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What do eACH, ACHe and ACH mean? Is there a difference?

The eACH (aka ACHe) age has arrived. Here’s what it means for your organization & how it differs from ACH.

What are ACH and eACH, and what do they measure?

ACH, which stands for "Air Changes per Hour," measures how often a volume of air within a space is added, removed, or exchanged with filtered air. In HVAC terms, it's essentially how often air enters and exits a room via a ventilation system. For example, an office space with an ACH of 4 means the entirety of the room’s air is replaced four times per hour or every 15 minutes.

To conceptualize the idea of ACH, imagine how many times per hour the windows are rolled down to replace the in-vehicle air with fresh, clean air. Put into practice, The American Society of Heating, Refrigerating, and Air-Conditioning Engineers, or ASHRAE, provides ACH recommendations based on an indoor space’s size, occupancy level, and use. As an example, most healthcare facilities require a minimum ACH of 6, with patient and surgical rooms requiring significantly more.

eACH and ACH both should be measured in zones (for action needed), and an overall building number (for context).

Unlike other common HVAC measurements, such as Seasonal Energy Efficiency Ratio (SEER) or Energy Efficiency Ratio (EER), which measure a system's heating or cooling performance under a given set of circumstances (indoor temperature control relative to the outside temperature, for example), ACH benchmarks the frequency of how often an entire location's air is completely replaced with new clean outdoor or filtered air. Because of this, ACH measurements often correlate to increased indoor air quality and safety, as airborne pathogens can linger regardless of temperature and can only be eliminated via filtration, adding fresh air to a space—and other disinfection technologies, such as UVC light, which improve a space’s ACH via an Air Changes per Hour Equivalency measurement called eACH. However, in most cases, increasing a space’s ACH requires additional energy expenditure to filter air in—not to mention the energy needed for temperature adjustment. All of this adds up to increased energy costs and decreased sustainability.

What is the difference between ACH and eACH?

When the COVID-19 pandemic began, guidance for mitigating the virus's transmission in indoor spaces centered heavily on maximizing fresh, clean air via ventilation to clear spaces of all sizes and types from airborne pathogens lingering in the air and on surfaces. Experts agreed that best practices at the time called for ventilation as the most critical defense strategy, with filtration as a complimentary or backup option. In early 2021, about a year after the pandemic started, ASHRAE released an updated set of "core recommendations" based on a more robust understanding of how the airborne transmission of COVID can be controlled within various indoor spaces. In these recommendations, ASHRAE addressed the limitations of an HVAC system's ability to pump in more outside air and the dramatic energy costs associated with increasing air capacity. In light of this, ASHRAE referenced healthy air strategies based on "equivalent" or "effective" air changes per hour (eACH), providing indoor spaces with the recommended ACH level without solely relying on ventilation.

According to ASHRAE, "The equivalent clean air calculation indicates that the outdoor air can be calculated using the combination of the actual clean air, impact of filtration or air cleaning technologies on recirculated air, and the impact of air cleaning technologies in the space." In practice, this means that a small indoor space requiring an ACH of 4-6, such as an office, classroom, or small store, could achieve this with any combination of clean air ventilation, recirculated air that passes through a filter with a MERV value of 13 or higher, portable HEPA filters, or UVC disinfection—all of which contribute significantly to improved indoor air quality and lower virus transmission.

eACH validates other methods of air health improvement beyond HVAC.

The emphasis on indoor air health born out of the pandemic has left building operators, mangers, and facilities professionals scrambling to improve indoor air quality in spaces of all sizes, uses, and ages, from indoor factories to classrooms. Traditionally, HVAC and filtration upgrades have been the answer to improve indoor airflow; however, these systems and their maintenance and upgrade costs prevent many organizations, such as schools, from making these changes quickly. Additionally, some older buildings are unable to make these upgrades because their structure itself cannot support these additions without a heft remodel.

With a better understanding of how to increase ACH levels without relying solely on HVAC, indoor spaces of all types can have healthier indoor air using vetted methods that cost significantly less and often reduce energy cost and consumption.

Health equity is more achievable with new, cost-effective technology.

