There are a range of reasons why mask wearing has not been recommended by the WHO for the general public. These are explained below.
False sense of protection - Wearing masks is not normal for most people. Putting one on can give people a false sense of protection against the virus and may make them less likely to practice other key preventative behaviours such as handwashing with soap and physical distancing. However in some settings mask-use has been found to be associated with other positive hygiene measures.
Potential self-contamination - Improper mask use (and re-use) could pose an infection risk if they become contaminated and proper hand hygiene is not followed. Surgical masks and N95 respirators are in short supply globally and so at the moment lots of people are buying and reusing them for extended periods of time or over the course of multiple days. Not changing masks when they become wet or soiled could allow bacteria to grow.
Prioritising masks for those who need them most - The demand for surgical masks and N95 respirators has increased dramatically with the spread of COVID-19 and the WHO and governments around the world are struggling to meet this supply. It is critical that public use of these items does not prevent them being available to those who need them most.
Mask use tends to be sporadic - Studies among people at risk of infection have shown that people do not wear masks consistently, limiting their benefit. However, recent studies in Hong Kong demonstrated that residents believed masks to be protective and that mask use in public spaces increased over the course of the outbreak.
Mixed evidence on effectiveness for public protection - The evidence we have about the benefits and limitations of masks are largely drawn from studies on influenza, studies in healthcare facilities, and studies of surgical masks or respirators, so it is hard to know how applicable they are to this pandemic. For more information, see ‘What do we know about the effectiveness of masks to prevent and COVID-19 transmission in community settings?’.
Improper disposal - Littering of masks or respirators could pose contamination risks for waste management workers or others who come into contact with them.
Difficulties related to wearing masks - People with hearing impairments may struggle to communicate if they rely on lip reading (see our section on inclusive fabric masks). Mask wearing may be difficult for other sub-groups of the population including children, people with cognitive or intellectual disabilities, people with respiratory problems (unrelated to COVID-19) or those living in hot and humid environments.
Mixed evidence about mode of transmission - Most COVID-19 infections are a result of being in close contact with an infectious person (for example, living with an infected person). At present, the WHO does not currently consider airborne transmission via aerosols to be a main route of transmission for SARS-CoV-2 among the general population but has recently acknowledged that it cannot be ruled out in crowded indoor settings with poor ventilation. While the possibility of airborne transmission is possible and may occur in some settings, the WHO maintains that COVID-19 is largely spread via large droplets emitted by infected individuals into the air. What they mean by this is that transmission occurs when an infected person coughs, sneezes or speaks and then releases large droplets into the air around them. These droplets are heavier than the air around them so they tend to fall on floors, surfaces or people in the surrounding area. For airborne transmission to occur, a virus must be suspended in smaller, lighter droplets (also termed ‘aerosols’) that can remain in the air for some time (e.g. even after the infected individual has left the room).
There is currently limited evidence as to whether airborne transmission of SARS-CoV-2 is occurring and to what extent, but it appears plausible and the idea is supported by a group of multidisciplinary scientists and engineers. Humans may expel aerosols during coughing and sneezing as well as normal speaking and breathing and some larger droplets expelled may evaporate to form smaller, lighter aerosols which can remain airborne for longer. Different sized particles may reach different areas of the lungs when inhaled, with smaller particles having greater potential to penetrate the lower respiratory tract. For such aerosols to cause COVID-19, expelled particles and aerosols must contain infectious SARS-CoV-2 particles. Experimental studies have demonstrated that infectious SARS-CoV-2 can remain in the airborne for at least 3 hours and potentially up to 16 hours in laboratory-generated aerosols. While useful to understand the potential for airborne transmission, the results of laboratory-based studies must be interpreted with caution because they do not reflect real-life circumstances and laboratory-generated aerosols may not be representative of the pattern of aerosols expelled during coughing, speaking, or breathing. Evidence of SARS-CoV-2 in air samples from healthcare and community settings is mixed and limited. Several studies have detected low concentrations of SARS-CoV-2 genetic material in air samples taken from hospital settings (Study 1, Study 2, Study 3, Study 4), while others have been unable to detect SARS-CoV-2 in air samples from patient hospital rooms (Study 1, Study 2, Study 3, Study 4). To date, only a single unpublished study has detected low levels of infectious virus in air samples collected from hospital rooms of COVID-19 patients. It is unclear if the level of virus detected is high enough to cause infection. There is an important distinction between the detection of genetic material, which can persist after a virus has been inactivated (‘killed’) and viable (‘live’) virus which may be capable of causing infection. To understand this further read this resource. Additional reviews summarizing the evidence of airborne transmission for SARS-CoV-2 are available here and here.