The New York Times - USA (2020-11-15)

6 N THE NEW YORK TIMES, SUNDAY, NOVEMBER 15, 2020

Tracking an OutbreakThe Science

The public health debate on masks is settled, said Joseph G. Allen, the director of the Healthy

Buildings program at Harvard. When you wear a mask, “you protect yourself, you protect others, you

prevent yourself from touching your face,” he said. And you signal that wearing a mask is the right

thing to do. With coronavirus cases still rising, wearing a mask is more important than ever.

This article is by Or Fleisher, Gabriel Gianordoli, Yuliya Parshina-Kottas, Karthik Patanjali, Miles Peytonand Bedel Saget.

No Matter the Mask, Here’s How It Protects

Cotton Mask N95 Respirator

The fibers present a dense

forest that the particles must

navigate as they move with the

air stream. The laws of physics

influence how the particles

interact with the fibers and how

well the mask can capture them.

Largerparticles are easier to

trap.They slam straight into the

fibers and get stuck when the

air stream brings them within

touching distance, or when their

momentum causes them to

veer off course.

LARGE

> 0. 5 micron

COTTON FIBERS

SMALL

< 0.1 micron

MEDIUM

< 0 .1 to 0. 5 microns

The smallest particles are

bounced around by air

molecules in a random zig-zag

pattern, increasing the time

they spend in the fiber forest

and their chances of getting

captured.

N95 FIBERS

Here are the fibers of an N 95
respirator. They are made of
synthetic material, vary in size
and are arranged randomly.
And they have an extra feature:
an electrostatic charge that
attracts and captures particles
of all sizes.

The material can filter at least
95 percent of the elusive
medium particles and even more
of the large and small ones.

A well-fitting N95 is the gold
standard, but don’t worry if you
can’t get your hands on one.

“It’s become clear that cloth
masks, even though they’re not
as effective as the N95s, are still
effective at reducing transmis-
sion,” said Linsey Marr, an
aerosol expert at Virginia Tech.
“Even if you’re not achieving that
95 percent reduction, something
is better than nothing.”

Medium-size particles are the

hardest to filter. They evade

capture because they follow the

air flow, twisting and turning

around the fibers.

Masks come in many styles and materials, but they
generally work in the same way. Layers of fibers
capture large respiratory droplets and smaller
airborne particles known as aerosols that can carry
the coronavirus. Thisprocess is known as filtration,
and a material’s ability to trap particles is called
filtration efficiency.

Tightly woven cotton outperforms most common
fabrics. A nonwoven material like that of an N 95
respirator is most effective. Let’s take a closer look at
how filtration works at the microscopic level.

Trapped by a Fiber Forest

AIR

FL

O

W

A good mask will have a large surface area, a
tight fit around the edges, and a shape that
leaves space around your nostrils and mouth.

When everyone wears a

mask, the combined

filtration efficiency

increases. Let’s say your

mask filtershalf of the

particles you exhale.

The particles that

escape disperse through

theair. Good ventilation

and social distancing

further reducethe risk of

transmission.

The particles that

eventually reach

someone else’s mask

get filtered again,

reducing the number

that get through.

A larger breathing zone

helps catch theparticles

and increases the chance

they will encounter a fiber.

Smaller
breathing
Loose-fitting zone
masks or those
pressed against
facial hair allow
aerosols to leak.

Better Mask Design
An augmented-reality experience
lets you stand inside the magnified
fibers of an N95 respirator to see
how it filters aerosols. Launch
aerosols of different sizes and see
how they are captured by the fibers
of the respirator.
To experience this in your space,
you will need the Instagram app. To
view on Instagram, open the cam-
era on your device and point to the
QR tag below.

Sources:Audrey Dang and Brent Williams, Department of Energy, Environmental & Chemical Engineering and the Center for Aerosol Science and Engineering, Washington University in St. Louis; Linsey Marr, Department of Civil and Environmen-
tal Engineering, Virginia Tech; Joseph G. Allen, T.H. Chan School of Public Health, Harvard University; Chris Zangmeister, Material Measurement Laboratory, National Institute of Standards and Technology; Edward Vicenzi, Smithsonian Institution’s
Museum Conservation Institute;Christian J. Kähler, Institute of Fluid Mechanics and Aerodynamics, Bundeswehr University Munich; Jesse V. Jokerst, Department of Nanoengineering, University of California, San Diego; Frank Drewnick, Particle
Chemistry Department, Max Planck Institute for Chemistry; Richard Stutt, Epidemiology and Modelling Group, University of Cambridge