Cloth masks may reduce the transmission of viruses such as SARS-CoV-2, which causes COVID-19, only if they are washed daily at high temperatures, according to a study.
"Both cloth masks and surgical masks should be considered 'contaminated' after use," said Professor Raina MacIntyre, from the University of New South Wales in Australia.
"Unlike surgical masks, which are disposed of after use, cloth masks are re-used. While it can be tempting to use the same mask for multiple days in a row, or to give it a quick hand-wash or wipe-over, our research suggests that this increases the risk of contamination," Maclintyre said.
The researchers analysed data from a randomised controlled trial (RCT) they published in 2015.
The latest study, published in the journal BMJ Open, is still the only RCT ever conducted on the efficacy of cloth masks in preventing viral infections, they said.
The researchers did a deep dive into the 2011 data on whether the health workers in their study washed their masks daily, and if so, how they washed their masks.
The team found that if cloth masks were washed in the hospital laundry, they were as effective as a surgical mask.
Given the study was conducted over five years ago, the researchers did not test for SARS-CoV-2.
Instead, they included common respiratory pathogens such as influenza, rhinoviruses and seasonal coronaviruses in their analysis.
It is based on self-reported washing data and was conducted by health workers in high risk wards in a healthcare setting.
"While someone from the general public wearing a cloth mask is unlikely to come into contact with the same amount of pathogens as healthcare worker in a high risk ward, we would still recommended daily washing of cloth masks in the community, Maclntyre said.
According to the analysis, handwashing the masks did not provide adequate protection.
Healthcare workers who self-washed their masks by hand had double the risk of infection compared to those who used the hospital laundry, the researchers said.
The majority of people in the RCT handwashed their masks, and this may be why the cloth masks performed poorly in the original trial, they said.
"The WHO recommends machine washing masks with hot water at 60 degrees Celsius and laundry detergent, and the results of our analysis support this recommendation," said Maclntyre.
"The clear message from this research is that cloth masks do work -- but once a cloth mask has been worn, it needs to be washed properly each time before being worn again, otherwise it stops being effective," said Maclntyre.
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