99.97 percent of particles sized 0.3 micrometres must be captured for a filter to earn the name HEPA. This figure is not a random selection by marketing departments, but rather a rigorous physical threshold established by the United States Department of Energy.
99.97%
The Rigorous HEPA Threshold (0.3μm)
Because this number is so precise, it provides a sense of security to a parent standing in a retail aisle in . The parent is looking at a box because their child is struggling to breathe during the pollen season. What the parent desires is a medical guarantee that the child will sleep through the night without coughing.
Because the law prohibits manufacturers from making specific medical claims for household appliances, the industry must sell the mathematics of the machine instead of the biological outcome of the user. A micrometre is one millionth of a metre, and this unit of measurement becomes the primary currency of the transaction.
The Intake: A Study in Pressure
Because the internal fan of the device creates a pressure differential, the air from the room is forced into the intake vents. This process is the first stage of any climate control cycle. In a store like
a shopper can see dozens of machines that all perform this same basic physical action.