Air purifiers using HEPA (high-efficiency particulate air) filters have become commonplace in hospitals and healthcare facilities with the primary goal of reducing airborne transmission of SARS-coV-2. The benefits of portable air filtration in hospitals include reducing room-to-room transmission and increasing room safety for staff and patients.
As air purifiers gain popularity in healthcare settings, it is important to take a closer look at filtration efficiency to ensure that these devices are functioning properly at the highest possible rate of capture of aerosols containing SARS-CoV-2 or other pathogens.
At the most basic level, an air purifier will work just as well as internal filters. If you are dealing with filtering viruses from the air, you need to choose an air purifier with a medical grade HEPA filter. Medical grade refers to top-level H13 or H14 efficiency-rated filters that capture a minimum of 99.95–99.99% of particles 0.3 microns (PM 0.3) or larger. These are the same filters that are used in isolation rooms and operating rooms.
Types of HEPA filters
HEPA air purifiers typically use either a synthetic pleated HEPA type filter or medical grade HEPA paper with an EN1822 rating.
HEPA-type filters, usually synthetic, are made of polypropylene media with an electrostatic charge. The charge increases the efficiency of the filter, so fewer particles pass through the filter media. Synthetic filter media is used for 2 reasons; this is a lower manufacturing cost and lower pressure drop, so the fan in the air cleaner can be smaller.
The electrostatic charge on the filter material essentially magnetizes the filter material to hold and trap more dust and particles, however it should be noted that the charge dissipates over time and results in reduced efficiency. In tests we performed on synthetic filters, we saw a drop from 99.95% to less than 75% within six months of use.
Synthetic filters cannot usually be certified as some ultra-fine particles penetrate the filter and thus fail the stringent EN1822 efficiency tests. In contrast, HEPA paper, also known as glass paper, maintains the same very high efficiency throughout the life of the filter.
- Most air purifiers are not medical grade filters (H13/H14 efficiency) and contain synthetic filters with lower efficiency E11 – E12.
- Buyer’s note: Synthetic HEPA filters using materials such as polypropylene do not provide the claimed lifetime performance, HEPA paper is the only material guaranteed to maintain performance throughout the lifetime of the filter. Synthetic filters use an electrostatic charge on the filter material that helps increase efficiency, but over time the charge is lost and so is the efficiency.
- Bigger is better: the larger the filter surface (usually measured in m2) increases the efficiency of the filter due to a larger contact area. When comparing air purifiers, look for the largest filter size.
- The Clean Air Delivery Ratio (CADR) advertised by many mass air purifiers is not a measure of overall filtration efficiency.
- Air purifiers that direct airflow in all directions tend to recirculate air at lower speeds, making them less efficient.