Indoor Air Quality

Air Changes Per Hour (ACH)

Air changes per hour (ACH) is one way to measure the indoor effectiveness of an air filtration system.

July 27, 2021
Last Updated On:
April 13, 2022
Last Updated On:
April 13, 2022

The pandemic has reminded everyone of the importance of indoor air quality. However, air quality predates the concerns of the COVID-19 and viral mitigation. Recommendations by the CDC, Health Departments, ASHRAE, and OSHA include improvements to building ventilation and portable air filtration

A determining factor in your HVAC or portable air filtration system's effectiveness is the air change rate (ACH). Calculating the rate of your air changes per hour is essential to maintaining a proper air exchange and air filtration equilibrium and ensuring the air is exchanged or turned over to provide the highest quality air within indoor spaces. Read on to learn what air changes per hour are and what they do.

What Are Air Changes Per Hour (ACH)?

Air changes per hour measure how often air volume will filter or turn over in an hour. This measurement, which is also known as the "air exchange rate" (ACH) or "air change per hour" (ACPH), measures the amount of air that will be added, removed, or exchanged within a room or building and is a critical factor of indoor air quality. ACH is vital for indoor air quality because it gauges how effectively the indoor air is replaced each hour within any given space.


In measuring air quality, cubic feet per minute (CFM) is another measurement directly associated with ACH. CFM is different from ACH in what is being measured. CFM measures the volume of air, in cubic feet, for each minute it moves. For portable air purifier systems, a higher CFM can filter more air and cover a larger room. In short, cubic feet per minute gives you an understanding of the coverage area a filtration system covers.

On the other hand, ACH calculates the number of times that air is cycled or replaced. How often is air moving through an HVAC or portable air filtration system (through air exchanges) so that you can understand how often clean air is being introduced into a space. ACH and CFM are closely associated because they are both critical for assessing indoor air quality. Knowing both the ACH and CFM helps you understand if your air filtration system(s) meets the minimum requirements for clean air and viral mitigation. 

Calculating Air Changes Per Hour

To calculate the air exchange rate, you will need to know your system's CFM and the total volume of the space you are trying to cover.

Air Exchange Calculation: ACHP = 60(Q)/Vol. 

  • ACPH = number of air changes per hour; higher values correspond to better ventilation
  • Q = Volumetric flow rate of air in cubic feet per minute (CFM)
  • Vol = Space volume L × W × H, in cubic feet
High ACH Air Purifier.

Understanding Air Quality: Outdoor Air vs. Filtered Indoor Air

Most of the air we breathe in indoor spaces is ventilated through heating, ventilation, and air conditioning (HVAC) systems. However, when possible, one should increase the amount of outdoor air within buildings. 

Adding fresh, clean, or outdoor air is sometimes required, although this depends on the setting. However, letting in outdoor air is not always a good thing. For example, some settings like hospitals and government buildings need filtered air to minimize the risk of individuals contracting diseases. 

Why is Air Changes Per Hour Important?

If you cannot open the windows to let in the fresh air, how often the air is being exchanged and replaced in an hour is an important measurement to understand. Air change rate recommendations can vary based on the facility, industry, local regulations, building codes, and more. For example, in specific sectors, VOCs, ozone, and electrodes are higher due to manufacturing equipment, building materials, or chemicals.

Regardless of space, a good rule of thumb should have a minimum of at least 3-4 ACH. However, most areas do not meet the requirements and have only 1-3 ACPH. Many high VOC environments, like bakeries, boiler rooms, computer rooms, paper mills, manufacturing plants, and the healthcare industry require ACHs upwards of 15 ACH, even as much as 50 ACH. 

Air Quality Controls Coporate Office Conference Room.

HVAC System Air Changes

You should prioritize understanding your HVAC system and know how often it is exchanging air (the ACH), how efficiently the filters perform (the MERV rating), and if the system can achieve the appropriate air exchanges for the building. 

Select industries, like medical industries, will often purchase specialized HVAC systems with high-volume filtration and filtration upgrades with a higher MERV rating. Unfortunately, HVAC systems often fall short of the recommended air exchange guidelines and lack the proper filtration using a MERV rating of MERV 13. The inefficiency leads to an increased risk of exposure to harmful chemicals, particulate and virus circulation through the air. 

ActivePure HVAC Induct Air Purification

Will Increased Air Changes Reduce COVID Transmission?

