As winter approaches, this is an article regarding the ASHRAE-approved position paper, "Infectious Aerosols," in October 2022.

ASHRAE Position Document On Infectious Aerosols

The paper focused on six major areas of engineering controls for mitigating aerosol transmission: 1. Ventilation, 2. Filtration, 3. Air Cleaning technologies, 4. Indoor Airflow Pattern, 5. Humidity and temperature control, 6. Demand-control ventilation.

Here is a highlight of each focused area.

Ventilation

"Ventilation is the process of supplying air to or removing it from a space by natural or mechanical means," according to what ASHRAE defines. It is recommended to have a minimum outdoor airflow rate of 10 L/s-person (21.2 cfm/person) for non-healthcare facilities and 60 L/s-person (127 cfm/person) for most spaces in healthcare facilities.

While the natural ventilation system is relatively low in both first and operation costs, it could be challenging to have precise control, especially on air temperature and humidity.

Filtration

The purpose of filtration is to remove particles from the air within a defined space. Various schemes classify these filters; two common types of filters in the market are MERV (Minimum Efficiency Reporting Value) and HEPA (High-Efficiency Particulate Air).

MERV is a scale of effectiveness for air filters that ASHRAE established. The scale ranges from 1 to 16. The larger number indicates higher efficiency. The performance of the filters are estimated in three range: Range 1 indicates 0.3 to 1 µm (microns), range 2 means 1 to 3 µm( microns), and range 3 specifies 3 to 10 µm( microns). ASHRAE standard 62.1 generally requires filters in HVAC systems of at least MERV 8 and recommends upgrading to MERV 13 during the respiratory-related pandemic if possible.

HEPA filters are tested 99.97% or more efficient in filtering 0.3 µm particles and may be used under more critical situations. While it is understood that those filters are assessed based on 0.3 µm particles or larger, they are, in fact, able to capture much smaller particles.

Air cleaning technology

Many technologies can inactivate airborne microorganisms or reduce aerosols from the air. One of the technologies is GUV (germicidal ultraviolet disinfection), also called UVGI (Ultraviolet germicidal irradiation). UVGI and other similar electric air cleaner products deactivate microorganisms by producing ions, hydroxyl radicals, and peroxides to damage the structure of nucleic acids and proteins.

However, the main byproduct of electronic air cleaners is ozone. Therefore, ASHRAE standard 62.1 -2019b requires all-electric air cleaners to pass the UL 2998 standard, which requires no more than five ppb (parts per billion) ozone concentration in the emission of air cleaners. In addition, ozone is not the only byproduct concern. Recent research has found that various chemical contaminants and aerosols are reactive when the air cleaner is used. The electronic cleaner also uses a germicidal wavelength of 254 nm UV-c, produced by mercury vapor or amalgam lamps. This wavelength could potentially lead to eye and skin irritation or damage. Thus, it is advised to use these applications during the lease of occupants in the building.

Indoor airflow pattern

Indoor airflow patterns can affect the flow path of aerosol from the source. However, physical testing and real-time measurement of existing ventilation performance could be time-consuming and labor-intensive. Suppose building operators could use advanced techniques such as CFD (computational fluid dynamics) analysis to predict the airflow patterns and flow paths, which often happens in the early stage of the building design phase. After the spatial diagnosis, building operators could properly separate the rooms/zones or place exhaust devices accordingly.

Besides those mentioned above, a personalized ventilation system that provides a local air source is the immediate method to alter the airflow so the fresh filtered air can go directly to the occupant's breathing zone without other mechanical operation.

Humidity and temperature control

HVAC systems are typically designed to control temperature and moisture, which can influence the transmissibility of infectious agents. A common understanding is that the human respiratory-immune system is the strongest when the relative humidity (RH) is between 40% and 60%. However, according to the positioning paper, individual pathogens respond differently to varying temperature and humidity conditions. Therefore, modifying the risk only through these measurements would require much work.

Demand controlled ventilation

Ventilation requirements are usually based on either the maximum number of occupants in the space or the volume of the room. These estimates are static and are only sometimes comparable as the number of occupants changes. Therefore, the carbon dioxide concentration as the measurement for the ventilation rate per occupant is more precise and often used to modulate the airflow.

With different HVAC systems, this CO2 assessment approach is only sometimes feasible in a given space. Hence, the direct measurement of carbon dioxide concentrations lies in the new sensor technologies. These new sensors allow the direct measurement of fine airborne particle matter, potentially including infectious aerosols. While sensors cannot distinguish infectious aerosols and other fine particles, it is still essential to indoor air quality assessment. It can also be used for modulating the airflow or control of the air-cleaning system.

By the Way, What is ASHRAE?

ASHRAE stands for the American Society of Heating, Refrigerating, and Air Conditioning Engineers. It is a professional association committed to advancing heating, ventilation, air conditioning, and refrigeration systems design and construction. It currently has over 55,000 members worldwide.

ASHRAE position papers are approved by their Board of Directors. These documents provided objective, reliable, and scientifically proven information to whoever cares about such industry issues.

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