Not all cloth masks are equal, but better than no protection from coronavirus
In the coronavirus pandemic, medical masks should only be used by high-risk health workers. Authorities have advised the public to rather use cloth masks. But how well do these masks protect us, and our communities?
Scientists’ understanding of the new coronavirus increases with each passing week. We now know, from small scale reports from around the globe, that people without symptoms or those still in the incubation phase can spread the virus.
Several health authorities have recently updated their advice on using masks. Dr Zweli Mkhize, South Africa’s health minister, has recommended that people wear cloth face masks when they leave their homes. The US Centers for Disease Control also recommends masks. The World Health Organization is yet to follow this trend.
Our previous analysis concluded that the use of N95 respirators and surgical face masks could slow the spread of coronavirus. But there’s a worldwide shortage of this equipment, so it’s best that it’s only used by people at high risk of infection, such as medical workers. Other people should continue to wash their hands often and practise social distancing.
The alternative is cloth masks. But what protection do they offer? And how do they compare to medical masks? We looked at the available research.
Can normal fabric filter a virus?
The first question is, can materials we have at home actually filter any virus?
In 2013 a team from Public Health England constructed a closed tube and blew air full of particles of a virus less than half the size of the new coronavirus through different types of filter material. They then measured how much virus got through.
They found that surgical mask material – used as the control – filtered nearly 90% of the virus. Vacuum cleaner bag material also did well, filtering 86% of the virus, but was difficult to work with and not particularly useful for making a mask.
The other standouts were tea towels (72%) and cotton blends (70%). Even a single layer of a basic cotton T-shirt could filter almost 51% of the virus particles.
A 2010 study by the US National Institute for Occupational Safety and Health had similar findings, although it wasn’t as specific about the materials used.
Can household fabrics act as a basic viral filter? The answer, technically, is yes.
Lab conditions don’t perfectly mimic the real world
But laboratory studies may not always apply in the real world. The experiments differed from the real world in two ways.
First, they looked at aerosolised viruses. These are virus particles directly blown down a tube, much like very fine dust – not a virus spread in little droplets of mucus.
They also blew all the virus particles directly through the filter, without allowing for leaks. Recreating this in the real world, at home, with a self-made mask is bound to be more challenging.
How much does a cloth mask protect you?
Moving on from the materials, let’s look at how well cloth masks themselves have performed in the ideal conditions of laboratory tests.
A team from the National Institute for Public Health and the Environment in the Netherlands compared the amount of particles in the air and behind a mask being worn by a person. This ratio is known as the “protection factor”. The higher the protection factor, the more effective the mask.
N95 respirators are the go-to respiratory protection in medical procedures with a high risk of viral contamination. They can be expected to have a protection factor of 66 to 113, giving the wearer a high level of protection from outside particles.
In the study, a leaky surgical mask had a protection factor of 4 to 5, and a common tea cloth a protection factor of 2 to 3.
More advanced, multilayered cloth masks have achieved protection factors nearing that of the N95 respirator. A team from the University of Pittsburgh in the US describe a mask made from eight layers of cotton T-shirt achieving protection factors of 13 to 67, depending on how securely it fit the face of the wearer.
So not all cloth masks are equal. Their filtration rate depends on a number of factors. These include:
- The fabric’s thread count: denser weaves provide greater filtration
- The number of layers: more layers provide more filtration
- Fabric: some fabrics provide greater filtration, but not all fabrics are easy to breathe through
- Fit: the mask must fit snugly, but also be comfortable enough to wear for long periods
- No randomised clinical trials on reusable cloth face masks
- What happens when we analyse the effects of cloth masks in the real world?
Frustratingly, this is where the hunt for good data begins to come apart at the seams. There have been no randomised clinical trials on the use of reusable cloth face masks.
A 2004 study in China examined behaviour that put people at risk of catching the severe acute respiratory syndrome caused by Sars-1, a different strain of coronavirus. The virus has similar characteristics to the new coronavirus.
In the study, the risk of infection in a group of people who always wore masks outside was reduced by 70%. But the study did not specify the type of mask used, although cloth masks have long been more popular in Asia than elsewhere.
A study from Hong Kong analysing 1,192 cases of infection had similar findings.
Cloth masks ‘better than no protection’
The available research suggests that cloth masks – store-bought or home-made – have a role to play in slowing the spread of the new coronavirus. At worst, they seem to be slightly less effective than surgical masks. At best, they could dramatically outperform surgical masks.
The Public Health England team concluded that a “homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection”.
Even a basic mask with gaps, such as one made from a tea towel, will reduce the amount of virus an infected person sheds. By catching some of the droplets expelled when coughing, sneezing or talking, it will help protect the community.
But more complex designs are needed for a cloth mask to give its wearer any real level of protection. This would likely require multiple layers of fabric and a secure fit that doesn’t allow air currents to sneak around the mask.
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