Health workers facing critical shortages of masks in the US and other countries are searching for evidence on their use. Agencies such as the US Centers for Disease Control (CDC) recommend use of home made cloth masks as a last resort.  The only randomised controlled clinical trial (RCT) of cloth masks currently available was published by our research group in 2015.  At the time, guidelines on personal protective equipment did not mention cloth masks, despite health care workers in Asia commonly using them, which was the rationale for conducting our study. The RCT found that cloth mask users are at higher risk of infection than medical mask users, and even the control arm. The rate of infection was highest in the cloth mask arm compared with the medical mask arm, ranging from 1.67 times higher for laboratory confirmed viral illness to 13 times higher for influenza-like illness. Penetration of the cloth mask by sodium chloride droplets was 97% compared to 44% for medical masks. Cloth mask wearers had higher rates of infection than even the control arm. The mask used in the study was one used commonly in hospitals in Vietnam, the study site, was manufactured locally in Vietnam, made of cotton or cotton polyester (50/50) mix and had two layers. The medical mask was a disposable, single-use product made in Vietnam.
Some subjects in the control arm wore medical masks, which may have reduced the apparent performance of cloth masks compared to the control group. However, in a post-hoc analysis presented in the 2015 RCT of all cloth and medical mask wearers (including those in the control arm), the higher rate of infection in cloth mask users persisted. Other reasons cloth masks may have performed badly include long duration of use and frequency and type of washing or cleaning. Cloth may become damp and contaminated, posing an infection and self-contamination risk, if not washed daily before re-use. A medical mask, in contrast, has a fluid resistant outer layer designed to prevent a stream of liquid entering the mouth. Cloth fabrics may also vary in quality. Ideal properties of a cloth mask include hydrophobic material, at least three layers, good fit around the face and good particle filtration.
Cloth masks made of cotton, gauze, silk or woven, have been used since the early 20th century to protect HCWs from various respiratory infections. [3-5] Cloth masks have one or multiple layers.  Most have strings or ties, however cloth masks used in community settings generally have ear loops. Cloth masks do not have a flexible nose piece to fit over the nose bridge. Homemade masks made from cotton T-shirt material may also provide a good fit and a measurable level of protection from a challenge aerosol. 
The filtration capacity of cloth masks is low compared to medial masks and respirators and depends on closeness of the threads, number of layers and type of gauze/cloth. [8,3] A study showed that sweatshirt fabric may be more protective than others due to less penetration.  Generally, the filtration capacity improves when the number of threads increases and the mesh becomes finer compared to coarse material with lower thread counts. Two or three layer masks provide better protection compared to a single layer mask. One study compared the filtration of a range of home-made materials with medical masks and showed that a medical mask filtered 0.02 micron particles better than any home-made product.  A vacuum cleaner bag was the best option among home-made products and a scarf was the worst; cotton blend was better than pure cotton.
The filtration of medical masks, however is not regulated. Only respirators are regulated on their ability to filter airborne particles, and their filtration efficacy is far higher than a medical mask.  There is evidence that SARS-COV-2 can persist in aerosols for 3 hours, and that seasonal coronaviruses can be aerosolised even by normal tidal breathing. [12,13]
Health workers facing shortages of respirators and medical masks during the pandemic of covid-19 are searching for evidence about the use of cloth masks, and are asking if they should wear no mask at all rather than a cloth mask.
We recommend as a matter of work health and safety that HCWs should not be caring for covid-19 patients without proper respiratory protection. Cloth masks are not a suitable alternative for HCWs. The physical barrier provided by a cloth mask may afford some protection, but likely much less than a medical mask or a respirator. Some HCWs may choose to work in inadequate personal protective equipment (PPE) such as cloth masks, but sterilising and re-using a respirator may be a better option. However, recent research showed that viable SARS-CoV-2 persists for seven days on the outer layer of a medical mask.  Another study showed poor performance of both medical and cloth masks worn by SARS_Co-V2 patients in filtering the virus.  High rates of illness or even death in HCWs, in addition to the personal toll, could substantially affect health workforce capacity. For these reasons HCWs should be given optimal respiratory protection. Other PPE such as gloves and eye shields are also important given the likely multimodal transmission of SARS-CoV-2.
As the covid-19 pandemic grows globally, universal face mask use has also become a topic of discussion. The US Centers for Disease Control (CDC) have recommended cloth mask use by community members.  However, the WHO have discouraged community use of masks, instead encouraging other measures such as social distancing and hand hygiene.  WHO suggest that mask use in the community has no benefit, and should only be used by sick patients (also referred to as “source control” ). In general, the results of community RCTs show protection for well community members in settings of intense transmission such as households and university campus settings. [19-22] In trials that studied mixed interventions such as hand hygiene, health education and masks, masks were more effective than hand hygiene alone, hand hygiene alone was not protective and both together are protective. [19-21]
It would be timely to collect data on universal face mask use adjusting for other disease control measures, and study whether this strategy can ease social isolation and allow earlier return to employment. It is essential that community members be provided with guidelines on the best fabrics and designs for home-made masks, as the filtration of different fabrics varies widely.  There is enough evidence that in settings where covid-19 is poorly controlled and there are few other options, that universal face mask use in households and crowded public areas might make a difference to individual protection and population disease control.
Raina MacIntyre is professor of Global Biosecurity at the Kirby Institute, UNSW Australia and leads a research programme in epidemic control including personal protective equipment.
Abrar Chughtai is a lecturer in the School of Public Health and Community Medicine UNSW and doing research on infection control in hospital settings.
Chi Dung Tham is a medical doctor, PhD in Epidemiology, previously worked in the National Institute of Hygiene and Epdemiology, coordinated epidemiological researches, surveillance, prevention and control programs for infectious diseases (EPI, SARS-CoV, A/H5N1, A/H1N1, dengue, HPV) and currently in the Ministry of Health, Vietnam
Holly Seale is a senior lecturer at the School of Public Health and Community Medicine and has been involved with the program of research focused on improving engagement with personal protective equipment and non-pharmaceutical interventions.
Competing interests: The authors have no competing interests. The cloth mask RCT was conducted in 2011 and funded by an Australian Research Council Industry Linkage Grant, in which 3M was the industry partner. We have no ongoing or current relationship with 3M.
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