Aerobiological engineering

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Aerobiological engineering is the field that designs buildings and systems to control airborne pathogens and allergens indoors. It’s important in places like offices, homes, and especially hospitals, where the indoor climate can help some germs survive and spread. Hospitals face many immunocompromised patients, so designers worry about viruses, bacteria, fungi, and other microbiological elements such as endotoxins, mycotoxins, and microbial volatile organic compounds.

Tiny particles can stay in the air for long periods, so ventilation and filtration are essential to dilute and remove contaminants. Regularly checking and replacing filters helps prevent the spread of disease and can also be used to control airflow in a building. However, ventilation can also bring in outdoor microbes or create conditions where microbes grow on damp filters and then become aerosolized through the system.

Hospitals use high air turnover in many rooms to keep air clean. Industry guidelines suggest roughly 12–25 air changes per hour (ACH) in treatment and operating rooms, and about 4–6 ACH in intensive-care areas. For rooms housing tuberculosis patients, the CDC recommends 6–12 ACH with the air exhausted through HEPA filters before it goes outside.

Isolation rooms are designed with either positive or negative pressure. Positive-pressure rooms protect very vulnerable patients by delivering HEPA-filtered air at high pressure into the room, which pushes air out to the hallway and prevents contaminants from entering. Negative-pressure rooms focus on containing pathogens by drawing air out of the room, helping keep dangerous aerosols away from staff and other areas; these rooms are commonly used for tuberculosis cases.

Filtration usually starts with less-efficient filters outside the air-handling unit, followed by HEPA filters after the unit. HEPA filters remove particles as small as 0.3 micrometers with at least 99.97% efficiency. Some systems also sterilize the air leaving isolation rooms by heating it to about 300°C for a few seconds. Ultraviolet germicidal irradiation (UVGI) uses UV light to destroy microorganisms. When UVGI is used with HEPA filtration, the combination is very effective: HEPA traps larger spores, while UV light kills smaller microbes.