Study measures effectiveness of different face mask materials when coughing
It’s a matter of finding the right balance – we want the materials to be effective at filtering particles, but we also need to know they don’t put users at risk of inhaling fibres or lint, which can be harmful
Eugenia O'Kelly,
University of Cambridge
Previous studies have
only looked at a small selection of fabrics when the wearer is breathing
normally, when particles are expelled at lower speed. Studying more fabrics and
testing them at higher speeds provides a more robust evidence base for the
effectiveness of fabric masks.
The results, reported in the journal BMJ
Open, show that most of the fabrics commonly used for non-clinical face
masks are effective at filtering ultrafine particles. N95 masks were highly
effective, although a reusable HEPA vacuum bag actually exceeded the N95
performance in some respects.
As for homemade masks,
those made of multiple layers of fabric were more effective, and those which
also incorporated interfacing, which is normally used to stiffen collars,
showed a significant improvement in performance. However, this improvement in
performance also made them more difficult to breathe through than an N95 mask.
The researchers also
studied the performance of different fabrics when damp, and after they had gone
through a normal washing and drying cycle. They found that the fabrics worked
well while damp and worked sufficiently after one laundry cycle, however
previous studies have shown that repeated washing degrades the fabrics, and the
researchers caution that masks should not be reused indefinitely.
“Fabric masks have
become a new necessity for many of us since the start of the COVID-19
pandemic,” said first author Eugenia O’Kelly from Cambridge’s Department of
Engineering. “In the early stages of the pandemic, when N95 masks were in
extremely short supply, many sewers and makers started making their own fabric
masks, meeting the demands that couldn’t be met by supply chains, or to provide
a more affordable option.”
While there are
numerous online resources which help people make their own masks, there is
little scientific evidence on what the most suitable materials are.
“There was an initial
panic around PPE and other types of face masks, and how effective they were,”
said O’Kelly. “As an engineer, I wanted to learn more about them, how well
different materials worked under different conditions, and what made for the
most effective fit.”
For the current study,
O’Kelly and her colleagues built an apparatus consisting of sections of tubing,
with a fabric sample in the middle. Aerosolised particles were generated at one
end of the apparatus, and their levels were measured before and after they
passed through the fabric sample at a speed similar to coughing.
The researchers also tested how well each fabric performed in terms of breathing resistance, based on qualitative feedback from users.
“A mask which blocks particles really well
but restricts your breathing isn’t an effective mask,” said O’Kelly. “Denim,
for example, was quite effective at blocking particles, but it’s difficult to
breathe through, so it’s probably not a good idea to make a mask out of an old
pair of jeans. N95 masks are much easier to breathe through than any fabric
combinations with similar levels of filtration.”
In preparation for the
study, the researchers consulted with online sewing communities to find out
what types of fabric they were using to make masks. Due to the severe shortage
of N95 masks at the time, several of the sewers reported that they were experimenting
with inserting vacuum bags with HEPA filters into masks.
The researchers found
that single-use and reusable vacuum bags were effective at blocking particles,
but caution that the single-use bags should not be used in face masks, as they
fall apart when cut, and may contain component materials which are unsafe to
inhale.
“It’s a matter of
finding the right balance – we want the materials to be effective at filtering
particles, but we also need to know they don’t put users at risk of inhaling
fibres or lint, which can be harmful,” said O’Kelly.
The researchers
caution that their study has several limitations: namely, that they did not
look at the role which fit plays in filtering particles. In a related project,
O’Kelly has been studying how the fit of masks in healthcare settings can be
improved. In addition, many viruses are carried on droplets which are larger
than those looked at in the current study.
However, O’Kelly says
the results may be useful for sewers and makers when choosing which fabric to
use for making masks. “We’ve shown that in an emergency situation where N95
masks are not available, such as in the early days of this pandemic, fabric
masks are surprisingly effective at filtering particles which may contain
viruses, even at high speeds.”
Further information
about the research can be found at: www.facemaskresearch.com
Reference:
Eugenia O’Kelly et al. ‘Ability of
fabric face mask materials to filter ultrafine particles at coughing velocity.’
BMJ Open (2020). DOI: 10.1136/bmjopen-2020-039424