The Counter-Intuitive Science of Air Purifier Filters

You might think that your air filters work like a fine-mesh strainer: particles larger than the “strainer” holes get captured, and smaller ones make it through.

If you do think that, it’d be understandable. But, you’d be wrong. The science of modern air filters is far more complex, and can seem somewhat counter-intuitive.

Air filters are exposed to particles of a wide size range, from less than 1/100th of a micron to above 10 microns. Below is a rough categorization of the types of particles and their sizes.

The physics and chemistry of very small particles behave differently from those of the larger particles. Oftentimes, the smallest particles represent a greater threat, because they cannot be seen and they silently get lodged in the tissues of our internal organs, such as the lungs and brain. Asbestos particles as small as 0.1 micron, for example, have caused tens of thousands of cases of mesothelioma.

As the above EPA chart shows, only HEPA filters (the blue line at the very top), like those used in Metalmark’s self-cleaning Tatama air purifier, capture 99.97% or more of particles of a vast range.

Typical HVAC filters (MERV 10) are not designed to capture particles in the range of 0.1 to 1 micron. They only capture around 25% removal efficiency in the most penetrating particle size range, which is what the industry calls the MPPS of filters. These filters are inexpensive and decent at large particle removal, but they leave most people, in the space where those filters are in use, vulnerable to a host of dangerous air pollutants.

The same upside down bell curve pattern holds true for other particle filters, but to a much, much smaller degree for HEPA. So, why is this? Why would filters capture fewer particles around 0.3 micron than  larger or especially smaller particles??

It turns out that air filters do work as you’d expect for PM1 and larger particles: the bigger the particle, the more likely it is to be strained out. The right half of the chart illustrates this pattern (although some MERV filters are poor at capturing even PM10 particles). 

However, there are more forces at work than simply straining. As particles travel across the filter, they get stuck on the media via different mechanisms due to their size differences, relative to the media fiber diameter, material characteristics, charges, and more. For the most part, larger particles tend to float in more of a linear fashion and become efficiently trapped in the media by mechanisms of interception and straining. On the opposite end of the spectrum,very fine particles (in the nanometer range) churn with random Brownian motion. Their movements are hard to predict, and they are more likely to be attracted to and stick on fiber media due to the forces of molecular attraction. As the particles increase in size from infinitesimal 0.01 micron and smaller up to 0.1 micron, the media becomes increasingly inefficient, as represented by the left half of the chart above.

As the graphs show, the combination of mechanisms is weakest in the range of 0.1-1 micron, and that’s where the bottom point of the filtration efficiency curve, or MPPS, is generally found. 

Because of these factors, the size range in which the greatest number of particulates are produced by our activities (<1 micron) is also where typical air filter media fall short. It’s a terrible coincidence, and a technological gap that requires different thinking, something that Metalmark has done a lot of, from a new angle.