Reducing the Spread of Airborne Pathogens and Winter Illnesses with Air Filtration

With COVID restrictions lifted, people are returning to work and school, and spending more time in restaurants and other venues. As people come in closer face-to-face contact in inside environments, they share the same air, increasing the risk of transmission of viruses and bacteria. 

Adults get two to four colds annually, with people in the United States suffering from 1 billion colds per year.1 In many areas of the country, more people gather indoors to escape the cold of the winter months. This further increases the rate of viruses passing between individuals.  

When someone who is sick or has a virus sneezes, coughs or just exhales, they expel mucus and saliva droplets that travel in the air and can land on or be inhaled by others standing nearby. That’s called droplet transmission. People don’t need to touch another person with a virus to catch it. They can be infected when nearby – typically less than six feet. One sneeze from an infected individual can spread about 40,000 liquid droplets that may contain pathogens.2 Those droplets can travel up to 150 feet per second, over 100 miles per hour.

When droplets travel as aerosols, they can linger in the air or travel on air currents for long periods of time before contacting a person. Airborne pathogens can also land on surfaces, spreading infection when people touch them.

Besides the common cold, aerosol transmissions can cause winter illnesses such as the flu, which can have deadly results. According to the Centers for Disease Control and Prevention, the 2021-2022 season had an estimated 9 million flu illnesses, 5,000 of which resulted in death.3   

Business and commercial enterprises as well as educational institutions and hospitals have a responsibility to ensure the safety of their occupants.  Healthy and safe indoor environments with good air quality are imperative to eliminate the risk of infectious diseases.  

How Can Airborne Pathogens Be Prevented?    

As masks are no longer mandated in many buildings, droplets from sneezes and coughs are more likely to become airborne, attaching themselves to other occupants. The right mix of ventilation, filtration, and air purification in a building can reduce the concentrations of viral particles in indoor environments to improve air quality and reduce the risk of illnesses. 

While ventilation dilutes the concentration of airborne respiratory droplets, high-quality filters can remove particles, dust, allergens, and pollution. Air purifiers can supplement HVAC ventilation and filtration systems to achieve optimum air quality and create a healthier work environment.  

How Ventilation is Useful for Preventing Airborne Diseases

Ventilation is an effective engineering control that supplies, distributes, and circulates fresh air and removes stale air throughout indoor spaces. In doing so, it improves the air quality of indoor environments and helps to control temperature and humidity levels for greater comfort.

Natural ventilation using fresh air is the best way to improve air quality in indoor spaces. However, some buildings have limited direct access to fresh air. Also, cold winter months often make it infeasible to heat fresh air due to energy costs. As a result, most HVAC systems mix fresh air and recirculated air. 

Ventilation effectiveness is measured by determining how often air changes per hour (ACH) in an indoor area. Higher ACH ratings denote more frequent air replacement that reduces the presence of contaminated microorganisms. The formula for calculating the ACH rate is:

ACH = CFM (airflow cubic feet per minute) x 60 (hour)/ (area of the space x height of ceiling)4

So, a room measuring 12’ x 10’ with a ceiling height of 10’ with an HVAC system having a ventilation rate of 65 cubic feet per minute (CFM) of supply air would have an ACH rating of 4. That number is considered the minimum air exchange for any commercial or industrial building.5 The higher the ACH rating, the more times the ventilation is exchanging air in a room to make it cleaner.

The United States Environmental Protection Agency (EPA) states that increasing air ventilation can reduce the concentration of airborne contaminants.6

Importance of an Air Filtration System

Ventilating an indoor space with recirculated air will not entirely eliminate airborne pathogens. The EPA notes that ventilation is not enough on its own to protect people from viruses such as COVID-19 transmission, especially in high-traffic areas.6 

Air filtration is needed to remove particles, including dust and aerosols, from an air stream before it is introduced or returned into a building space. Filtering air along with ventilation is the best way to improve indoor air quality to reduce the spread of infectious agents in buildings.  

