COVID-19: Respiratory droplet or aerosol transmission?
Respiratory droplets are large particles (> 5 µm in diameter) that tend to be pulled to the ground or furniture surfaces by gravity. Respiratory droplets do not circulate in the air for an extended period of time. Droplet nuclei are small particles less than 5 µm in diameter. These small particles may become aerosolized and float through the air for an extended period of time. The CDC and WHO believe that under ordinary conditions, SARS-CoV-2 is transmitted through respiratory droplets and contact routes.2 In an analysis of more than 75,000 COVID-19 cases in China there were no reports of transmission by aerosolized airborne virus. Therefore, under ordinary conditions, surgical masks, face shields, gowns, and gloves provide a high level of protection from infection.8
In contrast to the WHO’s perspective, Dr. Harvey Fineberg, Chair of the National Academies of Sciences, Engineering, and Medicine’s Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats, wrote a letter to the federal Office of Science and Technology Policy warning that normal breathing might generate aerosolization of the SARS-CoV-2 virus and result in airborne transmission.9 A report from the University of Nebraska Medical Center supports the concept of airborne transmission of SARS-CoV-2. In a study of 13 patients with COVID-19, room surfaces, toilet facilities, and air had evidence of viral contamination.10 The investigators concluded that disease spreads through respiratory droplets, person-to-person touch, contaminated surfaces, and airborne routes. Other investigators also have reported that aersolization of SARS-CoV-2 may occur.11 Professional societies recommend that all medical staff caring for potential or confirmed COVID-19 patients should use personal protective equipment (PPE), including respirators (N95 respirators) when available. Importantly, all medical staff should be trained in and adhere to proper donning and doffing of PPE. The controversy about the modes of transmission of SARS-CoV-2 will continue, but as clinicians we need to work within the constraints of the equipment we have.
Certain medical procedures and devices are known to generate aerosolization of respiratory secretions. These procedures and devices include: bronchoscopy, intubation, extubation, cardiopulmonary resuscitation, nebulization, high-flow oxygen masks, and continuous- and bilevel-positive airway pressure devices. When aerosols are generated during the care of a patient with COVID-19, surgical masks are not sufficient protection against infection. When an aerosol is generated maximal protection of health care workers from viral transmission requires use of a negative-pressure room and an N95 respirator or powered air-purifying respirator (PAPR) device. However, negative-pressure rooms, N95 masks, and PAPRs are in very short supply or are unavailable in some health systems. We are lucky at our hospital that all of the labor rooms can be configured to operate in a negative-pressure mode, limiting potential airborne spread of the virus on the unit. Many hospitals restrict the use of N95 masks to anesthesiologists, leaving nurses, ObGyns, and surgical technicians without the best protective equipment, risking their health. As one action to reduce aerosolization of virus, obstetricians can markedly reduce the use of oxygen masks and nasal cannulas by laboring women.
Universal use of surgical masks and mouth-nose coverings
During the entire COVID-19 pandemic, PPE has been in short supply, including severe shortages of N95 masks, PAPRs, and in some health systems, surgical masks, gowns, eye protection, and face shields. Given the severe shortages, some clinicians have needed to conserve PPE, using the same PPE across multiple patient encounters and across multiple work shifts.
Given that the virus is transmitted by respiratory droplets and contaminated surfaces, use of face coverings, including surgical masks, face shields, and gloves is critically important. Scrupulous hand hygiene is a simple approach to reducing infection risk. In my health system, all employees are required to wear a surgical mask, all day every day, requiring distribution of 35,000 masks daily.12 We also require every patient and visitor to our health care facilities to use a face mask. The purpose of the procedure or surgical mask is to prevent presymptomatic spread of COVID-19 from an asymptomatic health care worker to an uninfected patient or a colleague by reducing the transmission of respiratory droplets. Another benefit is to protect the uninfected health care worker from patients and colleagues who are infected and not yet diagnosed with COVID-19. The CDC now recommends that all people wear a mouth and nose covering when they are outside of their residence. America may become a nation where wearing masks in public becomes a routine practice. Since SARS-CoV-2 is transmitted by respiratory droplets, social distancing is an important preventive measure.
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