ASHRAE has been involved in a concerted effort to include references to international standards in ASHRAE Standards. The purpose is to provide consistency across borders and simplify international construction and building operations. A case in point is the proposed addition of provisions of ISO 16890 into ASHRAE 62.1-2016. Unfortunately this can lead to problems. Of particular concern is the following section:
“Proposed Addendum m to ASHRAE 62.1-2016, Ventilation for Acceptable Indoor Air Quality
188.8.131.52 Particulate Matter Smaller than 10 Micrometers (PM10). In buildings located in an area where the national standard or guideline for PM10 is exceeded, particle filters or air-cleaning devices shall be provided to clean the outdoor air at any location prior to its introduction to occupied spaces. Particulate matter filters or air cleaners shall have:
a. a minimum efficiency reporting value (MERV) of not less than 8 where rated in accordance with ASHRAE Standard 52.2
b. or a minimum efficiency of ISO ePM10 when rated in accordance with ISO 16890.”
In order to understand why the above addendum is problematic, let’s first take a look at the procedures in ISO 16890 used to classify filters. First, a clean filter is tested for efficiency in size ranges from 0.3um to 10um. Then another “identical” filter is conditioned using IPA vapor to eliminate any electret charge. This conditioned filter is then tested for efficiency in the same size ranges. The results are averaged. The averages are then multiplied by typical “urban” outdoor particle distributions to calculate ePM1 and ePM2.5 values. In the case of ePM10 the averages are multiplied by a typical “rural” outdoor particle distribution. The reporting values for ePM1, ePM2.5 and ePM10 all start at 50% and go up in 5% increments.
Critical components in these calculations are the particle distributions. If they are not accurate, the reporting values are not accurate. The designations ePM1, ePM2.5 and ePM10 are names for reporting groups. They may or may not have a relationship to outdoor PM1, PM2.5 or PM10.
Volume Distributions from ISO 16890-2016
The above distributions are taken for the ISO 16890 Standard. As you can see, the “urban” distribution is skewed toward smaller size particles while the “rural” distribution is skewed toward larger size particles.
In a recent paper entitled “Evaluating the Sensitivity of the Mass-Particle Removal Calculations for HVAC Filters in ISO 16890 to Assumptions for Aerosol Distributions” the author, Dr. Brent Stephens of the Illinois Institute of Technology, found that the particle distribution data used in ISO 16890 was outdated and inaccurate. The paper was published in the peer reviewed Journal Atmosphere 2018, 9(3), 85. A review of this article can be found at:
One of the key points of the article concerned the “rural” particle distribution used to calculate ePM10. This distribution did not match any of the over 190 particle distributions analyzed. These included urban, rural and semi-urban areas from around the World.
The reason for the lack of consistency with other distributions could be that this “rural” distribution is based on a study done by measuring particles in the troposphere over the central part of the United States. It was literally “pulled out of thin air.” As used in ISO 16890, it is not technically or scientifically accurate.
Furthermore, the use of the terminology “ePM10” is misleading. It implies that the calculation has something to do with PM10 designated areas. The wording of “Addendum m” of ASHRAE 62.1 (quoted above) confirms this fact. An examination of the EPA data on PM10 “Designated Areas” shows a much different picture.
The vast majority of PM10 designated areas in the United States are certainly not “rural.” According to the EPA “Green Book” data, ninety-five percent of the population in PM10 designated areas live in counties with a population of 200,000 or more. In addition, over half of the people living in PM10 designated areas are also in areas designated PM2.5.
The biggest “outlier” of an urban county that is designated PM10, but not PM2.5, is Maricopa County (home of Phoenix, Arizona). This means that building owners in Phoenix (home of 3,800,000 people) – the air conditioning capitol of the World – should select filters based on calculations using a “rural” particle distribution.
Now, there are some counties on the EPA PM10 Designated Areas list that are clearly “rural.” For example, Mono County, California (Population – 7,500); Archuleta County, Colorado (Population – 2,400); Shoshone County, Idaho (Population – 9,700) and Rosebud County, Montana (Population – 1,500) would surely be classified as “rural.” Yet, I don’t think the drafters of ISO 16890 (or for that matter ASHRAE 62.1) had these areas in mind when creating the Standard.
What difference does it make to use “rural” particle distributions for selecting filters in urban areas? As we have shown in other articles on this website, some common filters are far better at removing sub-micron sized particles than others. For example:
Generally, an electret MERV 8 pleated filter is 4 to 6 times more efficient at removing sub-micron sized particles than either a mechanical-only MERV 8 pleated filter, a MERV 8 Ring Panel or a MERV 8 polyester pad. In the “Preliminary investigation of particle size distributions assumptions in ISO 16890” paper by Dr. Stephens, he compared ePM10 calculations using a mechanical-only MERV 8 pleated filter and an electret MERV 8 pleated filter. Using the “rural” distribution, the MERV 8 mechanical filter had an ePM10 efficiency of 56.5% while the MERV 8 electret had an efficiency of 69.9%. On the other hand, when using the more realistic “urban” distribution the ePM10 was 37.6% for the mechanical MERV 8 while the electret MERV 8 had an ePM10 efficiency of 51.2%. The “cut-off” of ePM10 for being eligible for use under the proposed change to Addendum m of ASHRAE 62.1 is 50%. Interesting.
The bottom line is that the ePM10 calculation using the “rural” particle distribution in ISO 16890 is not technically or scientifically correct. It needs to be changed. Unless and until it is, ISO 16890 should not be included in any ASHRAE Standard.
While it is an admirable goal to include international standards in ASHRAE Standards, all such efforts should go through the same rigorous analysis as any other modification. The default cannot be to “accept it because it is an international standard.” The default must always be – the international standard has to be proven to be accurate and unbiased to be included in any ASHRAE Standard.