The Complexities of Bioaerosols

By Ed Rutkowski

The COVID-19 pandemic touched off a surge of interest in bioaerosols. But the understandable focus on protecting people from infectious viral diseases obscured the fact that bioaerosols encompass much more than viruses. As Cheri Marcham explained in a session at AIHce EXP 2023, bioaerosols do not only include viruses but also fungi, bacteria, spores, pollen, mites, and cell membrane components, as well as endotoxins from gram-negative bacteria, antigens and allergens, and microbial volatile organic compounds.

“The problem with bioaerosols is they’re ubiquitous,” Marcham told her audience. People are constantly exposed to bioaerosols, usually at low levels, and determining whether an exposure is hazardous is not a simple matter. Bioaerosols can cause a wide range of health effects, little information exists on the dose-response relationship, and responses can vary widely depending on a receptor’s health status, sensitivity, immunity, and other factors.

Jack Springston, Marcham’s co-presenter, discussed the necessary elements of an assessment for bioaerosols. The many relevant factors include the age and history of the building and the makeup of the structure itself—for example, what material the building envelope is composed of. It is also possible that the heating, ventilation, and air conditioning (HVAC) system may pull contaminants into the building. Humidity levels, the activities of building occupants, and the tasks performed by operations and maintenance personnel are all relevant factors, as is the surrounding outdoor environment; cooling towers, for example, are reservoirs for Legionella.

After completing a visual inspection, the OEHS professional may wish to conduct sampling. Several factors complicate this task. First, there are no Threshold Limit Values for bioaerosols that OEHS professionals can compare sampling results against, Springston said. Because personal sampling for bioaerosols is often not achievable, OEHS professionals typically use area samples as imperfect surrogates for personal exposures. In addition, the mere presence of bioaerosols doesn’t necessarily indicate a health risk. Given these complications, Springston warned that samples should only be collected if the OEHS professional suspects the results will support or refute their hypothesis about health effects experienced by building occupants.

For OEHS professionals who decide to collect samples, Springston urged them to document indoor and outdoor conditions. In general, levels for fungal samples collected in a building with good mechanical ventilation should be approximately one-tenth of levels found in samples collected outside, Springston said. If the building is naturally ventilated, the indoor and outdoor fungal levels should be roughly equal. For bacterial samples, comparing indoor and outdoor levels is rarely useful since natural sources of bacteria exist both indoors and outdoors.

One important control for bioaerosols is general ventilation, which can be used either to displace air or to dilute it. Marcham explained that displacement ventilation exhausts air to the outdoors and replaces it with conditioned, filtered air, while dilution ventilation mixes indoor air with outdoor air. For a bioaerosol such as SARS-COV-2, displacement ventilation—that is, getting the virus out of the building—is typically preferred to dilution ventilation. But if the building uses ventilation in combination with a disinfection method such as ultraviolet germicidal irradiation (UVGI), dilution ventilation may be more effective at moving the virus into the range of the UVGI devices, which are typically installed in the upper reaches of rooms.

Many considerations should inform the decision to use UVGI. Accidental overexposures to building occupants may cause eye and skin problems. UVGI lamps that use different wavelengths may take longer to inactivate viruses. Some biological agents are more resistant to UV light than others; UV doesn’t work as well on fungal spores as it does on viruses, Marcham said. The UVGI lamps need to warm up for as long as 30 minutes and typically need to be replaced within one or two years.

Another device that can be used to control bioaerosols is a portable air cleaner. The efficiency of these devices is characterized by the clean air delivery rate, or CADR, which is a standard defined by the Association of Home Appliance Manufacturers. Portable air cleaning device manufacturers typically report the CADR for three test agents: smoke, dust, and pollen. For viral hazards, the smoke CADR is the most applicable.

With so many factors to consider, Marcham’s caveat about UVGI devices seems to apply to controlling bioaerosols in general: “You have to think about how all this is going to work,” she said.

Ed Rutkowski is editor in chief of The Synergist.

AIHce EXP coverage on related topics includes “Clearing the Air on Air Purifiers” and “The Many Challenges of Bioaerosol Sampling.”