ASHRAE Position Document on Infectious Aerosols

A story on wall street journal regarding how to use ventilation system to battle Covid reminded me of a position paper from ASHRAE (1). I want to summarize the article here for your reference so you can strategically prepare your current space (home, school, or office).

In this paper, ASHRAE recognized transmission of SARS-CoV-2 through the air is likely caused by airborne exposure. It indicated that controlling airborne exposure by modifying building operations, including heating, ventilation, and air-conditioning systems, could reduce the risk of transmission through the air.

Here is the summarize of ASHRAE's implications.

1. Air Distribution Patterns

Suppose building operators could use advanced techniques such as computational fluid dynamics (CFD) analysis to predict the airflow patterns and flow paths (which often happens in the early stage of the design cycle). In that case, building operators could properly separate the rooms/zones or place exhaust devices accordingly.

2. Differential Room Pressurization

Similar to the first strategy, differentiating room pressure is important for controlling airflow between zones in a building. Placing airborne infection isolation rooms (AIIR) with extra exhaust/ capture ventilation will also help reduce the risk of airborne transmission. (2)

3. Personalized Ventilation

Personalized ventilation systems that provide local exhaust source control and supply 100% outdoor, highly filtered, or UV-disinfected air directly to the occupant's breathing zone (3) could protect against contaminated air exposure (4).

4. Central and Local Filtration

The use of highly efficient particle filtration in centralized HVAC systems reduces the airborne load of infectious particles.

There are two types of filters in the market: MERV and HEPA

MERV (Minimum Efficiency Reporting Value) is an effectiveness scale for air filters that ASHRAE established. The scale ranges from 1 to 16. The following table summarized different filters at the MERVE scale. (source: ASHRAE 52.2-2017)

For example, a MERV 10 filter needs to be more than 80% efficient in filtering particles between 3 and 10 microns; it is more than 50% efficient at filtering particles between 1 and 3 microns.

Typically, the residential buildings use MERV 1-4, and commercial building uses 5-8. However, during the coronavirus infection, ASHRAE recommends upgrading the air filter to at least MERV 13.

HEPA (High Efficiency Particulate Air) can capture most airborne germs. However, many ventilation systems are not designed to use HEPA, such as those furnaces in single houses (intended for the filter thickness about 1 inch). Therefore, The HEPA filters are also available as portable air cleaners. According to latest “Safety First Covid-19 Response Credit Guide“ published by USGBC, use HEPA filtration to achieve at least 5 air changes per hour in each occupied space is recommended.

5. Controlling Temperature and Relative Humidity

There is increasing scientific literature that shows the human respiratory-immune system is the strongest when the relative humidity (RH) is between 40% and 60% (5). HVAC systems are typically designed to control temperature and moisture, which can influence the transmissibility of infectious agents.

6. Using UVGI Devices

Ultraviolet germicidal irradiation (UVGI) inactivates microorganisms by damaging the structure of nucleic acids and proteins. In comparison, ASHRAE does not recommend or against UV energy in air systems to minimize the risks from infectious aerosols. Centers for Disease Control and Prevention (CDC) has approved UVGI as an adjunct to filtration to reduce tuberculosis risk and has recognized a guideline on its application (6).

Also, ASHRAE categorizes their recommendations on Level A, and Level B. Below is the summarized chart for their opinion.

Notes and References:

“How to Use Ventilation System to Battle Covid”, from Wall Street Journal, November 23, 2020
“ASHRAE Position Document on Infectious Aerosols” accessed April 14.2020
Conventionally, the "breathing zone" is defined as the zone within a 0.3 m (or 10 inches) radius of a worker's nose and mouth. But ASHRAE has its own definition. According to this institution, the region within an occupied space between plane 3 and 72 inches above the floor and more than 2 feet from the wall or fixed air conditioning equipment. You can see an illustration here.

Personalized ventilation are specially useful when there is a CO2 concentration situation in a space. High indoor CO2 concentrations indicate inadequate ventilation or crowding. High CO2 concentration has also been correlated with reduced cognitive performance and alertness. Recent scientific literature such as (Azuma, 2018) summarized such effect.
Coarse particles less than 10 microns are considered aerosol. Typical respiratory droplet is larger than 5 microns, which suspended in the air for at least about half hour to an hour. Therefore, higher graded filer such as MERV 9 and up can effectively catch those droplets. Yet the corona virus is 0.1 micron, therefore MERV13 and up could catch those particles when the droplets dries on the filter. HEPA filter can filter up to 99.9% of 0.1-0.3 microns, which is the most affective application for avoiding spreading the virus. If you are a home owners, placing a portable air cleaner by zones will also an economical method.

The earliest literature I found was in 1986 “Indirect health effects of relative humidity in indoor environments.” by Environmental Health Perspectives; 65: 351-361.
Recent years, Dr. Stephanie Taylor has published several findings on how indoor air moisture impact our health. One of her recent published paper is “Low indoor-air humidity in an assisted living facility is correlated with increased patient illness and cognitive decline.” By Taylor, S., and M. Tasi. 2018. Proceedings, Indoor Air 2018 744:1–8. You can get more Dr. Taylor’s insight through her website at 40to60RH.
If you use an air purifier or have HVAC devices, you will see CFM on the product specification. CFM means Cubic Feet per Minute. Let’s say if you have a product with 250 CFM and put it in a 250 square feet room with ceiling height of 10 feet. You will get 6 air changes per hour (ACH). Here is how it calculated:

250 CFM x 60 (minute)=15,000 Cubic Feet per Hour.

Your room volume is 250 (square feet) x10 (feet)=2,500 cubic feet

15,000 (Cubic Feet per Hour)/2500 (Cubic Feet) =6 times per Hour (means every 10 minutes the entire space will refresh once through the filter. )