Aerosolized virus

Summary of recent changes Updates as of May 7, This science brief has been updated to reflect current knowledge about SARS-CoV-2 transmission and reformatted to be more concise.

Modes of SARS-CoV-2 transmission are now categorized as inhalation of virus, deposition of virus on exposed mucous membranes, and touching mucous membranes with soiled hands contaminated with virus.

Although how we understand transmission occurs has shifted, the ways to prevent infection with this virus have not. All prevention measures that CDC recommends remain effective for these forms of transmission.

On This Page. The largest droplets settle out of the air rapidly, within seconds to minutes. The smallest very fine droplets, and aerosol particles formed when these fine droplets rapidly dry, are small enough that they can remain suspended in the air for minutes to hours.

Infectious exposures to respiratory fluids carrying SARS-CoV-2 occur in three principal ways not mutually exclusive : Inhalation of air carrying very small fine droplets and aerosol particles that contain infectious virus. Risk of transmission is greatest within three to six feet of an infectious source where the concentration of these very fine droplets and particles is greatest.

Deposition of virus carried in exhaled droplets and particles onto exposed mucous membranes i. Risk of transmission is likewise greatest close to an infectious source where the concentration of these exhaled droplets and particles is greatest. Touching mucous membranes with hands soiled by exhaled respiratory fluids containing virus or from touching inanimate surfaces contaminated with virus. The route of virus exposure is crucial to understanding transmission.

Aerosols and droplets sprayed on the body and its mucous membranes, a kind of contact transmitter, while aerosols particles are inhaled by the respiratory system Dhand and Li, This distinction now follows disease severity, infectious dose, and control strategies.

At a close range, both inhalation and contact transmission pathways are possible, but in a longer range, when the droplets have settled rapidly transmission through the inhalation pathway is important. Also, the aerosols may be formed through the resuspension of static aerosols or settled dust Duguid, ; Prather et al. The virus is found in saliva or respiratory fluid, which varies in size from aerosols and droplets from 0. In general, these aerosols particles and drops are produced through breathing, speaking, and talking with aerosols, coughing, and rapid drops accompanied by coughing Dhand and Li, ; Heyder et al.

Individuals produce droplets and aerosols in a wide range of concentrations and sizes. The smallest airborne aerosols particles are probably to be caused by the bursting of bronchial fluid film or laryngeal vibrations related to singing and speech. Larger droplets and aerosol particles are related to specific articulation movements of speaking as well as sneezing and coughing Dhand and Li, ; Stadnytskyi et al. Aerosols exposure is far greater than droplets exposure, except for coughs, at intervals of less than 0.

A large proportion of the SARS-CoV-2 transmission involves asymptomatic peoples who do not cough or sneeze, suggesting that aerosol particle exposure is an important pathway of transmission Prather et al.

The production of aerosol is very widely for different activities and people. For example, speaking louder than whispering or softer talking produces more aerosol material Stadnytskyi et al.

Activities such as singing which involves deeper breathing emit more aerosol particles and droplets from the lower respiratory tract.

More research is needed on these changes in which respiratory aerosols are produced and the activities that most lead to aerosol production.

To explain the diversity in the production of infectious aerosols, it is necessary to better understand the mechanisms of production and composition of fluids that lead to different sizes and numbers.

Immediately after inhalation, due to the evaporation of water and liquids in them, droplets and aerosols begin to shrink. The virus in aerosols or droplets is subject to biological decay because of ultraviolet UV light exposure, desiccation and or other factors Schuit et al.

The droplets settle quickly, while the aerosols can be transported over long distances. These aerosols are transported by a stream of exhaled gas emitted by individuals and can then be carried quickly over large distances Bourouiba, ; Van Doremalen et al.

In the indoor space, by reducing the velocity of the exhalation cloud that carries them, background ventilation airflow scatters the particles remaining in the air. Eventually, these aerosols particles "deposit" or land on the surfaces.

If these land on sensitive cells such as in the eyes, bronchi, distal lungs, they may cause infection Bourouiba, That is not yet clear how many infectious droplets or aerosols are needed to cause an infection. All breathing, exhaling, talking, singing, coughing and sneezing, create persistence of droplets and airborne aerosol particles suspended in exhaled air multi-phase turbulent gas cloud. Exhaled clouds, with airborne particles and droplets of any size, can move to 8 m if sneezing Bourouiba, ; Yao et al.

All exhalations emit the high-momentum clouds with polluted droplets inside when breathing, talking, singing, coughing, and sneezing, but the arrival distance is different, and the size of the aerosol and droplets within them are different. Another pathway of transmission can be by regeneration dust or aerosol particles containing the virus from on the clothing or other surfaces and or on floors, as well as aerosolization of fomites.

Half of the airborne aerosols particles in a room can be due to resuscitation by walking on the floors. Contaminated tissue rubbing produces particles thousands, and has been shown guinea pigs transmit viruses through their fur Asadi et al. SARS-CoV-2 infected humans can produce the infectious aerosols particles that may transmit the disease after adequate exposure. Although positive RNA samples indicated the infectious virus possibility, except in sub-micron-sized particles, the results of cell culture were inconclusive Santarpia et al.

