CADR, ACH, eACH: Understanding Indoor Air Quality

Indoor clean air has become a subject of great interest around the world. As the pandemic began spreading in 2020, it didn’t take long for scientists to understand that the quality of indoor air was, in part, responsible for the rapid spread of the disease. Due to a lack of innovation in IAQ solutions to improve the quality and health of indoor air, many companies chose to have employees work remotely to help keep them safe. While there were some benefits to taking this step (such as reducing the spread of the virus at work), there were also a number of drawbacks. Many teams suffered from the inability to connect in person, brainstorm together, or tackle problem-solving in real time.
Indoor air quality

Now there is a growing movement to return to the office and to get back to “normal.” In order to help employees feel confident about coming back to work, and to help create a healthier indoor environment, businesses are paying close attention to the processes and equipment that allow them to improve the air quality in their offices. In March of 2022, President Biden announced the Clean Air in Buildings Challenge. This proclamation is a call to action for leaders and building owners and operators of all types to assess their indoor air quality and make ventilation and air filtration improvements to help keep occupants safe.

Employers are legally bound to provide safe workspaces for their employees. Schools are also obligated to ensure the air in their buildings is healthy for students, faculty, and staff. There are a number of considerations concerning air quality. These include ventilation (including the lack of outside air), issues with controlling temperature, high or low humidity, recent remodeling, and other activities in or near a building that can affect the fresh air coming into the building.

Breathing Easier

Evaluating equipment that promises to provide cleaner air comes down to three sets of letters: CADR, ACH, and eACH. Clean air delivery rate (CADR) is the rate at which an air cleaning device or piece of equipment delivers clean, purified air to a room or a space. The rate is measured in cubic feet per minute (CFM). It is an essential parameter to determine the indoor air quality of any indoor space.

ACH stands for air changes per hour. ACH is a calculation of how many times per hour the entire volume of air in a given space is replaced. It is also sometimes referred to as the “air change rate” or “air exchange rate.” One air change will remove ~63% of microorganisms in the air, and each subsequent air change removes 63% of what’s left, and so on. Having a consistent supply of fresh air is beneficial to mental performance and physical health as well. According to the Lawrence Berkley National Laboratory, there are many serious effects of poor indoor air quality. The more frequently air is exchanged in a given space, the healthier the air quality becomes.

Understanding the difference between these terms:


a calculation of how many times per hour the entire volume of air in a given space is replaced.

Airflow in CFM x 60

Room Area sq. ft. X Ceiling Height


the rate at which an air cleaning device or piece of equipment delivers clean, purified air to a room or a space.


equivalent air changes per hour. Air is changed through disinfection rather than replacement.

To determine the ACH, identify the size of the space you’re looking at. For example, an average U.S. elementary school classroom is recommended to be a minimum of 900 square feet. The volume of a classroom with these dimensions would be 8550 cubic feet. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends six air changes per hour for classrooms of this size. ACH is determined by this calculation:

For air purifiers and air disinfection devices, the measurement is equivalent air changes per hour (eACH). Essentially, though the air is not being changed in the same manner as it is with an HVAC system, it is still being changed through disinfection. These devices work with an HVAC system to help improve indoor air quality, adding eACH to provide healthier indoor environments.

One example of a disinfection device that provides exceptional eACH rates is the upper-room ultraviolet germicidal irradiation device (UVGI) known as Beam. Beam is mounted near the ceiling in areas such as hallways, cafeterias, or large conference rooms to provide an additional level of disinfection against airborne microorganisms. Beam adds +12eACH in the space where it is installed. It runs continuously and provides safe, chemical-free disinfection in occupied spaces for improved indoor air quality at far less operating cost than HVAC.

R-Zero has several ways to improve the health of your indoor spaces while reducing the use of chemical cleaners and sanitizers. We invite you to take a look at our innovative line of disinfection devices and sensors. Contact us today.