Working as part of an HVAC system, commercial air filtering systems actively clean contaminants from the air before it is circulated to occupied spaces. Various air filters are available to trap different-sized particles as air flows through them. For example, High-Efficiency Particulate Air (HEPA) filters trap and remove the smallest particles associated with viruses. According to the Institute of Environmental Sciences and Technology (IEST) standard IEST-RP-CC001, a HEPA filter is at least 99.97% efficient at capturing submicrometer particles.7

Air Filter Efficiency Ratings

ASHRAE Standard 52.2.2017 is the method used to test the performance of filters used in HVAC systems.  Filters receive a Minimum Efficiency Reporting Value (MERV) from 1 to 16, with a higher rating indicating their ability to trap smaller particles. The values help to compare and choose among different filters. Below is a chart published by the EPA.8 

MERV Rating Average Particle Size Efficiency in Microns
1 – 4 3 to 10 less than 20%
5 3 – 10 less than or equal to 20%
6 3 to 10 less than or equal to 35%
7 3 to 10 less than or equal to 50%
8 1 to 3 less than or equal to 20%

3 to 10 less than or equal to 70%

9 1 – 3 or less than or equal to 35%

3 – 10 less than or equal to 80%

10 1 to 3 less than or equal to 50%

3 to 10 less than or equal to 80%

11 0.3 – 1 less than or equal to 35%

1 to 3 less than or equal to 50%

3 to 10 less than or equal to 80%

12 0.3 – 1 less than or equal to 35%

1 to 3 less than or equal to 80%

3 to 10 less than or equal to 90%

13 0.3 – 1 less than or equal to 50%

1 to 3 less than or equal to 85%

3 to 10 less than or equal to 90%

14 0.3 – 1 less than or equal to 75%

1 to 3 less than or equal to 90%

3 to 10 less than or equal to 95%

15 0.3 – 1 less than or equal to 85%

1 to 3 less than or equal to 90%

3 to 10 less than or equal to 95%

16 0.3 – 1 less than or equal to 95%

1 to 3 less than or equal to 95%

3 to 10 less than or equal to 95%

HEPA 99.97% of particles in the 0.3-micron range

Particles that are larger or smaller than 0.3 microns are captured with a greater than 99.97% efficiency


According to this chart, a MERV 9 filter will filter out 80% of 3 – 10-micron particles, while a MERV 13 will filter out 90% of them. While MERV 7 – 11 filters generally cost less, they are less effective in decreasing the risk of infectious disease transmission.  

The World Health Organization reports that respiratory droplets are > 5 – 10 µm in diameter size while droplet nuclei are <5μm in diameter.9  To eliminate these pathogens, ASHRAE recommends using a filter with a minimum rating of MERV 13 that is 85% efficient in capturing infectious particles ranging in size from 1 µm to 3 µm.  But, a MERV 14 or better is preferred.1

Choosing the Right Air Filter

While air filters with higher MERV ratings do a better job in removing pathogen-laden airborne particles, they can reduce airflow and negatively affect HVAC system performance. That’s why it is important to consider different parameters of your HVAC system to choose the best air filter. Camfil, the leading manufacturer of commercial and industrial systems for air filtration and air pollution control, provides these 5 tips:

  1. Choose an air filter appropriate for the type of HVAC unit and space to which it supplies conditioned air. Upgrading a filter may not be feasible and it will damage the HVAC system. Many standard HVAC filtration units cannot accept HEPA filters as the fan is not capable of overcoming the HEPA filter’s higher resistance to airflow.  
  2. Know that every space does not need HEPA-filtered air. Other non-HEPA, but high-efficiency air filters can remove viruses, bacteria, and other microorganisms associated with airborne particle transmission. Lower-efficiency air filters are designed to remove larger particles, such as dust and pollen.  
  3. Ensure the HVAC system is properly installed and maintained according to the manufacturer’s instructions. This may include monitoring the condition of the coils, keeping the drain lines clear, and checking for leaks, or other issues.
  4. Ensure that the HVAC system’s fan is running while people are in the building and one hour after they leave, even while heating or cooling. Running the fan as much as possible allows the air filters to continuously remove airborne pathogens.
  5. Regularly communicate with employees, customers, and others about the use of high-efficiency air filters and their value in preventing the spread of airborne pathogens. Building awareness and support for air filtration systems can protect people’s health.