Recently, an important study was able to measure the infectious virus using the "gentler" collection method Lednicky et al. Generally, these studies suggest that COVID patients may produce aerosols with reproducible capable intact virions in the cell culture.

The half-life of viruses in the aerosol is 1. The Goldberg drum is used to hold airborne aerosols and particles and measure the stability of the infectious virus. In other studies, infectious aerosols particles after 16 h found in the Goldberg drum that demonstrating virus stability Tang et al.

UV light greatly reduces the stability of the virus, and humidity and lower temperatures may increase the stability Schuit et al. According to studies, the half-life of the virus decreases with increasing humidity. In another study, the virus half-life was reduced to 55 min, due to high humidity Matson et al.

Studies show that in warmer temperatures the virus infectivity disappears faster. The sunlight effect on the coronavirus was investigated using simulation of UV light with sunlight in spring, summer and autumn. Using simulated saliva in a laboratory experiment, the virus was found to be unstable at the high relative humidity in the tissue culture, while another study did the opposite Smither et al. The size of the aerosol and droplets containing the influenza virus on exhalation can differ depending on the strain of the virus, and the size can affect the infection.

Smaller aerosols particles may play a significant role in some influenza virus strains than other strains, which indicating strain variations in the predominant mode of transmission. Aerosols are an important transport route for SARS-CoV-2, as aerosols particles can contain the infectious SARS-CoV-2, and remain suspended in the air for hours, which may be up to several meters transported from the source. Proving the aerosol particles transmission path is challenging and requires further research.

Particularly, methods are needed that can sample large volumes and different sizes of air without harming the virus. For further understanding, a unified method is needed to quantify respiratory flow propagation across studies. Also, there is a need for risk assessment and a better understanding of transmission pathways to develop multi-scale models different scales at models coupling that take into account the physical processes related to transmission dynamics.

To better understand the conditions that can increase the transmission risk, more study is needed to determine the range of human infectious dose, the difference in contamination rate with aerosol size, and the impact of the external factors on the disease severity. It is important to know how the infectious dose and inoculation pathway affect transmission.

Awareness of the size distribution of viral aerosols is important in the exhaled breath, and knowledge of the mediated aerosols size range is also important for transmission.

The limitations of the animal models must be considered when predicting human effects. Additionally, the impact of the repeated exposure, as well as the impact of the virus deposition site, must be determined. For disease severity, further research is needed on a combination of the underlying health issues such as sex, age, genetic and other factors.

There is limited data about sex and age and is needed more human information to determine these factors' effect on the risk of transmission and the rate of infection. More aggressive mitigation strategies should be used in high-risk populations; treatment research should be analyzed in terms of age and gender, and other external items such as air pollutants on susceptible individuals should be considered.

It would also be helpful to do more research in these cases:. Research on the characteristics of human-made SARS-CoV-2 infectious aerosols and variability in the infectious aerosols particle production rate from person to person. The infectious aerosols production during illness; and the relationship between aerosol particle dose and response via the aerosol pathway. Also, there is a need to design aerosol collectors that optimize the preservation of intact viruses.

This is true for some other viruses , including those found in infants. Lab workers at the University of Nebraska have published their finding that they, too, have identified coronavirus RNA in the air. Bahnfleth noted several examples, including a restaurant in Guangzhou, China, where multiple people without direct contact with one another became infected from a single individual, and a choir practice in Washington state where presumed droplet and aerosol spread from singing sickened 53 people, two of whom died.

In an e-mail interview, researcher Bjorn Birnir shared his work, published in a preprint a non—peer reviewed paper , that demonstrated how an infected person continues to exhale a cloud of droplets and aerosols. Much of the solution to the challenge of aerosol and droplet transmission in indoor areas is ventilation. The goal of ventilation, instead, is to exhaust air from inside a building—along with whatever contaminants it contains—and replace it with clean air from the outside.

One great way to do this, according to Yao and the CDC, is to increase outdoor ventilation by opening windows and doors. Experts say this can make a huge difference in air flow rates in buildings.

In some instances, Bahnfleth says, a fan can be placed near one window while other windows are left open in order to create forced ventilation. Nothing is perfect, of course. Ventilation can be expensive, depending upon what is needed, and some of the recommendations may use significant amounts of energy.

High efficiency air filtration and disinfection are important. Filters should be upgraded to the extent possible in HVAC systems without diminishing airflow. And if HVAC units cannot use higher-grade filters, consider using portable air cleaners with HEPA high-efficiency particulate air filters to disinfect the air further.

A word about ultraviolet light. UV fixtures can be mounted on the ceiling or walls, or placed inside ventilation ducts to neutralize viruses and bacteria. The biggest limitation is that the irradiation can be a health hazard, to both skin and eye , which is why the fixtures are placed up high, away from people. On this front, there is promise. A newer ultraviolet technology that employs a lower wavelength of light, called far-UVC light, appears to work without the potential health issues.