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Airborne Infections

Potential actions to reduce the health risks from indoor airborne transmission (see below for details on choosing the best actions depending on conditions):
  1. Wear a well-fitted mask and maintain physical distance.
  2. Increase natural ventilation by opening windows and doors.
  3. Operate HVAC system in “all fresh” mode when possible (i.e., 100% outdoor air).
  4. Install and maintain HVAC filters rated MERV 13 or above
  5. Install portable air cleaner(s).
  6. Consider installing Ultra-Violet Germicidal Irradiation (UVGI)

For current California recommendations on improving indoor air quality (IAQ) to reduce the spread of COVID-19, see the California Department of Public Health (CDPH) Interim Guidance for Ventilation, Filtration and Air Quality in Indoor Environments.

For a less technical discussion on COVID-19 and IAQ for workspaces than what is presented below, see CDPH COVID 19 & Improving Indoor Air Quality at Work.

For detailed analysis on the relative effectiveness of wearing masks, ventilation and filtration, see the Air Quality Section (AQS) report: The role of building ventilation and filtration in reducing risk of airborne viral transmission in schools, illustrated with SARS-Co V-2 (2020)(PDF, 1.02MB)

Airborne Infection Control in Indoor Environments 

COVID-19 is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). COVID-19 health effects in infected persons range from undetectable to fatal, but most commonly include fever, dry cough, and fatigue. The first known case was identified in December 2019. After that, the disease quickly spread worldwide.

The World Health Organization (WHO) declared the outbreak a public health emergency of international concern in January 2020, began referring to it as a pandemic in March 2020, and ended its declaration of COVID-19 as a global health emergency in May 2023.

It is now widely acknowledged that COVID-19 is an airborne transmissible disease. This means that (1) larger virus-containing droplets, which do not stay in the air for long distances or amounts of time, can be transmitted at short-range to a susceptible person near an infected person, but also that (2) smaller virus-containing droplets or aerosols, which can stay suspended in the air and remain infectious over longer distances and amounts of times, can cause both short-range and long-range transmission. Lowering the concentration of airborne viruses in indoor air is known to reduce the risk of transmission. More information about COVID-19 can be found at the CDPH COVID-19 Home Page.

The COVID-19 pandemic has reminded us that poor ventilation and IAQ  in buildings can increase the risk of transmission of airborne respiratory infections.

The use of personal respiratory protection, such as masks, helps to reduce both short-range and long-range transmission of airborne respiratory infections. Avoiding crowding, especially in indoor settings, can help reduce short-range transmission only. However, this strategy is not always possible or practical. As we emerge from the pandemic, preventive IAQ control measures become more important and should be continuously implemented to maintain good IAQ and reduce the risks from exposure to airborne agents, especially for long-range transmission. Buildings should have readiness plans that include instructions on how heating, ventilation, and air conditioning (HVAC) systems and procedures should operate to address the control of infectious aerosols. 

Common IAQ control measures include increasing natural ventilation by opening windows and doors, increasing the outdoor ventilation rate supplied by mechanical HVAC systems, removing particles by filtration, or disinfecting indoor air using ultraviolet germicidal irradiation.

Natural ventilation through open windows is not always an option: outdoor air quality may be poor (e.g., during wildfires or dust storms or in polluted urban areas), weather may be inclement (e.g., in seasons of heat, cold, snow, or rain), or there may be noise, safety concerns, or security risks. In those cases, mechanical ventilation can bring in outdoor air that is filtered, without the concerns about safety, security, or outside noise.

Effective filtration of virus-laden aerosols from indoor air can be accomplished within HVAC systems with filters rated at Minimum Efficiency Reporting Value (MERV) 13 or higher, or with properly selected in-room portable air cleaners (PACs). Additional information on PACs and HVAC operation can be found at COVID-19 & Improving Indoor Air Quality at Work.

When applied appropriately, ventilation and filtration can reduce the indoor air concentrations of various pollutants of concern, including bioaerosols (e.g., influenza and common cold viruses as well as allergenic and irritant plant, animal, and mold particles), other particulate matter (e.g., combustion vehicle emissions or wildfire smoke), and volatile organic chemicals and particles generated from various indoor sources and activities. 