Will an Air-Cleaning Device Help?

When the goal is removing harmful microscopic particles, you can achieve the best results by adding an air purifier with a HEPA filter to the HVAC system. For example, the Camfil City M Air Purifier includes molecular and particulate filters to effectively lessen the spread of airborne pathogens and winter illnesses.

Camfil offers these 4 tips to choose and operate an air purifier to keep the air in a designated space clean to reduce the risk of people getting sick: 

  1. Choose the right air purifier for the size of the space you want to purify. For example, the Camfil City M Air Purifier is designed for medium-sized rooms and spaces, such as offices, classrooms, and small apartments.
  2. Install air purifiers according to the manufacturer’s instructions. Be sure to position them where they can most effectively purify the air in the entire space.
  3. Turn on the air purifier and set it to the fan speed appropriate for the space. It does not do any good to install a purifier with a loud fan that will be turned off because it is distracting. Find an air purifier like City M that is extremely quiet, even in the highest settings.
  4. Change out air filters according to manufacturer instructions. When used in typical environments, City M air filters last 18 months. 

Clean Air Strategy 

It takes a multilayered strategy to create a healthy indoor environment void of airborne pathogens, allergens, and other pollutants. The United States Environmental Protection Agency has cited a “Clean Air to Buildings Challenge” that outlines best practices to improve ventilation and indoor air quality to reduce risks associated with airborne viruses. 11  Action items include:

  • Create a clean indoor action plan.
    • Determine how clean outdoor air is supplied and circulation in the building.
    • Understand how the HVAC system works. Assess and inspect systems for ventilation, filtration, and air cleaning.
    • Determine how much clean air is needed and supplied.
    • Assess the direction of air flows.
    • Schedule regular inspections and maintenance.

  • Optimize fresh air ventilation.  
    • Verify if outdoor air is clean enough for use in the ventilation system. 
    • Identify what devices can efficiently and cost-effectively increase fresh air ventilation.
    • Operate HVAC systems during occupied hours.
    • Verify the functionality of exhaust fans in bathrooms.
    • Increase the volume of clean, outdoor air during high-risk times.

  • Enhance air filtration and cleaning.
    • Install MERV-14 or better air filters.
    • Enclose gaps around air filters.
    • Use air cleaners to increase cleaning rates.
    • Increase filtration/ventilation in highly populated or high-risk areas with greater emissions of airborne particles and aerosols

  • Conduct community engagement, communication, and education
    • Communicate your action plan to improve indoor air quality and reduce disease transmission.
    • Provide feedback mechanisms such as maintenance requests.
    • Host building walkthroughs, hang signage, or communicate through other means of building work and how individuals can help to keep buildings operating optimally. 

While these initiatives do not completely eliminate the risk of exposure to particles and aerosols, they dramatically reduce it. Other protocols such as hand and surface cleaning can help reduce their presence and enhance environmental safety conditions. 

1Common Cold, WebMD, February 5, 2022,,60%20have%20fewer%20than%20one%20cold%20a%20year.

2Passing Pathogens, Arizona State University,  

3Preliminary Estimated Influenza Illnesses, Centers for Disease Control and Prevention, October 4, 2022,

4Ventilation in Buildings, Centers for Disease Control and Prevention, June 2, 2021,  Ventilation in Buildings | CDC 

5Building Ventilation – the Proper Air Changes Per Hour, Atlantic Environmental Corporation,

6Ventilation and Coronavirus, United States Environmental Protection Agency, Ventilation and Coronavirus (COVID-19) | US EPA

7IEST-RP-CC001: HEPA and ULPA Filters, IEST, IEST-RP-CC001: HEPA and ULPA Filters

8What is a MERV rating? United States Environmental Protection Agency, What is a MERV rating? | US EPA

9Modes of Transmission Virus Causing COVID-19, World Health Organization, March 29, 2020, Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations (

10Filtration and Disinfection, ASHRAE, Filtration and Disinfection FAQ (

11Clean Air in Buildings Challenge United States Environmental Protection Agency, March 2022, Clean Air in Buildings Challenge | US EPA

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