SARS-CoV-2 and Health Care Worker Protection in Low-Risk Settings: a Review of Modes of Transmission and a Novel Airborne Model Involving Inhalable Particles

Abstract

Since the beginning of the COVID-19 pandemic, there has been intense debate over SARS-CoV-2's mode of transmission and appropriate personal protective equipment for health care workers in low-risk settings. The objective of this review is to identify and appraise the available evidence (clinical trials and laboratory studies on masks and respirators, epidemiological studies, and air sampling studies), clarify key concepts and necessary conditions for airborne transmission, and shed light on knowledge gaps in the field. We find that, except for aerosol-generating procedures, the overall data in support of airborne transmission-taken in its traditional definition (long-distance and respirable aerosols)-are weak, based predominantly on indirect and experimental rather than clinical or epidemiological evidence. Consequently, we propose a revised and broader definition of "airborne," going beyond the current droplet and aerosol dichotomy and involving short-range inhalable particles, supported by data targeting the nose as the main viral receptor site. This new model better explains clinical observations, especially in the context of close and prolonged contacts between health care workers and patients, and reconciles seemingly contradictory data in the SARS-CoV-2 literature. The model also carries important implications for personal protective equipment and environmental controls, such as ventilation, in health care settings. However, further studies, especially clinical trials, are needed to complete the picture.

Keywords: COVID-19; SARS-CoV-2; bioaerosols; infection prevention; inhalable aerosols; low-risk settings; personal protective equipment; respiratory protection; ventilation.

FIG 1

A broader airborne model involving inhalable aerosols for SARS-CoV-2 transmission in low-risk health care settings. (A) Worst-case scenario: no protection on either the sick patient (source) or the health care worker (exposure), emission of particles of various sizes (droplets and aerosols) during natural respiratory activity (breathing, talking, and coughing), entry of infectious inhalable aerosols, and impaction in the nose where viral receptors are abundant and infectivity is greatest. (B) Best-case scenario and NIOSH hierarchy of controls: source control (mask-wearing by the sick patient), engineering control (optimal ventilation), and exposure control (droplet-contact PPE worn by the health care worker) to prevent short-range droplet and inhalable aerosol transmission.