Aerobiological engineering

Aerobiological engineering is the science of designing buildings and systems to control airborne pathogens and allergens in indoor environments. Most commonly focused on environments include commercial buildings, residences, and hospitals. This field of study is important because controlled indoor climates in generally tend to favor the survival and transmission of contagious human pathogens as well as certain kinds of fungi and bacteria.

Aerobiological engineering in healthcare facilities

Since healthcare facilities can house a number of different types of patients who potentially have weakened immunes systems, aerobiological engineering is of significant importance to engineers of hospitals. The aerobiology that concerns designers of hospitals includes viruses, bacteria, fungi, and other microbiological products such as endotoxins, mycotoxins, and microbial volatile organic compounds (MVOC's). Bacteria and viruses because of there small size readily become airborne as bacterial aerosols and remain suspended in air or hours, and because of this adequate precautions and mitigation techniques need to be taken with indoor air quality (IAQ) in hospitals dealing with infectious diseases.

Ventilation systems

At a minimum, ventilation systems provide dilution and removal of airborne contaminants, which in general leads to improved IAQ and happier occupants.If filters are checked and replaced as need they can form an integral component of an immune building system designed to prevent the spread of diseases by airborne routes. They can also be used for pressurization of areas within buildings to provide contamination control.

Biocontamination in ventilation systems

Ventilation systems can contribute to the microbial loading of indoor environment by drawing in microbes from outdoor air and by creating conditions for growth. When microbes land on wet a filter that’s been collecting dust, they have the perfect medium to grow on, and if they grow through the filter they have the potential to be aerosolized and brought throughout the building via the HVAC system.

Dilution rates

Bacteria in hospitals can be aerosolized from sick patients coughing and sneezing and because of the frequency of the germs being aerosolized it is necessary that air changes per hour (ACH) rates remain high in treatment and operating rooms. ASHRAE typically recommends 12-25 ACH in treatment and operating rooms and 4-6 ACH in intensive care rooms. For rooms containing tuberculosis patients Center for Disease Control (CDC) recommends an ACH of 6 to 12, with exhaust air being sent through HEPA filters before being sent outside.

Pressurized isolation rooms

In order to keep patients safe, hospitals use a range of technologies to combat airborne pathogens. Isolation rooms can be designed to feature positive or negative air pressure flows. Positive pressure rooms are used when you have patients who are extremely susceptible to disease like HIV patients. For such patients it is paramount to prevent the ingress of any microorganisms, including common fungi and bacteria that may be harmless to healthy people. Theses systems filter the air before delivery with a HEPA filter and then pump it in to the isolation room at a greater rate than exhausted which forces air from the isolation room out into the hallway keeping it free of microbes. In a negative pressure system the focus is keeping infectious disease isolated buy controlling the airflow and directing harmful aerosols away from health care workers and other occupied areas. Negative pressure isolation rooms keep contaminants and pathogens from reaching external areas. The most common application of these rooms in the health industry today is for isolating tuberculosis patients. To do this, the air is exhausted from the room at a rate that’s greater than air being delivered to the room. This makes it difficult for airborne disease to go from a contaminated area to a hospital hallway by having air constantly being drawn in from the hallway.

Air sterilization processes

The norm for filtration in healthcare facilities is low efficiency air filters outside the air-handling unit followed by the HEPA (High Efficiency Particulate Air) filters placed after the air-handling unit. To be HEPA certified, filters must remove particles of 0.3 µm diameter with at least a 99.97 percent efficiency. Air burners sterilize air that is leaving contaminated isolation rooms by heating air to 300 degrees Celsius for six seconds. Ultraviolet germicidal irradiation is also another technique for special purpose air sterilization. Ultraviolet germicidal irradiation (UVGI) is defined as electromagnetic radiation in the range of about 200 to 320 nm that is used to destroy microorganisms. When HEPA filters are used in conjunction with UV sterilization tools the results can be extremely effective, since the filter will remove the bigger hardier spores, all that is left are the smaller microbes which are killed much easier with the high intensity UV light than the larger spores.

References

#C.S. Cox "The Aerobiological Pathway of Microorganisms". Chichester G.B.: John Wiley & Sons 27, 118-119
#Godish, Thad. "Indoor Environmental Quality". Boca Raton, FL, USA: Lewis Publishers, 2001. p 190
#Kowalski, Wladyslaw Jan. "Aerobiological Engineering Handbook". Blacklick, OH, USA: McGraw-Hill Professional Publishing, 2005. p 6, 185, 231, 260, 528, 530