Guidelines for Indoor Air Quality: CDC Revised Guidance for Air Ventilation

Guidelines for Indoor Air Quality: CDC Revised Guidance for Air Ventilation in 2023

The Centers for Disease Control and Prevention (CDC) published new guidance in May 2023 on the ventilation of buildings. These important new indoor air quality regulations include specific targets that should drastically help to reduce viral transmissions of flu, and also help slow the spread of other pandemic-causing viruses, like Covid-19 in buildings.

In the same month, ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, a body that regularly sets the standards for safety and performance of HVAC technology and applications, also released a critical update. ASHRAE published its first-ever standard for the maintenance of healthy indoor air quality. The May publication was a draft of the new standards, however in July 2023, the final version was published as the ASHRAE Standard 241P, Control of Infectious Aerosols.

This blog post will review these new changes, explain what they mean and look at how they will help stop the spread of viruses through indoor air spaces.

CDC air quality concerns

The US Center for Disease Control and Prevention (CDC) states that particle pollution and smog–in particular ‘ground-level ozone’, which is the main part of smog–are two of many causes of a decline in outdoor air quality, which has a detrimental impact on public health in the US. 

Indoors, the situation can be much more hazardous. Current heating, ventilation and air conditioning (HVAC) systems are primed to circulate toxins, pollutants, and even viruses like Covid-19. In fact, in recent years, it has become understood that viral transmission is markedly affected by indoor air quality levels; good air quality indoors is key to reducing transmission. Buildings with poor ventilation can actually trap pathogens, like the Covid-19 virus, and recirculate them, causing the air quality to remain low.

Virus particles, like those of Covid-19, are transmitted from person to person in two ways. In one instance, a person can come into contact with larger particles of the virus when an infected person exhales, coughs or sneezes. Large droplets can spray into the air and will then fall to the ground or onto other surfaces. If a person comes into contact with those droplets they can contract the virus. In another instance, the same cough, sneeze or exhale by the infected person will also produce much smaller particles, which remain suspended in the air for hours afterwards.

Unlike the outdoors where air can move a lot faster and further, indoors virus particles can linger in the air and become trapped, unable to disperse further afield. Consequently, despite many people’s best efforts to wear masks, lather their hands in antibacterial gel, and stay meters apart from each other, the risk of transmission between people sharing indoor spaces is high, and many people contracted the Covid-19 this way. 

What is sick building syndrome (SBS)?

The medical disorder of sick building syndrome (SBS) is not new to Covid-19, but the very essence of SBS–that people can become ill from the air within certain buildings–applies to viral pandemics too. SBS was first identified by the World Health Organisation some fifty years ago, and additional research has since confirmed that some buildings present as “sick”. The air contains airborne particles that cause people to experience a host of symptoms, from coughs and colds to headaches and fatigue. Upon leaving the building, these symptoms somewhat miraculously abate. The cause of sick building syndrome often cannot be identified, whereas the cause of building-related illnesses often can be identified.

Taking steps such as improving ventilation, allowing more fresh air into spaces, and improving filtration can significantly slow the transmission of life-threatening pollutants, bacteria, and viruses. In fact, a building that has a well-working HVAC system that regularly cycles fresh air into indoor spaces can be classified as a “healthy building”. Air purifiers can also combat poor quality air in buildings too. A building in this category is able to reduce the transmission of viruses, improve asthma symptoms, and even boost cognitive functioning for inhabitants too.

What are the key air ventilation updates made by the CDC?

  • Key information on “how much ventilation is enough” states that in occupied spaces 5 air changes per hour is recommended at the very least.
  • The central HVAC filter efficiency has been updated to the Minimum Efficiency Reporting Value (MERV) of 13 or better. Previously, the minimum rating requirement was equivalent to a MERV-8. MERV ratings are given to show a filter’s ability to capture particulate matter between 0.3 and 10 microns (µm). 

Additional CDC building ventilation updates include:

  • Updated guidance on requirements for flushing building air once occupants have left the building.
  • Additional information concerning energy cost, up-front costs, and maintenance costs for ventilation strategies.
  • Additional FAQs covering matters related to DIY air cleaners and generally providing information in a much more succinct format.

The CDC’s revised guidelines focus on improving ventilation and subsequently improving air quality too, and in particular contain guidelines for indoor air quality in schools, and also for office buildings and other commercial spaces.

Buildings will now have to install MERV-13 filters, replacing their MERV-8 filters which captured around 20% of airborne particulate matter. MERV-13 filters capture 80-90% of airborne particulate matter.

How many air exchanges per hour are recommended by the CDC?

A key question that the CDC wanted to answer with their spring 2023 update was “how much ventilation is enough?”. The answer lies in the number of air changes per hour (ACH), which the CDC recommended to be 5, at a minimum.

