A quiet virus filter for as little as 35$ with scavenged battery and hose-less design
Channel: Enjoy Physics
Date: 2020-07-11

Please use updated design with no hose.
https://www.youtube.com/watch?v=Wd1i55ngqK8

The design in the above link is more affordable, quieter and lighter.
And there are fewer parts, so it is easier to make.

Archived Description Below (out of date, do not use):

This shows how to create a quite powered air purifying respirator. 
Or pump blowing filtered air into a bag over your head.
The initial model that I made with a 1 watt fan
blew air between 1 and 1.5 liters per second. That's equivalent to 3 breaths a second.
The idea is that 
as long as filtered air enters the bag over your head faster then you breathe, 
you will only breath filtered air.

The parts listed for the build are 94 dollars. 
Making the hood from scratch and finding a battery can reduce the build cost to less than 50$.
All parts may be found in trash to reduce build cost to time and effort for sourcing materials.

The main design is:
hepa filter
to
pump to move air (at least 1 watt, preferably 2 or more and as quiet as possible)
to 
hose
to
bag over head with clear plastic window to see through.

Update: 
A 2 liter bottle can be used as a face shield and attached to a heavy duty trash bag to make a hood (save 23$): 
https://youtu.be/dDM1uRHQENk
or
https://www.youtube.com/watch?v=sLhCSukWZ8g
Also, the motor and filter are light, the power pack is the heaviest part.
So the motor and filter could be attached directly to a bike helmet and bag so that no hose is needed, saving 10$ and increasing air flow (not yet tried, if you beat me to it, please share).
If you make your own hood in a hose-less design and scavenge a power supply (a used motorcycle battery or battery from an old compute), 
the cost would be about   35 dollars or 2 large pizzas.

Below are links to materials for making this device.
The starred, *, items were used for build cost estimate of 94$.
It's a little less if the parts ordered in quantity (filters, 6 for 8$,  voltage regulators, 10 for 12$, are split or resold).

Glue:
*The glue that I used (8$ for 2oz, keep a window open or work outside for the fumes, takes a few days to cure completely):
https://tinyurl.com/ybp3y26x
Plumbers epoxy which may be better (6$ for 2oz, recommend wearing gloves): 
https://tinyurl.com/yc5d8ama

Filters:
Any HEPA filter should filter 99.97% of virus particles for air that passes through the filter medium. Of course any air that leaks around the filter will not be filtered so poorly manufacture filters may work better if removed from original housing and sealed around the edges into a new housing.
*Filters for a roomba (7$ for 6 or 1.33$ each if splitting with others).
https://tinyurl.com/y7vbp6ez

Pumps (only one needed, could be salvaged from some graphics cards ).
*A quiet fan (9$ 12V@0.15A=1.8W, should pump 1.5 to 2 liters per second through 22mm tube):
https://tinyurl.com/yahzfptu
A more powerful and nosier fan (unless regulated to lower voltage or current), a voltage or current regulator can be used to make it quieter (9$ 12V@0.35A=4W or more than twice the power of the above fan, the out put should be above 2 L/s through 22mm tube, though the narrow tube restricts air flow much more the faster the air moves, so double power does not mean double flow unless a wider tube is used).
https://tinyurl.com/y6wj9ble

Voltage regulators (only one is needed):
These will connect a USB power supply to your pump.
The ones listed here assume use of a 12V fan, though the adjustable model will work with any fan (within the limits of its available voltage and current).
Simple model USB to 12V, not adjustable (8$)
https://tinyurl.com/y7366qwx
*Adjustable (12$ for 10 or 1.2$ each if splitting, needs a usb to micro usb cable which can probably be found from people discarding old phones and such).
https://tinyurl.com/ycakrhpy
Adjustable regulator with display for voltage and current. Also has on off button (15$):
https://tinyurl.com/y6ut4hqx

Hose:
*22mm CPAP or corrugated aerosol tubing recommended in linked video (advantage is that it's medical grade and meant to be used for breathing, 10$ for 6 feet or about 2m):
https://tinyurl.com/y8ahq4ql
A wider tube made with EVA (ethylene-vinyl acetate) may be safe and provide more air flow (may also require cutting a wider hole if using the prefabricated hood).
I did not try this yet but it may work well (16$ with shipping, 35mm, 2m or about 6 feet).
https://tinyurl.com/ya9gv734

Hood (could be made from a bag with clear plastic for a window):
*Hood as recommended in linked video(23$):
https://tinyurl.com/y7x2ds6o

Power Supply:
The more watt hours the better.
* 24Ah (amp hours) * 3.7V (internal battery voltage) = 89 Wh if it meets the averted specifications this will run a 1.8 Watt motor for almost 2 days (40 hours). 25$
https://tinyurl.com/y8mop37n

Credits:
I based my initial build on the mask form this video:
https://www.youtube.com/watch?v=JOT9pyQvvYU
I found that the mattress pump was loud, 
and a computer turbine fan was quieter for the amount of air flow.