HEPA filters are very popular in air purifiers and vacuum cleaners, for good reason.  They capture more small particles that other filters miss.  You might have seen our small tutorial on “What is HEPA?” on our FAQ page, but we wanted to do a deeper dive on these filters here.

There are four main ways that mechanical filters capture particles.  These are explained in our article here. 

• HEPA is one of many standardized rating systems for mechanical filters.  
• HEPA is not the end all be all for every situation of course, specifically having next to no impact on gases (VOCs / odors) and it does not have a germicidal effect or effect outside of the filter. 
• The HEPA rating system is based on it's efficiency to remove 0.3 microns, which is what's called the "Most Penetrating Particle Size" (MPPS).  This could also be considered the worst case scenario.  Smaller particles and larger particles in general are easier to capture.  I've put together the attached with more info on this.  ANY true HEPA of an H13 grade should be able to filter out much smaller particles, however sensing equipment and relevance of testing data is not as available for those particles, and intentionally confusing marketing practices like "  __x times smaller than the HEPA standard muddy the waters".  I've linked to a nasa study below that goes into some greater detail on this and how even packs of carbon filters can begin to have near HEPA grade of filtration of these ultra fine particulates.  Attached are a few screenshots from the NASA study that illustrates the mechanisms for HEPA and why the MPPS is a thing. 
• Some companies add marketing buzzwords (TM or (R)) to the HEPA but this means nothing.  Look for the grade (H11, H12, H13, H14) to see its efficiency.  H14 (HEPA) is NOT the highest grade either, ULPA is a higher grade (i.e. U15 +). A "super-duper-mega H13 HEPA filter" is still a lower grade filter than an H14.  
• A higher grade of HEPA is not always better for homes.  Typically the cost to force a sufficient amount of air through the filter becomes increasingly expensive and loud.  In many cases we'd recommend an H13 with sufficient air flow and noise instead of trying to spend a bunch more money on H14 or U15 filters with similar air flows and noise.  Sometimes even lower grades of filters (Low grade HEPA) with higher airflow will have in general a better effect on particles or dust because it has so many more passes through the filter
• A ~$1,000 HEPA on turbo speed moves maybe 5-20% of the air that a normal home's HVAC system moves all the time. As such we emphasize a good quality filter (changed regularly) in the HVAC (with carbon for chemical absorption) and running the blower regularly (low energy cost big benefit).  You also do not want to put too dense a filter in your HVAC as the pressure will be too great for the fan, typically the only way around this is increasing surface area of filters by adding more returns, or using a "V" or "VV" formation if space allows. We also have proprietary solutions we're working on to solve for this. 
• A HEPA on "quiet mode" is more practical, but is only giving you a fraction of the benefits that were marketed to you on turbo speed. 
• The diameter of a fan directly impacts how much air it can move.  A giant slow moving (RPM) fan will have less noise, but move more air, then a smaller fan moving at high RPMs.  A tiny fan in most air purifiers simply can't even come close to matching a ceiling fan or HVAC blower.  You are also buying control systems, wifi, plastic, etc., when you buy a plug in HEPA air purifier, so you want one that's big and bulky enough to be effective without all the bells and whistles that do nothing directly for air quality. 
• Not replacing HEPA filters regularly (due to cost) can result in a negative benefit, especially when you factor in things like full carbon filters that begin to off gas VOCs, or biological contaminants like mold that may be growing in a hepa filter with no germicidal technologies.

Check out the following links to do more research:

• Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials - NASA Technical Reports Server (NTRS)
• https://en.wikipedia.org/wiki/HEPA#Mechanism

Photo by Nubelson Fernandes on Unsplash