The best IAQ index for the purpose of airborne infection control is the equivalent outdoor flow rate of “clean,” i.e., pathogen-free air, per individual (eOAi), in units of CFM (cubic feet per minute) per person. The eOAi is based on the total amount of “clean” air introduced by any combination of central mechanical ventilation, natural ventilation, or additional devices that provide pathogen-free air (such as MERV 13 filters or portable air cleaners/PACs) divided by the number of occupants.

It is sometimes also referred to as “equivale” outdoor air flow rate per person. The eOAi, which describes the rate of cleaned or exchanged air provided per person, is related mathematically to another concept, the total equivalent air change rate in units of air change per hour (eACH), which refers to the rate at which the entire volume of an indoor space is cleaned or exchanged. Because eACH does not directly reflect the amount of clean air per person, it is a less precise number for controlling infectious disease transmission. 

When a building is operating in infectious risk management mode, another important control measure in addition to ventilation and air cleaning is to limit occupancy. Lowering permitted occupancies will reduce the required eACH rate  and may reduce the risk of close-range transmission if people are generally further apart. 

In California, the CDPH Interim Guidance for Ventilation, Filtration and Air Quality in Indoor Environments (developed and updated with AQS participation), is the main guidance document on IAQ for infection control. This guidance applies to non-healthcare organizations in general (including schools) and was jointly developed by CDPH, the California Department of Health Care Access and Information (HCAI), and the California Division of Occupational Safety and Health (Cal/OSHA). 

The guidance document helps prevent employee exposure to COVID-19 in California workplaces not covered by the Cal/OSHA Aerosol Transmissible Diseases standard (PDF,  2.60MB).

In workplaces, employers and employees are subject to either the Cal/OSHA COVID Non-Emergency Regulations or the Cal/OSHA Aerosol Transmissible Diseases (ATD) standard (PDF, 2.60MB) and should consult those regulations for additional applicable requirements. 

For healthcare specific guidance, see CDPH guidance on Best Practices for Ventilation of Isolation Areas to Reduce COVID-19 Transmission Risk in Skilled Nursing Facilities, Long-Term Care Facilities, Hospices, Drug Treatment Facilities, and Homeless Shelters.

Introduction to Common Airborne Infections

Airborne infectious diseases are those that are spread through the air. Airborne diseases can be spread by a variety of agents, including bacteria, viruses, and fungi. These organisms may be transmitted directly from person to person through breathing, sneezing, coughing, or talking, or indirectly through the spraying or splashing of liquids, the spread of dust, or any other activity that generates particles in the air. Some common airborne infections in addition to COVID-19 and other respiratory infections are briefly introduced below. More information about airborne diseases and precautions can be found in this book available from National Library of Medicine

Examples of infections spread through inhalation:

Common colds

Common colds, which almost everyone has experienced, are the main illness causing children to miss school and adults to miss work (adults have an average of 2-3 colds per year, and children have even more). A sore throat and runny nose are usually the first signs of a cold, followed by coughing and sneezing. 

Cold viruses are spread through respiratory droplets that are released when an infected person breathes, talks, coughs or sneezes. These droplets can enter another person’s body if they breathe them in, or if they touch a surface contaminated with the virus and then touch their eyes, nose, or mouth.

This can happen when people shake hands with someone who has a cold or touches an object contaminated by a cold virus, then touches their eyes, mouth, or nose without washing their hands. 

There is no vaccine to protect you against the common cold because many different viruses can cause colds. More information about common colds can be found on the CDC website.


Influenza (the flu) is a contagious respiratory disease caused by influenza viruses that infect the nose, throat, and sometimes the lungs. Influenza can ca use mild to severe illness, and at times can lead to death. 

Similar to cold viruses, flu viruses can spread through respiratory droplets that are released when an infected person breathes, talks, coughs or sneezes. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Less often, a person might get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose, or possibly their eyes.

Anyone can get the flu (even healthy people), and serious flu-related problems can happen at any age, but some people are at higher risk of developing serious complications if they get sick (for example, people 65 years and older, people with certain chronic medical conditions, pregnant women, and young children). The best way to prevent the flu is by getting vaccinated every year and practicing everyday preventive measures such as staying away from people who are sick, covering coughs and sneezes, and washing hands frequently. More information about the flu can be found on the CDC website  and the CDPH Influenza page.

Tuberculosis (TB)

Tuberculosis is caused by a bacterium called  Mycobacterium tuberculosis. The bacteria usually attack the lungs but can attack any part of the body. However, not everyone infected with TB bacteria becomes sick. If not treated properly, TB disease can be fatal. 

