What is a Good Air Change per Hour (ACH)?

Different air purifying and cleaning technologies can exist as part of a structure’s central heating, ventilation and air conditioning (HVAC) system — or they can be portable. Portable units come in various sizes and are intended for use in a localized area, such as a single room.

Portable air cleaning devices with HEPA filters can remove dust, mold, bacteria, pollen, and other particles from the air by trapping them in the filter. According to Public Health Ontario, a portable air cleaning unit should have the ability to remove small airborne particles in the range of 0.1 to 1 µm in diameter in order to be effective in eliminating viruses in an indoor space. Viruses may also be carried in larger droplets. 

To ensure a portable air cleaning device is operating to the best of its ability, it’s important to understand what air changes per hour are and what the optimum air change rate for your space is.

What are Air Changes per Hour (ACH)?

Air changes per hour (ACH) is also known as the air change rate. This is a measure of the volume of air added to or taken from a space in one hour, divided by the volume of the space. Air changes per hour is a measure of how many times the air within a specific space is replaced every hour if the air in the space is uniform or perfectly mixed.

To reach equilibrium air pressure, the amount of air exiting the space and entering the space has to be the same. In many air distribution situations, air is not uniform or perfectly mixed. The percentage of a space’s air which is exchanged is dependent on the efficiency of the airflow in that space, as well as the ventilation methods used.

Why are Air Changes per Hour important?

Contaminants of all sorts — from common dust to Volatile Organic Compounds (VOCs) — are present in homes, workplaces, and many other indoor spaces. To reduce the presence of these pollutants and achieve better air quality, sufficient ventilation and air exchange is required. Air changes per hour is an important measurement when determining how effectively indoor air is exchanged or filtered.

Several regulatory bodies have air changes per hour recommendations in place. These differ from facility to facility depending on the size and purpose of the environment. For example, the Canadian Standard CSA Z 317.2 "Special Requirements for Heating, Ventilation, and Air Conditioning (HVAC) Systems in Health Care Facilities" states that in an operating room, the minimum total air changes should be 20 air changes per hour.

How does it manage infection risks during in-person dental care?

Dental instrumentation, salivary and respiratory sources, ultrasonic scalers with no rubber dam, and air-water syringes can all produce aerosol droplets. Distant contamination can occur in a dental clinic as aerosol droplets can remain in the air and travel through it. There is an additional risk of disease transmission once an infected patient has left the dental practice.

Increasing the air changes per hour in a room lowers the risk of airborne infections and decreases patient turnover time.

Is it mandatory for clinics?

There are currently no mandates requiring portable air cleaners or air purifiers in dental clinics in Canada. However, Public Health Ontario has published a document answering frequently asked questions about the use of portable air cleaners and the transmission of COVID-19. The document states:

“An overall risk mitigation strategy against COVID-19 transmission indoors includes public health measures (e.g., physical distancing, masking, cough and sneeze etiquette, cleaning and disinfection) and may be supported by indoor air quality improvement using ventilation and filtration. Portable air cleaners often provide localized (as opposed to central) filtration and can play a role in this strategy, particularly in areas where ventilation is inadequate. Although no direct scientific evidence was identified that demonstrates the effectiveness of portable air cleaners in reducing COVID-19 transmission, based on what we know, portable air cleaners equipped with a HEPA filter could remove COVID-19 virus particles from indoor air and potentially reduce exposure.”

While air purifiers or portable air cleaning devices are not legally required in dental clinics, they can certainly play a key role in a larger strategy for better indoor air quality and reduced risk of disease transmission.

How do you calculate Air Changes per Hour (ACH)?

In order to calculate air changes per hour, you must first know the cubic feet per minute (CFM) airflow your air purifier or air cleaning device provides. This information can be found on the device itself or in your user manual.

You must also know the area size and height of the room in which your air purifier or air cleaning unit is intended to be used in. Then, use the following formula to calculate air changes per hour: CFM x 60 / (area x height).

As an example, we’ll calculate the air changes per hour for an air purifier with 200 CFM in a 150 sq ft room with an 8ft ceiling. 200 multiplied by 60 gives us an airflow of 12,000 cubic feet per hour. The volume of the room is its area (150 sq ft) multiplied by its height (8ft), which is 1,200 cubic feet. We then divide the cubic feet per hour airflow (12,000) by the volume of the room (1,200), which equals ten. This means the air purifier in this room is capable of ten air changes per hour.