June 22nd, 2022 | Posted in Air Cleaners
If you are looking for an air cleaning unit for your environment, what measurement is there to determining how efficient any particular unit is, and what does that data mean? The two standards you will probably come across are ASHRAE Standard 52.2-2017 and ISO 16890. They are different in what they report, but they can be compared to help you understand how effective an air filtration unit is.
The American Conference of Governmental Industrial Hygienists (ACGIH) has noted that “Exposure to excessive amounts of particulates can cause adverse health effects as a result of pulmonary clearance overload[i].” Larger particles are generally trapped by the mucociliary escalator and we clear those particles out when clearing the throat or blowing the nose. Smaller particles, however, go past the mucociliary escalator and travel deeper into the lung where, in addition to overloading the normal clearance mechanism, they can have toxic effects associated with the chemistry of the individual particles. The human body has built-in mechanisms called “macrophages” for removing these unwanted very fine particles from the lung. Both the mucociliary escalator and the macrophages can be overwhelmed by excessive particles in the air we breathe with resultant possible adverse health effects.
Air filtration is the primary means of mitigating these harmful effects by reducing the concentration of particles in the air. The is no filtration mechanism that removes all of the particles from the air, but we need to have some means of evaluating air filter effectiveness. Consequently, standard-setting organizations have developed standards for evaluating the ability of air filters to remove particles of varying sizes from the air we breathe. In the United States, we are accustomed to using “ANSI/ASHRAE Standard 52.2-2017, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size” for this purpose. ASHRAE 52.2 uses a test aerosol consisting of potassium chloride (KCl) salt in a solution ranging in size from 0.3 microns (0.3µ) to 10 microns (10µ). The test aerosol is run through the filter being evaluated and the number of particles in each of 12 size ranges, both upstream of the filter and downstream of the filter, is counted by an analyzer. The reduction in the number of particles downstream compared to upstream is expressed as “efficiency.” For example, a filter that removes 95% of the particles of a given size range would be identified as 95% efficient at that size range. A composite of the “particle size removal efficiency” (PSE) across the range of particle sizes of interest is used to produce the “Minimum Efficiency Reporting Value”, or MERV, for the filter being evaluated. MERVs range from 1 to 16, with 16 being the most effective for smaller particles.
Not all nations use ASHRAE 52.2 and there are a number of differences in the assumptions, techniques and methods used for filter evaluation in different parts of the world. In order to provide a standard for global use, the International Organization for Standardization (ISO) has recently published ISO standard 16890: “Air Filters for general ventilation.” In that vein, some questions come to mind:
The National Air Filtration Association (NAFA) has published the “NAFA User’s Guide to ISO 16890” and the information below is, in part, summarized from that reference work.
Although these two methods differ in terms of what they report (52.2 uses number vs. ISO 16890 uses weight), they use the same analyzer size ranges and therefore may be roughly compared. The table below from the “NAFA User’s Guide to ISO 16890” provides a means of comparing the two methods. It’s not so much like comparing apples and oranges as it is like comparing Clementine oranges to Navel oranges.
|ISO 16890||MERV||Intended Particle Size Range, μm|
|ISO COARSE||1 – 6||>10.0|
|ISO COARSE > 95%||7 – 8||>10.0|
|ePM10||9 – 10||3.0 – 10.0|
|ePM2.5||11 – 12||1.0 – 3.0|
|ePM1||13 – 16||0.3 – 1.0|
Because we are a global economy, it is important to appreciate that international standards exist even if they do not directly impact your work. If, for example, you work solely in the United States, ISO 16890 is not likely to impact your business. If, however, you work outside the United States, then you can expect to be asked the question: “What’s the difference between these two standards?” Now you know: ISO 16890 uses mass of particles and ASHRAE 52.2 uses the number of particles. And ISO 16890 collates the test data into 5 groups while ASHRAE 52.2 collates the data into 16 groups.
[i] “Particulates (Insoluble) Not Otherwise Specified (PNOS),” Documentation of the Threshold Limit Values and Biological Exposure Indices 7th Edition, American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, 2001