Australian researchers have found cloth face masks only provide protection against viral infections when washed every day and at a high temperature.
The analysis by scientists at Sydney's University of New South Wales was published in medical journal BMJ Open on Friday.
They analysed data from a controlled trial of health workers in Vietnam in 2011 which looked into the efficacy of cloth masks in preventing viral infections including influenza, rhinoviruses and seasonal coronaviruses.
The report found that cloth masks must be washed daily to reduce the likelihood of contamination and transmission of viruses like SARS-CoV-2.
"Both cloth masks and surgical masks should be considered 'contaminated' after use," said professor Raina MacIntyre, who conducted the study.
"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."
But they also found that handwashing doesn't provide adequate protection. Instead, cloth masks need to be washed at high temperatures.
The healthcare workers who self-washed their masks by hand had double the risk of infection compared to those who used the hospital laundry.
"The WHO recommends machine washing masks with hot water at 60C and laundry detergent, and the results of our analysis support this recommendation," Prof MacIntyre said.
The researchers said if a cloth mask was washed in the hospital laundry - "they were as effective as a surgical mask".
But while the research for the study was completed over five years ago, the researchers emphasised it is still applicable to COVID-19.
"COVID-19 is a highly infectious virus, and there is still a lot that we don't know about it, and so it's important that we take every precaution we can to protect against it and ensure masks are effective," Prof MacIntyre said.