Clearing the Air on Air Purifiers

By Ed Rutkowski

The COVID-19 pandemic brought unprecedented attention to the air purifier industry as employers desperate for workers to return to their facilities sought ways to limit the risk of infection in workplaces. But as with the surge in counterfeit respirators, the urgency of the pandemic led to alleged abuses, including reports that manufacturers exaggerated the performance of their air purifiers. Even in cases where manufacturers acted responsibly, the sudden demand contributed to confusion about what air purifiers do and how organizations can pick the best one for a given application.

On Monday at AIHce EXP 2023, a representative from an air purifier manufacturer delivered a presentation that attempted to clarify how air purifiers work and what organizations should look for when selecting one. Jeff Abramson of Omni CleanAir opened his session by describing the basic function of an air purifier: a blower pulls air through a pre-filter and then a HEPA (high-efficiency particulate absorbing) filter and exhausts the cleaned, treated air into a space. Typically, the space will be sealed off from other areas and the device placed in the middle. Clean air may be piped through ducts.

Abramson explained that the pre-filters can be either carbon filters or MERV (minimum efficiency reporting value) filters, which are designed to capture larger particles. Some devices use both MERV filters and carbon filters as pre-filters. This design captures more particles but also slows the air moving through the device, increasing the time needed to clean the air in a space.

The key component is the HEPA filter, which can come in either a standard or a high-capacity version. The difference between the two is the efficiency with which they capture particles in the 0.3-micron size range: a standard HEPA captures 99.97 percent of these particles, while a high-capacity HEPA captures 99.99 percent. Perhaps unintuitively, air meets less resistance when moving through a high-capacity HEPA filter than through a standard HEPA filter, Abramson said, so the high-capacity filters provide more protection and allow the device to operate more efficiently.

Another consideration is that the design of the device should allow for the filters to be properly sealed, eliminating leakage, or air that bypasses the filters.

The amount of air moving through a device is measured in cubic feet per minute, or CFM, which depends on the strength of the motor and the pressure within the system. Blockages such as constricted ducting or loaded, dirty filters will affect CFM performance: “Over time, your 1,800 CFM machine will be performing at 1,500 CFM” if the blockage isn’t removed or the filters aren’t replaced, Abramson said.

A second metric associated with air purifiers is the clean air delivery rate. The CADR is a standard created by the Association of Home Appliance Manufacturers (AHAM) to measure the volume of treated air. CADRs are determined for three types of particles: tobacco smoke, dust, and pollen. The higher the CADR, the faster the device cleans the air. Ideally, Abramson said, the CADR of a device would be equal to its CFM. The greater the difference between these parameters, the less efficient the device. (More information on CADR is available from AHAM.)

Perhaps the most important metric is air changes per hour, or ACH, which indicates how many times the air is replaced in a room of a certain size over the course of an hour. Minimum ACH values are specified in ASHRAE standards. CDC also recommends minimum ACH values for certain healthcare facilities.

According to Abramson, unscrupulous manufacturers can deceive customers by listing specifications obtained during unrepresentative operating conditions. For example, the CFM of a device is greater when measured without the pre-filter in place. He also cautioned against manufacturers who claim that the CADR of a device doesn’t matter.

Abramson concluded his presentation by recommending that individuals or organizations interested in purchasing an air purifier consider the maintenance cost: a device that uses a smaller HEPA filter will be less expensive, but the filter will need to be replaced more often. While larger machines are usually noisier, they can often be placed in locations that minimize the disruption.

Ed Rutkowski is editor in chief of The Synergist.