behind us. People walking past fume cupboards force a huge oscillation in the air, which moves at four times the average fume hood face velocity. If the movement is brisk, the resulting turbulence has more force. The magnitude of the pressure pulse is much greater if laboratory coats are unfastened, as they fan out as we walk.

In our outdoor environment, we look forward to a mild breeze. In general, it requires about six miles (10km) an hour to fly a kite, or sail a boat. These activities can be enjoyed more often than not. So ordinary wind conditions often exceed the average fume hood containment face velocity more than five times over. This is why fume hood performance suffers when doors and windows are opened. Remember also that architectural features in the laboratory can channel and magnify these adverse air flows.

Even when internal doors are opened, they act as air pistons, and generate both a positive pressure and a negative pressure pulse on alternate sides of the door as it swings open, and this is reversed as it closes. The pressure pulses are proportional to the size of the door and the speed of its movement, and propagate through the laboratory at the speed of sound The effect of pressure pulses on fume hood velocity can be easily measured, even by basic instruments.

Often our laboratories are air conditioned, and we like to feel the benefits physically. Any draught felt by our skin is already greater than the average fume hood containment face velocity. Air conditioners are often set up with fast air flow to promote mixing with the room air and heat transfer. These air supply currents may be ten times the average fume hood containment face velocity. Air supply jets can be felt many metres away.

So the average fume hood face velocity, on which we rely for containment of fumes and our protection from harmful fume exposure, is vastly overpowered by actions and conditions which we accept as normal every day occurrences.

When we are aware of the significance of our actions, we can change to ensure we minimise adversity and get the best possible performance from our laboratory fume hoods.