A survey of US schools taken in 2022 by the CDC found quite inconsistent implementation of post-pandemic ventilation improvements. The vast majority of schools have systems built to facilitate around 3 air exchanges per hour. With a new target requiring an almost doubling of the ACH rate, schools can work towards positive change by improving both ventilation and filtration. There is also the option to use devices that provide the equivalent ACH (eACH) such as air purification devices, added to existing HVAC systems. The CDC recommends that this number of air changes should greatly reduce the number of germs in the air.

An air change is defined as either “supplying or exhausting an amount of air (use the larger of the two values but do not add them together) that is equal to all the air in a space” according to the CDC.  The final amount should be multiplied by 5 and if delivered over an hour, it equates to 5 ACH. See below for a general guide on how to calculate ACH and eACH.

How to calculate ACH or eACH

  • Determine (or measure) the airflow through the system in cubic feet per minute (cfm).
  • Determine the area of the room = length (ft) x width (ft)
  • Determine the height of the room (ft).
  • Calculate ACH:

Calculate ACH

  • When multiple strategies are used, repeat the ACH calculation for each system then add together for a total ACH value (which could be compared to the minimum 5 ACH recommendation).

The CDC also notes that this is a general rule that should be applied with a degree of common sense to different scenarios. For example, a larger space where only a few occupants frequent, such as a factory or warehouse floor, may not require 5 ACH. At the other end of the scale, spaces that hold much higher occupancy numbers, or where the occupants are very vulnerable to poor air quality, may require a significantly higher ACH rate.

ASHRAE guidelines for indoor air quality

ASHRAE has provided guidelines and standards for over a century specifically for engineers of HVAC systems. The ASHRAE Standard 241, Control of Infectious Aerosols, published in July 2023 applies to office buildings, as well as any other new or existing buildings, or any buildings undergoing renovations. The aim of this new standard is to establish the “minimum requirements aimed at reducing the risk of disease transmission”. In this context, infectious aerosols refer to particles that contain disease-carrying pathogens, and that are exhaled by infected individuals. The hope is that this new standard will help prevent buildings from spreading dangerous pathogens, like SARS-COV-2 virus, which was responsible for COVID-19.

Finding the right technology to improve air quality 

The CDC’s May 2023 on ventilation in buildings also included an FAQ on how to find air quality improvement devices. Importantly, the CDC does not recommend or endorse any specific technology or product. Instead, the CDC recommends using a product that employs proven technology and has demonstrable efficacy. This can be determined by independent research.

The CDC recommends a cautious approach towards any new or emerging technology, especially as new technology has a limited track-record for testing and side-effects of a product’s use, particularly when treating air in large indoor spaces or circulating through HVAC systems. Some devices may be classified as ‘pesticide’ devices by the US EPA. Consequently, it’s important to look for any EPA registration requirements that may apply to manufacturing sites and to also verify manufacturers efficacy claims with any supporting data available.

Following the advent of Covid-19, there has been an explosion in the market of products promising to tackle, treat, and eliminate the Covid-19 virus from the air. There are multiple different methods offered including chemical fogging, dry hydrogen peroxide, and ionization, or a combination of methods. Some of these methods have been proven effective, however it is important to verify that the technology using the method is itself effective.

Some technologies release chemicals, ions, or reactive oxygen species (ROS) into the air. This may pose a risk to certain vulnerable groups of people. Some independent testing bodies do provide advisory and compulsory limits for certain emissions from these technologies. The CDC also recommends checking for safe ozone emission levels, and in particular, verifying that products generating ozone meet the UL 2998 standard certification for zero ozone emissions

Get the TADIRAN AIROW 3™  to improve air quality indoors 

These new standards and recommendations implemented by ASHRAE and the CDC serve as an actionable guideline for managing indoor air quality in office buildings, schools, and all other commercial buildings such as factories and warehouses. As well as following these guidelines, there’s also the possibility of employing air purification technologies to help remove germs from indoor environments. The TADIRAN AIROW 3™ can be added to HVAC systems by a professional HVAC engineer. It works to remove pathogens by neutralizing them with hydrogen peroxide, which is disseminated throughout indoor spaces via the HVAC system.

In tests conducted at the Kitasato Research Center for Environmental Science in Japan, aerosolized Blue Mold (Penicillium Citrinum) was reduced by 99.4% in a 25 cubic meter chamber over a period of 180 minutes. Additional testing at the FDA-cleared Aerosol Research and Engineering Laboratory showed that only 0.006% of Staphylococcus Epidermidis bacteria was discoverable after just 60 minutes. Testing took place in a one cubic meter chamber. A separate test in the same laboratory demonstrated a 99.994% reduction of the RNA Virus MS2 Bacteriophage in just 60 minutes. These results show just how effective the TADIRAN AIROW 3™ can be at reducing harmful pathogens and promoting safe and healthy air indoors.

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