What’s Really In The Air?

Coarse particles, over 3 microns in size, are filtered by MERV-8. Looking at fractional efficiency curves and the dust that ASHRAE tells air filter companies to test with would make you think that these large coarse particles are the main source of pollutants in the air. However when sampling of air is done to determine the distribution of particles less than 1% of particles are even over 1 micron in size.

This disparity of what is done in a lab under 52.2 standards and what filters face within your facility makes evaluation and expectations of different filters difficult for you. Taking fractional efficiency curves from MERV testing and mating it with the distribution of particles that are in the air show a much different picture than looking at fractional efficiency charts alone will give you.

Dust holding capacity is the other major area where looking at a 52.2 MERV report will mislead you. Because the average particle size of the dust in the standard is 7.7 microns but the average size of dust a filter will encounter in your HVAC system is only 0.8 microns the loading capacity and characteristics will be much different in the lab. This means an air filter manufacturer has to make a decision: design a filter to look good in a lab for 52.2 MERV testing – or – design a filter that is going to perform better in real-world conditions.

Because there is no process and each HVAC is a little different from one another the only way for you to really know what you are getting for the money you are spending on filters is to do in-situ testing at your facility. This means monitoring the particle capture efficiency and changes in static pressure over time of filters. This should be done periodically to make sure your current vendor is giving you what you want and when testing different filters when evaluating a possible change to a new product or vendor.

Some good news: ISO 16890 is approximately 18-24 months from replacing the MERV testing procedure. This international filter testing procedure has been in development to create conditions that better replicate the real-world in lab testing; which will then more accurately address the shortcomings of the current 52.2 MERV standard and make your life easier.