Maintaining Indoor-Air Quality with Air Curtains

By Andy Ross, Mars Air System

This article appeared in the 2023 edition of AMCA inmotion magazine.

During the spring and summer of 2023, smoke from wildfires raging out of control in eastern and western Canada blanketed cities across North America, prompting local health officials to issue air-quality alerts and encourage people to stay inside,¹ renewing public interest in indoor-air quality (IAQ) sparked by the coronavirus disease 2019 (COVID-19) pandemic three years earlier and leading building owners and managers to take a hard look at the conditions inside their facilities.² With the retrofit or replacement of a ventilation system not always practical or feasible, smaller-scale solutions for maintaining IAQ are needed.

Typically installed at building entrances, air curtains (Photo A) have been used for decades to minimize the cross-migration of warm and cold air from buoyancy pressures and wind, saving on air-conditioning costs, and to provide environmental separation by repelling dust and dirt, fumes, odors, and flying insects.³ Now, evidence suggesting air curtains also can play a role in maintaining IAQ is emerging.

This article will discuss results of two experiments: one of an ultraviolet- (UV) lamp-equipped air curtain’s ability to disinfect air and one of an air curtain’s effectiveness in preventing airborne particulates from entering a building.

Air Disinfection

Through a process known as ultraviolet germicidal irradiation (UVGI), light in the UV-C spectrum has been used to neutralize pathogens for more than 100 years.^4,5 Traditionally, UV-C has been deployed away from high-traffic areas to minimize the risk of accidental human exposure. When paired with an air curtain, however, UV-C can be deployed safely in busy environments.

A typical UV package for an air curtain (Photo B) is designed so that no light from the bulb directly escapes the air curtain. Like all light, UV-C loses energy when reflected from most surfaces, so UV light reflected from the interior of an air curtain (often seen as a “blue glow”) is not a safety concern.

For added safety, UV-resistant control equipment and kill switches that automatically de-energize a light source if an air curtain is opened can be installed. Further peace of mind can be had by ensuring an entire UV-equipped unit is certified to conform to national and/or local safety codes and standards, as some air curtains are certified for only standalone use.

Experiment

In a test of the effectiveness of a UV-C-equipped air curtain under field conditions, a unit with two low-pressure lamps that generated light at a wavelength of 253.7 nm was installed above the door inside the walk-in cooler of a pizzeria in Los Angeles. The lamps were powered around the clock, while the fan operated at high speed when the cooler door was open and low speed when the cooler door was closed. The theory was that, through continuous circulation and exposure to the UV lamps, air within the cooler would be sanitized. What’s more, through the eradication of airborne pathogens such as mold and bacteria, the shelf life of produce and other perishable ingredients would be extended.

Open Petri dishes were placed at various locations throughout the cooler, with the air curtain operating as described. Over time, airborne pathogens settled on the Petri dishes. After seven days, the Petri dishes were removed from the cooler to incubate. A second set of Petri dishes then was placed at the same locations as the first inside the cooler, with the UV lamps in the air curtain turned off. After another seven days, the second set of Petri dishes was removed from the cooler and allowed to incubate. After seven more days, the growths on the two sets of Petri dishes were compared (Figure 1: Pathogen growth, walk-in-cooler experiment).

Results

As might be expected, pathogen concentration varied by cooler location. Most significantly, there was no detectable growth on any of the Petri dishes that were in the cooler with the UV lamps on, even with twice the incubation time of the control group.

Though UV-C traditionally is employed to disinfect air traveling at velocities lower than those seen in air curtains,⁶ the results validate the effectiveness of the test’s “hybrid” operational model—that is, UV lamps continually powered, with the fan running at high speed when the door is open and at low speed when the door is closed. And while more mold than any other kind of pathogen (subsequent analysis revealed at least four distinct varieties in the control group) was collected, the broad germicidal effectiveness of UV-C makes the operational approach viable in environments where virtually any airborne pathogen circulates.

Of course, a larger room may require additional units for the same level of effectiveness to be achieved. Nevertheless, the test results indicate a UV-equipped air curtain can serve as a viable sanitization device. What’s more, its built-in light shielding and air-circulation capabilities mean additional units could be installed wherever needed—not just over a doorway—in a larger space.

Experiment

A study was conducted to determine an air curtain’s effectiveness in preventing airborne particulates from infiltrating a building. To simulate airborne particulates from a wildfire, a commercial haze machine was used to generate a light fog of fine water droplets outside of a doorway. Outside and inside the doorway, air-quality sensors were deployed to estimate the concentration (in micrograms per cubic meter [μg/m³]) of airborne particles with diameters less than 2.5 microns (PM2.5). The door was actuated with an automatic opener, which, in turn, was controlled with a programmable timer (photos C and D).

“Outside” the doorway, the fog generated by the haze machine typically reached concentrations of 1,200 μg/m³ to 1,500 μg/m³, which served as a rough analog to the greater-than-800-μg/m³ concentrations of PM2.5 observed during extreme wildfire events.12 The concentrations occasionally dipped as low as 1,100 μg/m³ or went as high as 2,000 μg/m³ because of shifting wind patterns around the outside air sensor; this variance was counteracted by the use of fans at the doorway applying a controlled light wind load and ensuring a constant stream of particulates in the direction of the “inside” environment. Tests were performed with the air curtain off and the air curtain on and the door open 25 percent of the time and with the air curtain off and the air curtain on and the door open 50 percent of the time.

Results 

Running the air curtain reduced particulate concentration inside the room (Figure 2). As might be expected, absolute indoor particulates increased with the amount of time the door spent open; percent reduction in indoor particulates, however, appeared largely unaffected. When the door was open 25 percent of the time, indoor PM_2.5 concentrations were reduced by 54.0 percent, from a maximum of 658 μg/m³ with the air curtain off to a maximum of 303 μg/m³ with the air curtain on. With the door open 50 percent of the time, indoor PM_2.5 concentrations were reduced by 54.4 percent, from a maximum of 1,005 μg/m³ with the air curtain off to a maximum of 458 μg/m³ with the air curtain on.

While the testing involved particulate levels higher than those expected in most real-life scenarios, the consistency in percent reduction is indicative of the positive health impacts of air curtains in the field, as even a much lower (e.g., 20 percent) reduction can amount to a significant health benefit. With evidence of the harmful effects of airborne particulates continuing to emerge and nearly 6.7 million deaths worldwide a year attributed to the cumulative effects of air pollution,¹³ every little bit helps.

Conclusion

In this time of increased public interest in IAQ, building owners and managers would do well to consider all of their options. Empirical tests are demonstrating that better IAQ is attainable with a far less than capital investment. Air curtains, with their ability to both sanitize interior spaces (in combination with a UV package) and keep out exterior pollutants, are capable of contributing to IAQ in ways other auxiliary HVAC equipment is not.