TB bacteria are spread through aerosols in the air from one person to another. The TB bacteria are released into the air when a person with TB disease of the lungs or throat coughs, speaks, laughs, or sings. People nearby may breathe in these bacteria and become infected as the aerosols in the air can stay for several hours. TB is NOT spread by shaking someone’s hand, sharing food or drink, touching bed linens or toilet seats, sharing toothbrushes, or kissing. Not everyone infected with the bacteria becomes sick. People that have been infected but are not sick have latent tuberculosis infection (LTBI). People with LTBI can become sick with TB disease in the future if they do not take treatment for LTBI. More information about TB can be found on the CDC website  and the CDPH Tuberculosis page


Legionellosis (LEE-juh-nuh-low-sis) is a respiratory disease caused by  Legionella  bacteria from environmental sources rather than transmission from other persons.

Sometimes the bacteria cause a serious type of pneumonia (lung infection) called Legionnaires’ disease. The bacteria can also cause a less serious infection called Pontiac fever that has symptoms similar to a mild case of the flu. 

Legionella bacteria are found naturally in freshwater environments, such as lakes and streams. The bacteria can become a health concern when they grow and spread in human-made water systems, such as showers and faucets as well as cooling towers (which are air-conditioning units for some large buildings). Home and car air-conditioning units do NOT use water to cool the air, so they are NOT a risk for legionella growth. 

People can get Legionnaires’ disease when they breathe in small droplets of water in the air that contain the bacteria. Less commonly, people can get Legionnaires' disease by aspiration of drinking water (when water “goes down the wrong pipe”).

Most healthy people exposed to Legionella do not get sick, but older people, current or former smokers, and people with weakened immune systems may be at increased risk. In general, people do NOT spread Legionnaires’ disease and Pontiac fever to other people. More information about legionella can be found on the CDC’s website  and at Legionellosis (Legionella) ( .

Past AQS Work on Aerosol Transmission and Control of Infectious Agents

Inadequate ventilation in California classrooms is common and is linked to higher illness absence (Mendell MJ, et al. Association of classroom ventilation with reduced illness absence: A prospective study in California elementary schools. Indoor Air 2013. 23(6):515-528). Growing evidence suggests that transmission of viruses and bacteria can increase in crowded, poorly ventilated indoor spaces, through small aerosols that can remain airborne.

During the COVID-19 pandemic, AQS has developed:

  • a report on modeling the potential benefits for schools of adequate outdoor air ventilation and air filtration in reducing long-range airborne transmission of respiratory infections;
  • additional useful information on practical considerations for ventilation and air filtration inspection and improvement for reopened schools; 
  • a peer-reviewed journal paper on a more comprehensive multiplicative modeling framework addressing various indoor and outdoor scenarios to reduce risks of both short-range and long-range airborne transmission exposure; and
  • a peer-reviewed journal paper on the comparison of mask protection against inhaling wildfire smoke, allergenic bioaerosols, and infectious particles. 

See AQS Reports and supporting information (below) . These are part of AQS’s efforts to better understand aerosol transmission and to promote adequate ventilation and filtration in schools, which may have broad long-term health benefits.

More information on school ventilation and air filtration can be found at:

AQS Reports and Supporting Information

The role of building ventilation and filtration in reducing risk of airborne viral transmission in schools, illustrated with SARS-CoV-2 (2020)(PDF, 1.02MB)

Ventilation and Filtration to Reduce Long-Range Airborne Transmission of COVID-19 and Other Respiratory Infections: Considerations for Reopened Schools (2021) (PDF, 1.39MB)

Window Openable Area Inspection Datasheet (SI Units) (Excel file, see Appendix B for details)

Window Openable Area Inspection Datasheet (English Units) (Excel file, see Appendix B for details)

CO2 Decay Ventilation Measurement Datasheet (SI Units) (Excel file, see Appendix E for details)

CO2 Decay Ventilation Measurement Datasheet (English Units) (Excel file, see Appendix E for details)

Information for School Districts on Purchase of Filtration/Air Cleaning Devices (2021) (PDF)

Modeling the Impacts of Physical Distancing and other Exposure Determinants on Aerosol Transmission (2021) (Journal article)

Comparative Mask Protection against Inhaling Wildfire Smoke, Allergenic Bioaerosols, and Infectious Particles (2022) (Journal article)

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