For years, HVAC, filtration, and pumps were the key to keeping indoor air safe and healthy. The problem is that those systems are challenging to implement, adjust, and maintain—not to mention the energy costs—only organizations with the means to keep these systems running. With new technology, organizations with limited budgets, like schools and non-profits, can easily address healthy air concerns and meet ASHRAE eACH standards without draining an already limited budget. Additionally, organizations with limited budgets tend to have smaller facilities and staff, meaning upgrades could take months which can drastically impact revenue. With Poppy providing zone-by-zone air safety data, these organizations can address challenges like under-ventilation to raise their indoor health standards and over-ventilation, leading to increased and unwanted energy costs.

What are some sample eACH standards for common indoor spaces?

The below table is just a sample of the minimum eACH requirement to ensure adequate indoor air quality. Remember, an eACH of 6 means 99.99% of the room's air is refreshed every ten minutes or six times per hour.

​Space type

Suggested eACH


6 eACH+

Data centers

8 eACH+

Medical spaces

6 eACH+

Manufacturing spaces

10 eACH+

Waiting rooms

4 eACH+

With eACH as a valid measurement of how well air is cleaned within a space, businesses, schools, hospitals, retail shops, and countless other spaces can meet current and emerging building health standards. The key, however, is finding the right set of technologies to measure and validate that air health efforts are working and new disinfection tools that are easier to deploy than HVAC upgrades.

The importance of indoor air health is expanding—and supported by new technology.

Recently experts in air health, ventilation quality, and pathogen transmission have introduced a new term to not only understand how well the air is cleaned, but based on the efficacy of the cleaning and how many people is it safe to house based on that cleanliness.

Equivalent clean air refers to the concept of providing indoor spaces with ventilation that mimics the quality and characteristics of clean outdoor air. It involves supplying fresh air to indoor environments to maintain air quality and ensure a healthy and comfortable indoor environment.

When outdoor air is brought into a building, it undergoes filtration, conditioning, and sometimes treatment processes to meet specific indoor air quality standards. The goal is to remove pollutants, particulate matter, and contaminants while adjusting the temperature and humidity levels to create a comfortable and safe indoor environment.

Equivalent clean air can be achieved through various ventilation systems, such as natural ventilation, mechanical ventilation, or a combination of both. Natural ventilation relies on openings like windows, doors, or vents to allow clean air to enter and circulate through the building. Mechanical ventilation, on the other hand, uses fans, ducts, and air handling units to distribute and control the flow of clean air.

When people occupy a space the amount of exhalation bring new potential pathogens into the air, and therefore increases the need for greater ACH or eACH. This new term is called Equivalent Clean Air, measuring the amount of clean air needed to clear the air based on the number of people in the room.

By providing equivalent clean air, indoor spaces can minimize the buildup of pollutants, remove airborne pathogens, and maintain an overall healthier indoor environment. This is especially important in buildings where the windows cannot be opened or in areas with poor outdoor air quality due to factors like pollution, allergens, or extreme weather conditions.

ECA is calculated by the volume of fresh, clean air entering a space that equals the volume of delivered air (ft3/min, m3/hr, m3/min or L/s) times the % outdoor air. In practice, the formula looks resembles:

Volume Fresh Clean Air =Volume of delivered air x % Clean Outdoor Air of delivered air

This new measure is a more holistic picture of indoor air health because it encompasses all efforts to reduce indoor polluted air by replacing it with either sanitized or fresh air. While “outdoor air” might be the baseline term for adequate indoor air health, the term itself is just vernacular. In reality, “outdoor or fresh air” applied to buildings means that the air at any given time is considered clean, healthy, sanitized, or pollutant free.

About Poppy.

Poppy uses patented sensor technology to collect and interpret spacial airflow data in minutes, from measuring eACH and ECA to identifying overventilation and underventilation. Poppy partners with building owners, facility managers, and companies to quickly uncover ways to optimize, save energy costs, and align to new standards.

Partner with us to uncover ways to improve air health, optimize ventilation, sustainability, and reduce energy consumption and costs.


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