Using air-change-per-hour (ACH) rates in specifying ventilation for dilution can be counterproductive. Consider the following example:

A smoke bomb is lighted in a sealed room, releasing 1 million particles of purple smoke. The room is 500 sq ft, with an 8-ft ceiling. The steady-state concentration is 250 particles per cubic foot.

Now, assume the same scenario in the same room, but with a 12-ft ceiling. When the smoke settles, the steady-state concentration is 166 particles per cubic foot.

The point is this: Contaminants naturally dilute in larger volumes. Therefore, with larger volumes, less ventilation is required for the same concentration to be achieved. When we specify a ventilation rate in ACH, we use more ventilation (which involves larger systems, more capital, more energy, more carbon emissions, and more maintenance.)

But, that is not all. The ACH metric is, itself, problematic.

When engineers say “four air changes per hour,” non-engineers hear, “The entire volume of air in the room is replaced four times every hour, or once every 15 min.” That statement is nearly true for operating rooms and unidirectional, laminar-flow cleanrooms. However, most rooms in hospitals use ceiling diffusers and return grilles; the air is mixed. A more correct interpretation is, “A volume of air equal to the volume of the room is added four times per hour.” Of course, this is difficult to say and more difficult to explain.

Still confused? Try this: Color a glassful of water deep red with food dye. Then, add a glassful of clear water. There will be spillage, but the water in the first glass still will be colored, having lightened only to dark pink. You will need to add five or six glasses (depending on the shape of the glass, the location of the pour, the speed of the pour, and other factors) before the water looks clear. For an even more fun experiment, start with red vinegar, and see how many “water changes” until you no longer can taste it.