- [Reporter] Take a look at this map.

It shows the change in coronavirus cases

over the past two months.

You can see cases arising.

And as the weather gets colder and people had back indoors,

experts say it could get worse.

- As we get through the fall and into the winter

with the holiday season going,

we've got to do something different.

- [Reporter] Experts warn that many buildings,

like restaurants, schools, and homes,

are not equipped with the ventilation and filtration systems

needed to reduce the spread of COVID-19.

- So, by now, people are really familiar

with masking, hand-washing, distancing,

but fewer people are aware that ventilation

and filtration really matter.

- [Reporter] It's now clear that the coronavirus

travels through the air,

which is why indoor gatherings can be so deadly.

The virus can accumulate in aerosols,

tiny droplets that gather in the air,

as we breathe and talk.

- That virus is never naked in the air, is what we say.

It's always floating in respiratory droplets

that are much bigger.

- [Reporter] Indoors, those particles can add up,

increasing the risk of transmission.

And one way to mitigate that

is to pump in fresh air with a ventilation system.

- We have to think about the thermal conditions

like humidity and temperature,

but also just how much outdoor air is moving in.

For me, the priority is to increase the air exchange rate.

- [Reporter] The air exchange rate

determines how many times an hour

fresh air circulates through a building.

There's not an exact recommended rate,

but experts say that three to six exchanges per hour

is ideal.

But good flow of fresh air is only one part

of preventing the indoor spread of the coronavirus.

The other is filters,

and experts recommend using one called HEPA.

This type of pleated mechanical filter

is made from fiberglass, foam or cotton.

It can remove more than 99% of airborne particles,

including those carrying viruses like the coronavirus.

This is the same type of filter

used in hospitals and airplanes,

but there has been some confusion

over whether or not HEPA filters

can effectively capture airborne coronavirus.

That's because these filters are rated for particles

that are 0.3 microns in size, larger than the coronavirus.

People assume that they don't catch anything smaller,

but that's just not true.

- Filters are rated for the particle size

they perform worst at, and that's 0.3 microns.

As you get to bigger particles, and even smaller particles,

the HEPA filter will capture closer to 100%.

- [Reporter] Because the coronavirus tends to travel

in droplets that are larger than 0.3 microns,

HEPA filters can capture almost all of them.

The virus also travels in droplets that are smaller,

and HEPA filters can capture those too.

- Filters work not by straining out things

that are larger than the holes in them,

but they actually work by trapping particles in the air

as the air flows around the different fibers in the filter.

- [Reporter] When very small aerosols

bump into gas particles, they move in a random pattern.

That is called Brownian motion.

- Kind of like a drunk person stumbling around in the dark.

And because of that random motion,

they can crash into the fibers.

- [Reporter] The harder it is for particles

carrying viruses to pass through a filter,

the lower the chance of viral transmission.

But the problem is that not every building

has a system that can handle them.

- When you have a air system moving air

through the building, and then it runs through a filter,

you can imagine that a higher-efficiency filter

that's more tightly woven

is going to be harder to push air through.

Very few systems can push air through a HEPA filter

unless it's designed that way,

because HEPA is the most efficient filter we have.

- [Reporter] That is why experts recommend

that schools, businesses and homes

use a different type of lower-efficiency filter called MERV.

- If you can get up to something called MERV-13,

that's able to remove 80% or more

of viruses that might be in the air.

- [Reporter] This is important because this type of filter

can be installed in standard ventilation systems,

like what you might have in your home.

Still, there are plenty of buildings

that aren't able to adapt this type of filter either.

- Many people are starting to realize for the first time

that their buildings can't respond.

Their systems are not dynamic, they're not resilient,

they can't increase capacity.

- [Reporter] Older and underfunded schools

face some of the biggest challenges.

According to the U.S. Government Accountability Office,

41% of districts need to update or replace

their ventilation system in at least half of their schools,

which adds up to about 36,000 schools nationwide.

The Washington D.C. public school system

spent $24 million on upgrades

to handle higher-efficiency filters

in preparation for the return of students and teachers.

And it's not just pricey for schools.

Some restaurants that will need to spend over $30,000

to upgrade their systems, in addition to operating costs.

- You pay more for energy

to bring in more outdoor air and condition it,

and also work harder, your system will work harder

to push air through a higher-efficiency filter.

- [Reporter] The air pumped into buildings

also needs to be cooled or heated,

which adds to energy bills.

Even though not every building

can afford to make these adjustments,

experts say there are other solutions.

For older buildings with outdated systems,

opening windows is a good, cheap option.

But in the winter, this isn't always possible,

and some buildings have windows that are sealed shut.

- If you can't hit the targets,

consider supplemental air cleaning

through the use of a portable air cleaner

with a HEPA filter.

Now, if you size these correctly for the room,

for restaurant, a school classroom, a bedroom at home,

you can get four, five or six air changes per hour

of clean air, and it can be cost-effective.

- [Reporter] That's what the DC public school system did,

in addition to upgrading their ventilation systems.

The bottom line, we know that the coronavirus

can spread through the air,

and we know how to reduce the risk of transmission.

The biggest hurdle will be the cost

of putting the science into action.

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