A recent development affecting the air filter industry is the addition of Addendum k to Standard ASHRAE 62.2-2016 (Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings) to include air filter classifications from International Standard ISO 16890. The purpose is to add “an alternative method providing PM2.5 efficiency data” to the ASHRAE 52.2 MERV data already in the 62.2 Standard. The new Addendum also includes a Table providing the “Filtration Factor for Filters with a PM2.5 Designation.” A “Filtration Factor” is included for 35%, 50% 70%, 85%, 90% and 95% PM2.5 Efficiency Designations. On the surface this seems like a good idea. In reality it is a bad idea and could undermine the relevance of ASHRAE 62.2 recommendations. Here’s why:
First and most importantly, ISO 16890 does not provide PM2.5 efficiency data on filters. It provides ePM2.5 efficiency data. In ISO 16890, ePM1, ePM2.5 and ePM10 are calculated making assumptions about the make-up of the particles being filtered. We will talk about why those assumptions are questionable later in this article. The only time PM2.5 is mentioned in ISO 16890 is in the Introduction which talks about the adverse health effects of PM1 and PM2.5 outdoor air exposures. The inclusion of “PM2.5 Designations” in ASHRAE 62.2 illustrates a point we have been making for months – the terminology of ePM in ISO 16890 is misleading. The inclusion of PM2.5 Designations in this Addendum is proof positive of this fact. If the sophisticated, experienced and knowledgeable drafters of Addendum k are confused, what can we expect from the rest of the HVAC industry?
Secondly, the ePM data in ISO 16890 is based on 100% outdoor air particle counts. It does not take into consideration air recirculation through the HVAC system (and filter) and indoor generated particles. Virtually all residences in the USA use a recirculation system with return and supply ducts – not 100% outdoor air. Furthermore, in an occupied mechanically ventilated house, the majority of the mass of particles exposed to filters are of indoor origin. In a naturally ventilated house (ie. open windows), the particles do not pass through a filter. In either case, the calculated ISO 16890 ePM removal efficiencies do not give an accurate picture of filter effectiveness on PM levels in indoor residential environments.
Thirdly, ISO 16890 applies it’s “bimodal” ePM calculations to filter types – not outdoor air particle concentrations. In the ISO 16890 Standard, filters are tested without conditioning and with IPA conditioning and the average is then multiplied by the “typical” particle concentrations in multiple size ranges from 0.3um to 10um. These particle size ranges are defined as “urban” and “rural.” The major characteristics of these ranges is that the “urban” particle size ranges are skewed toward more particles in the smaller sizes while the “rural” particle size ranges are skewed more toward the larger particles. (The “typical” particle counts per range in ISO 16890 in both the “urban” and “rural” classifications are based on data that is at least 25 years old. Things have changed in the last 25 years – but that will be covered in a future post.)
Now, here is where it becomes really interesting. The “rural” and “urban” particle size distributions were developed and used in numerous studies to calculate the effect of outdoor air on indoor particles. Obviously, levels of outdoor to indoor particle transfer through infiltration and ventilation vary according to the outdoor air conditions. But in ISO 16890 the distributions are used to evaluate filter types:
“Fine filters, mostly designed to filter out PM1 and PM2.5 particle size fractions, are evaluated using a size distribution which represents urban areas, while the filters predominately designed to filter out the PM10 fraction are evaluated using a size distribution which represents rural areas.”
It is not clear what criteria are used to define filters that are “predominately designed to filter out PM10 particles.” It would seem that filter designers would seek to develop filters that do the best job at removing particles of all sizes given the limitations of materials, filter resistance and cost. But apparently not – according to ISO 16890.
From the limited data I have seen, it appears that pleated filters, as used predominately in the USA, would be evaluated using particle size distributions representing rural areas. The overwhelming majority of US residential filter housings use panel or pleated filters. Most of the US residential units are in urban areas. So the ePM2.5 efficiency designations create the rather bizarre situation that preclude the use of filters commonly used in the US residential market.
It is certainly understandable that members of the ASHRAE 62.2 Committee would like to have good PM2.5 data. It would be helpful for use in calculations of the effect of filtration on residential indoor air quality. But using ISO 16890 data is not an answer. Actually, it is an impediment that will obfuscate the need for additional research. There have been some good studies done on this subject. At least one is being conducted currently. It is time to consolidate this data and formulate realistic scenarios of the effects of filtration on indoor particle counts under different conditions. The result would be an Addendum with real value for ASHRAE 62.2 users.
To summarize, ISO 16890 efficiency data is not a good fit for ASHRAE 62.2. This data is inaccurate, incomplete and not applicable to the US residential market. It should be removed from Addendum k or at least modified to accurately reflect it’s shortcomings.