by John Young
One thing that has shocked me with the current pandemic is the number of doctor’s offices that have closed due to the fact they are not properly designed to handle patients with airborne communicable illnesses. Thus, if a patient comes into such an office, they risk infecting doctors, nurses, office staff and other patients. In fact, for the most part, doctor’s offices tend to be located in generic office space using unfiltered recirculated air shared with adjacent unrelated enterprises such as mortgage companies. So when a person sick with an airborne illness sneezes in the doctor’s office, people in the mortgage company are also exposed.
And this brings me to explain both positive and negative pressure spaces.
Operating rooms and clean rooms for repairing hard drives or manufacturing medications are designed to work at positive pressure. That is, they are maintained at a pressure higher than their surroundings with air fed into them via HEPA filtration. This is not usually the HEPA filters you buy for a home air filter, which filter out 99.97% of bacteria – but rather the type that will filter out 99.997% or even higher. They aren’t cheap. Typically, they are also preceded by high intensity germicidal UV light which kills both bacteria and viruses as well. The key idea with these designs is to keep whatever is inside the room from becoming contaminated. People who undertake mushroom culture as a hobby do this on a small scale using what is called a “laminar flow hood” which provides a stream of sterile air under which they work, so their cultures don’t get contaminated.
Rooms for containing dangerous organisms, including spaces containing Biosafety Level 2 or greater safety cabinets are also (or at least should be) operated at negative pressure. They are maintained at a pressure lower than their surroundings by fans that push air from the room at high speed, usually (again) feeding that air through high intensity germicidal UV and HEPA filters to the outside of the building. The point is to keep whatever is inside the room from accidentally getting out, and such rooms are usually (depending on biosafety level) equipped with at least a decontamination room for ingress and egress, and for the most dangerous organisms, even decontamination showers that use powerful germicides.
Cabinets for working with BSL-2 organisms are typically designed for a specialized airflow that both protects the contents from contamination, and protects the room from the contents. Cabinets for working with BSL-3 organisms are usually what they call “glove boxes,” and if you are working with BSL-4 organisms, you are wearing a space suit.
So what does this have to do with you?Unless you live in a cave, you know that the United States is currently leading the world in diagnosed Covid-19 cases, and that it is likely even more pervasive than those numbers indicate. You know that thousands have died, that hospitals are having to reuse equipment, and that even doctors and nurses are getting sick and dying. But you may also have to leave the house for work, or to go to the grocery store, where you are not only exposed to direct risk, but the possibility of dragging errant viral particles back home on your clothing.
I won’t go over the proper use of masks, gloves and face shields in this article – you can find such instructions on popular video sites. Instead, I am going to introduce EAU’s Negative Pressure Project. For many homes, you don’t need to get as fancy as this project — just a fan on exhaust in a window of the room where you enter the house will be an improvement.
In the context of your home, you enter via the negative pressure room where you remove all your clothes, disinfect your hands with rubbing alcohol, and then go straight to the shower where you will use generous amounts of soap and water. Then you re-dress, put on fresh gloves, and put your clothes (making sure they don’t touch you) straight in the washing machine.
The following is for technically inclined people with an understanding of how to program Arduino microcontrollers and do electromechanical interfacing.
The negative pressure project is conceptually simple, and it can be applied in any room that has a window to the outside or where a duct can be run to the outside. This could be used in a hospital, doctor’ office, or for a room that houses an entrance to your home.
As published here as a prototype, it uses two Arduino Uno R3 (or compatible) boards that communicate pressure information to each other via Xbee protocol, and the board inside the room uses a DAC (digital to analog converter) to change a voltage that controls the speed of a fan in order to maintain the room at negative pressure.
The parts for this are largely inexpensive and easily obtained via mail order. The only aspect requiring interfacing not covered in the source code is the interface to the fan, whose speed can be varied using various mechanisms. So to use this, you’ll have to adapt it to your needs.
In the interests of serving all who may benefit during a time of risk, this project has been released under the permissive 3-clause BSD license, which allows for both commercial and non-commercial use. The file including the needed libraries, documentation and source code can be downloaded here.
So if you need something more sophisticated than a one-speed fan in a window, now you’ve got it. Hopefully, you find this helpful. If you can’t personally use it, share it with someone who can!