Why is CDPH updating its definition of close contact in large indoor non-healthcare spaces? 

SARS-CoV-2 aerosols are generated and released by a person with COVID-19 through activities such as breathing, speaking, and coughing. These aerosols enter the air around the infected person and then spread out evenly throughout the air.

In indoor environments, exposure to SARS-CoV-2 aerosols can occur in two ways: 1) directly, through face-to-face interactions with a person with COVID-19 and 2) indirectly, by inhalation of aerosols that have spread out from the person with COVID-19 and accumulated in the air in a space.  Both types of exposures to SARS-CoV-2 aerosols can lead to infection and COVID-19.

The risk of infection from direct face-to-face interactions depends on the distance from the infected person, with the highest risk being within six feet. The risk of infection from exposure to aerosols that have accumulated in the air, however, depends on the size (volume) of the room and the levels of ventilation and air filtration as key factors amongst others.

In addition to the infection risk from face-to-face interactions in an indoor space, air quality models predict that spending 15 minutes anywhere in a 400,000 cubic ft indoor space or smaller with an infected person poses an infection risk from indirect exposure to aerosols that have accumulated in the air; the infection risk increases with duration of time spent in the space. 

For larger indoor non-healthcare spaces greater than 400,000 cubic feet, the infection risk from exposure to aerosols that have accumulated in the air is expected to be <10% even after 8 hours because of the large volume of air present. The infection risk in these large settings is thus mainly limited to direct, face-to-face exposure with the infected person. 

What is the difference between direct and indirect exposure?

Direct, short-range exposure occurs when someone inhales SARS-CoV-2 aerosols during face-to-face interactions with a person with COVID-19. The infected person generates and releases aerosols through breathing, speaking, coughing, and sneezing. The concentration of the aerosols containing SARS-CoV-2 is highest close to the infected person and decreases as the aerosols spread out through the air, especially in larger spaces where there is sufficient air volume to dilute the aerosols that may accumulate.

Indirect, long-range exposure occurs when someone inhales SARS-CoV-2 aerosols that have traveled away from a person with COVID-19 and accumulated in the air in an indoor space. The aerosols tend to mix evenly throughout a space because of dilution and air mixing. Smaller spaces will tend to have higher concentrations of accumulated aerosols than larger spaces, because there is less air to dilute the aerosols in a smaller space.

For both direct and indirect exposures, the risk of infection depends on the duration of exposure, whether the infected person has symptoms, and whether the infected and exposed persons were wearing a respirator or mask. The risk of infection from direct exposure also depends on the distance from the infected person, with the highest risk being within six feet. The risk of infection from indirect exposure also depends on the size (volume) of the room and the levels of ventilation and air filtration. Therefore, the risk of infection from indirect exposure is the about same for everyone in a smaller indoor space regardless of the distance from the infected person. This is because they are all exposed to about the same aerosol concentration after it mixes throughout the room.  In a large indoor space, SARS-CoV-2 aerosols get diluted, and the risk of indirect exposures is low.

How should healthcare facilities respond to a potential exposure when using this updated definition?

Healthcare facilities should continue to use the CDC's risk assessment framework to determine exposure risk for healthcare personnel (HCP) with potential occupational exposure to patients, residents, and visitors with COVID-19 in a health care setting. CDC provides additional considerations for assessing exposure risk for patients or residents exposed to HCP with COVID-19 in a health care setting. CDPH guidance for assessing community-related exposures should be applied to HCP with potential exposures outside of work (e.g., household), HCP exposed to each other while working in non-patient care areas (e.g., administrative offices), and for patients/residents exposed to other patients/residents or visitors in health care and non-patient care areas (e.g., waiting rooms, dining areas). Healthcare facilities should prioritize identifying and responding to such contacts based on their proximity to the case, duration or intensity of the exposure, and risk of severe illness or death from exposure, as described above.

Originally published on October 12, 2022