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Airborne Infections

Recent AQS Work on Aerosol Transmission and Control

Inadequate ventilation in California classrooms is common and is linked to higher illness absence. This is a cause for increased concern during the current COVID-19 pandemic caused by SARS-CoV-2. Growing evidence suggests that viral transmission can increase in crowded, poorly ventilated indoor spaces, through small aerosols that can remain airborne.

We have 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,
  • a peer-reviewed journal paper on a more comprehensive multiplicative modeling framework addressing both short-range and long-range airborne transmission exposure risk reductions for various indoor and outdoor scenarios, and
  • additional useful information on practical considerations for ventilation and air filtration inspection and improvement for reopened schools.

AQS has also participated in developing the State interim ventilation guidance: Interim Guidance document.  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).  It helps prevent employee exposure to COVID-19 in California workplaces not covered by the Cal/OSHA Aerosol Transmissible Diseases standard.

These works 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:

CDC’s Guidance for Ventilation in Schools and Childcare Programs

EPA’s Program for Healthy Indoor Air Quality in Schools

Practical Implications

Multiple protective strategies can help to substantially reduce the risk of long-range airborne transmission of SARS-CoV-2 in classrooms. These include:
  • Mask wearing: All individuals (teachers, students, staff, etc.) should wear masks— under all ventilation rate or air filtration conditions in the classroom, this practice reduces both short-range and long-range airborne transmission risk comparing to not wearing mask.
  • Outdoor air ventilation: The system should provide at least the code-required minimum ventilation rate (per California Title 24). In classrooms with no ventilation and no filtration, the risk of long-range airborne infection would be over six times as high as that for classrooms with code-required ventilation and a MERV 8 filter.
  • Filtration: Ventilation system filters should be MERV-rated at MERV 13 or better. They should also be properly installed (i.e., no gaps that would allow air to bypass the filter) and properly maintained (i.e., replaced as often as recommended). MERV-rated filters can provide substantial protection from long-range airborne infection, especially if ventilation is poor.
  • In-room (portable) air cleaners: Air cleaners used to reduce the risk of long-range airborne transmission should provide high-efficiency filtration and a sufficient “clean air delivery rate” (CADR) (i.e., at least 2/3 of the floor area). Such air cleaners can provide substantial additional protection, especially in naturally ventilated classrooms (in which air is supplied only through open windows or doors) or in classrooms with non-functioning or poorly functioning ventilation systems. Multiple devices per classroom may be necessary for sufficient total air cleaning.

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)

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

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)

Other Airborne Infections

Infections from environmental exposures

Legionellosis (LEE-juh-nuh-low-sis) is a respiratory disease caused by Legionella bacteria. 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 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 (air-conditioning units for 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.

Infections spread from person to person

Common colds are the main reason that children miss school and adults 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.

Viruses that cause colds can spread from infected people to others through the air and close personal contact. You can also be infected through contact with stool (poop) or respiratory secretions. This can happen when you shake hands with someone who has a cold, or touch a doorknob that has viruses on it, then touch your eyes, mouth, or nose.

You can help reduce your risk of catching a cold by (1) washing your hands often with soap and warm water; (2) avoid touching your eyes, nose, and mouth with unwashed hands; and (3) staying away from people who are sick. There is no vaccine to protect you against the common cold because multiple viruses cause colds.

You can protect others when you have a cold by (1) staying at home while you are sick; (2) avoiding close contact with others, such as hugging, kissing, or shaking hands; (3) moving away from people before coughing or sneezing; (4) coughing and sneezing into a tissue then throwing it away, or coughing/sneezing into your upper shirt sleeve, completely covering your mouth and nose; (5) washing your hands after coughing, sneezing, or blowing your nose; and (6) disinfecting frequently touched surfaces, and objects such as toys and doorknobs.  More information about common colds can be found on the CDC website.

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

Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people nearby. Less often, a person might also 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 high 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.

Tuberculosis (TB) is caused by a bacterium called Mycobacterium tuberculosis. The bacteria usually attack the lungs, but TB bacteria 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 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, or sings. People nearby may breathe in these bacteria and become infected.

TB is NOT spread by shaking someone's hand, sharing food or drink, touching bed linens or toilet seats, sharing toothbrushes, or kissing. More information about TB can be found on the CDC website.

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