The CDC updates its guidance on ventilation in buildings

June 5, 2021 – 5 Min

On June 2nd 2021, the Centers for Disease Control and Prevention (CDC) updated its guidance on infection control to prevent the transmission of Covid-19, recommending a layered approach to reduce exposure to the virus. One key ‘tool to improve ventilation’ is cited as ‘portable high-efficiency particulate air (HEPA) fan/filtration systems to enhance air cleaning’.

Key extracts in relation to HEPA are as follows:

Research shows that the particle size of SARS-CoV-2 is around 0.1 micrometer (µm). However, the virus generally does not travel through the air by itself. These viral particles are human-generated, so the virus is trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are larger. Most of the respiratory droplets and particles exhaled during talking, singing, breathing, and coughing are less than 5 µm in size. By definition, a High Efficiency Particulate Air (HEPA) filter is at least 99.97% efficient at capturing particles 0.3 µm in size. This 0.3 µm particle approximates the most penetrating particle size (MPPS) through the filter. HEPA filters are even more efficient at capturing particles larger and smaller than the MPPS. Thus, HEPA filters are no less than 99.97% efficient at capturing human-generated viral particles associated with SARS-CoV-2.

Portable HEPA filtration units that combine a HEPA filter with a powered fan system are a preferred option for auxiliary air cleaning, especially in higher risk settings such as health clinics, vaccination and medical testing locations, workout rooms, or public waiting areas. Other settings that could benefit from portable HEPA filtration can be identified using typical risk assessment parameters, such as community incidence rates, facemask compliance expectations, and room occupant density. While these systems do not bring in outdoor dilution air, they are effective at cleaning air within spaces to reduce the concentration of airborne particulates, including SARS-CoV-2 viral particles. Thus, they give effective air exchanges without the need for conditioning outdoor air.

In choosing a portable HEPA unit, select a system that is appropriately sized for the area in which it will be installed. This determination is made based on the air flow through the unit, which is typically reported in cubic feet per minute (cfm). Many portable HEPA filtration units are assigned a Clean Air Delivery Rate (CADR), which is noted on a label in the operators manual, on the shipping box, and/or on the filtration unit itself. The CADR is an established standard defined by the Association of Home Appliance Manufacturers (AHAM). Participating portable air cleaner manufacturers have their products certified by an independent laboratory, so the end user can be assured it performs according to the manufacturer’s claims.

Key extracts in relation to UV light are as follows:

Ultraviolet germicidal irradiation (UVGI), otherwise known as germicidal ultraviolet (GUV), is a disinfection tool used in many different settings, such as residential, commercial, educational, and healthcare settings. The technology uses ultraviolet (UV) energy to inactivate (kill) microorganisms, including viruses, when designed and installed correctly.

There is still a lot to learn about SARS-CoV-2 and the extent of airborne viral particles and spread. However, UVGI can inactivate viruses in the air and on surfaces. The design and sizing of effective UVGI disinfection systems requires specific knowledge and experience.

Seek consultation with a reputable UVGI manufacturer or an experienced UVGI system designer prior to installing UVGI systems.

Key extracts in relation to emerging technologies are as follows:

There are numerous technologies being heavily marketed to provide air cleaning during the ongoing COVID-19 pandemic. Common among these are ionization, dry hydrogen peroxide, and chemical fogging disinfection. Some products on the market include combinations of these technologies. These products generate ions, reactive oxidative species (ROS, which are marketed using many names), or chemicals into the air as part of the air cleaning process. People in spaces treated by these products are also exposed to these ions, ROS, or chemicals.

While variations of these technologies have been around for decades, relative to other air cleaning or disinfection methods, they have a less-documented track record when it comes to cleaning/disinfecting large and fast volumes of moving air within heating, ventilation, and air conditioning (HVAC) systems or even inside individual rooms. In the absence of an established body of peer-reviewed evidence showing proven efficacy and safety under as-used conditions, the technologies are still considered by many to be “emerging.”

As with all emerging technologies, consumers are encouraged to exercise caution and to do their homework. Registration alone, with national or local authorities, does not always imply product efficacy or safety. Consumers should research the technology, attempting to match any specific claims against the intended use of the product. Consumers should request testing data that quantitively demonstrates a clear protective benefit and occupant safety under conditions consistent with the intended use. When considering air cleaning technologies that potentially or intentionally expose building occupants, the safety data should be applicable to all occupants, including those with health conditions that could be aggravated by the air treatment. In transient spaces, where average exposures to the public may be temporary, it is important to also consider occupational exposures for workers that must spend prolonged periods in the space. Preferably, the documented performance data under as-used conditions should be available from multiple sources, some of which should be independent, third-party sources. Unsubstantiated claims of performance or limited case studies with only one device in one room and no reference controls should be questioned.


Study: Indoor air cleaners fall short on removing volatile organic compounds. Comparison of four oxidation-based cleaners suggests the devices produce their own pollutants and vary in effectiveness.


SARS-CoV-2 viral particles spread between people more readily indoors than outdoors. Indoors, the concentration of viral particles is often higher than outdoors. When indoors, ventilation mitigation strategies can help reduce viral particle concentration. The lower the concentration, the less likely viral particles can be inhaled into the lungs (potentially lowering the inhaled dose); contact eyes, nose, and mouth; or fall out of the air to accumulate on surfaces.

Protective ventilation practices and interventions, including portable HEPA air purifiers with high Clean Air Delivery Rates and 3rd party verification, can reduce the airborne concentrations and reduce the overall viral dose to occupants.

AHAM Air purification Air purifier CADR CDC covid-19 hepa hepa13 hospital grade HVAC IAQ Indoor Air Quality Rensair ROS SARS-CoV-2 updated guidance uvc UVGI ventilation

Get a quote

Describe your needs and get your quote within 24 hours

Get a Quote

Talk to an expert

Book a free consultation with one of our technical experts

Book Consultation