The World Health Organization (WHO) issued a scientific brief saying that environments heavily populated with inadequate or insufficient airflow raise infection rates. These studies are based on theories of airflow physics and SARS-CoV-2 aerosol transmission rates. 

COVID-19 is transmitted through respiratory droplets in the form of an aerosol. Droplets can range from >5-10 μm in diameter and occur when a person is in close contact (within 1 meter) with an infected person with or without respiratory symptoms. Poor ventilation and spaces with insufficient ACH increase the number of aerosol droplets in the air.

Therefore, anyone inhaling an aerosol with a sufficient amount of the virus in it that air would become infected. With a higher air exchange rate, the air is exchanged more frequently, reducing the density of aerosols within the air. Therefore, to reduce the risk of coronavirus transmission, improving the air exchange rate and air filtration rate can minimize transmission rates. 

Aeresol COVID-19 Virus Transmission.

Air Exchanges and COVID Transmission Rates

In general, WHO warns that transmission rates are higher in crowded places, close-contact settings, and confined and enclosed spaces with poor ventilation. In addition, the risk is ultra-high when all three of these settings overlap. There are no official CDC or ASHRAE recommendations for ACH to reduce COVID transmission (other than improving ventilation, increasing ACH, implementing MERV 13 rated air filters or HEPA filters, and portable air filtration). 

However, some researchers recommend at least 3-4 ACH in indoor settings, with 6 ACH being ideal. The Rhode Island Department of Health recommends that in a 30-foot by 30-foot classroom with 25 students in it, the air should be replaced at least every 15 minutes, which equals an ACH of 4. If the air is replaced at least every 10 minutes, there is an ACH of 6, which is better. 

If you want to use air change rates for minimizing contamination, research suggests that it should be based on a reasonable estimate of the occupied volume. In general, you want to increase the ventilation in enclosed spaces as much as reasonably possible. You can do this by opening a window or increasing air exchanges through a portable purifier system.

Air Filtration Recommendations for Reducing COVID Transmission

Here is a brief of what the CDC recommends for ventilation: 

  • Turn on fans. If you can have a fan in front of a window, this can help to introduce fresh air into the space. Try to avoid placing fans in a way that could potentially direct contaminated air to another person (see FAQ here). 
  • Ensure HVAC is working correctly for the space and occupancy. You may need to adjust the HVAC system and increase airflow. For example, turn off AC demand-controlled ventilation (DCV) controls (like the auto setting). Turn it to 'ON' so that the fan is always running, even if AC is not required. 
  • Improve central air filtration to as much as you can without reducing airflow designs. Make sure air filters are appropriately sized and within their recommended service life. In non-residential settings, run the HVAC system at maximum outside airflow for 2 hours before and after the building is occupied.
  • Inspect and maintain exhaust ventilation systems, especially in kitchens and cooking areas. Use these systems whenever the spaces are occupied.
  • Use portable high-efficiency particulate air (HEPA) fan/filtration systems to enhance air cleaning (especially in higher-risk areas). Make sure the air filters in place have higher-quality mechanical filters—MERV 13. Spaces without central air (or systems that cannot be upgraded) can add a portable air purifier to increase ACH and air filtration effectiveness. 
  • The use of ultraviolet germicidal irradiation (UVGI) can be added as a supplemental treatment to inactivate SARS-CoV-2. UVGI systems can provide air cleaning within occupied spaces, and in-duct UVGI systems can help enhance air cleaning inside central ventilation systems.

Once there is viral particulate indoors, bringing in the fresh air, changing over the air, and proper filtration will help reduce viral particulate.

HVAC Ductwork Repairs.

How to Increase Air Changes Per Hour With an Air Purifier

Improving your ventilation alone isn't going to eliminate the spread of the coronavirus. Using portable air filtration systems dramatically helps reduce the amount of virus circulating in the air. In addition, following local health regulations, such as wearing masks, washing hands, disinfecting surfaces, and maintaining a physical distance, can reduce transmission rates. Portable air purifiers are an easy and effective way to increase air exchanges within a building. The plug-and-play standalone devices can also provide a higher level of filtration that HVAC systems do not have. 

For example, air purifiers typically come with HEPA filters, capturing over 99.97% of airborne contaminants that are as small as 0.1 microns in size. Certain high-quality air purifiers will have multiple layers of filters, including pre-filters, activated carbon filters, and UV-C lights as well. Therefore, by upgrading your current air filtration system with an air purifier, you can increase air changes per hour and eliminate dust, pollen, dander, and reduce the number of airborne contaminants entering your space. 

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