This is an acronym. The HE stands for “High Efficiency,” but the PA could mean a number of things:
- High-Efficiency Particulate Air
- High-Efficiency Particulate Absorbing
- High-Efficiency Particulate Arrestance
Basically, it means “very good air filter.” Weirdly, there are many standards for (1) the particle sizes it has to stop, and (2) the amount of air flow it has to allow (note that “many standards” feels like an oxymoron).
(The amount of air flow allowed is an important point. An airtight seal would be the world’s most perfect “filter,” but if it doesn’t allow air to pass through, then it’s not helpful. You can make a filter better by making it more dense (?), but then you start restricting airflow. So, it’s a balancing act – it has to filter bad stuff out, while still letting enough good air pass through.)
For example, the US Department of Energy has nine “designations” for something to be called a “HEPA filter.” A lot of those specifications have to do with the construction and dimensions of the filter.
For the actual filtering properties, they defer to the American Society of Mechanical Engineers:
ASME AG-1 HEPA filter performance is based on efficiency for a given particle size. The HEPA filter must exhibit a maximum penetration of 0.03% when tested with an aerosol of essentially mono-dispersed 0.3-micrometer diameter test aerosol particles…
Essentially, the filter has to stop 99.97% of a particles of 0.3 micrometers in diameter. For comparison, a human hair is 80-100 micrometers in diameter. meaning these particles are 1/300th the width of a human hair. (How they actually test this, I have no idea.)
But this is just one standard. There seems to be no universally accepted standard (though the DOE standard is commonly accepted in the U.S.).
It’s important to note that “HEPA” doesn’t refer to methodology, or how the filter cleans the air. It only refers to the result. It’s a testing term, not a manufacturing or technology term. A filter could have millions of tiny little elves plucking contaminants from the air, and so long as they did a good job, it could be called a “HEPA filter.”
Of course, we’ve been exposed the phrase “HEPA Filter” so often, that it’s taken on some implication of advanced technology. But the term has been around since the 1950s. It was just a scientific term that a marketing executive thought sounded important enough that it could be thrust upon the populace as something to be associated with quality.