Contamination is a most persistent adversary. It stalks us in the dining room, the kitchen, the bath, in public buildings, factories and laboratories. With an ocean of air full of interesting particles, vapors and living organisms, it is always available to add something to the food we eat, the air we breathe, the products we make and the experiments we perform. It is so relentless, that protection from it is a big 
challenge, but preventing contamination with positive pressure ventilation is a powerful option. Our home, our vehicle, laboratory or factory and a host of other closed structures can avoid contamination by positive pressure isolation.
When a balloon is inflated, the positive pressure gas inside is retained by the latex envelope that contains that pressurized fluid. We know that the balloon will eventually deflate because balloons leak just a bit as all containers leak. But, that balloon interior is protected from contamination so long as the pressure inside is positive, higher than atmospheric pressure around it.
How positive pressure isolation blocks contamination
Air or helium in a balloon will slowly leak out, but the positive pressure keeps outside air, smoke, microbes or gas from leaking into the balloon. This is true for positive pressure in a balloon, a building, a room, a laboratory glove box or just about anything else. This is a form of positive pressure isolation. The mechanism is pretty simple. Every wall, latex membrane or other solid barrier has leaks in it, as it is made of molecules with lots of space between them, so there are
always a few holes, even if they are too small for us to see. If the barrier has good integrity, the holes are very small, so most microbial life, dust or other contaminants are blocked, but a few gas molecules and very small particles or microbes could clear the opening. But if the air or gas inside is positively pressurized, then the leakage flow through any hole will be from the inside to the outside. If the hole is small, no dust particle, gas molecule or microbe can get in, because they can’t swim upstream, against the flow of gas or air. The positive pressure guarantees that the flow direction is outward.
How to use positive pressure in a building or room
When we want to avoid the flow of pollution, cold or hot air or other contamination into a room, a house or a building, we can positively pressurize the structure. This is possible and practical because even a very slight positive pressure will provide isolation protection. A pressure isolated laboratory, house or building is generally kept at a positive 
pressure of less than 2 inches (5 cm.) of water; a fraction of the pressure we exert to drink through a straw!
This is fortunate, as most buildings have an awfully lot of leaks. They have leaks at doors, windows, pipes, wall panels and other places, so it takes substantial air flow to maintain the positive pressure. This flow is driven by the fan in a heating or cooling system or other ventilation equipment. Fans can propel a substantial flow of air at a small positive pressure that is sufficient to pressurize a room or building. In our post about avoiding air pollution at home, we discuss how to set up such a system.
Using positive pressure isolation in a laboratory or factory
Isolation can be critical in laboratories and factories, as the items handled can be very sensitive and susceptible to contamination. Food processing facilities have been shut down permanently, and companies put out of business because of contamination. In other industries, explosions, fires and other serious problems have been the result of contamination of the materials or processes inside. Clearly many types of manufacturing, processing and lab work must have this protection. Often, the rooms or buildings that require isolation can be pressurized similarly to buildings and rooms discussed above. In some cases, whole rooms or buildings are not an option.
Research experiments and clinical lab tests require high accuracy measurements that are not possible if specimens are contaminated. While some rooms, labs or buildings are isolated this way, often isolation must exclude the people working there, as they would be sources of contamination. Naturally, many researchers and lab 
technicians use pressurized or purged workstations to ensure clean processing, so they can perform experiments that work, and gather accurate data from them.
In some cases this requires use of highly filtered or treated air, to maintain sterile or ultra-clean work spaces. In other situations, air itself is a contaminant, as oxygen, water vapor, nitrogen or carbon dioxide can attack the materials or processes under study. In such situations the process must be contained in sealed laboratory glassware, tanks or barrier isolation glove box workstations that support controlled atmosphere processing in a non-air special environment. In any of these situations, positive pressure isolation is an important tool, as contamination must be avoided with a secure and reliable protocol.
Even when air is not a contaminant, and super-clean or sterile air is not essential for process integrity, pressurized isolation of an experiment may be critical. Minor 
variations in air movement, temperature, humidity or other parameters can make a difference in measurements required in almost any test or experiment. When the process being studied is isolated in a positive pressure controlled environment, variation in measurement is reduced by minimizing the mechanical, chemical and thermal “noise” that would otherwise influence experimental data. Pressurized isolation is a powerful tool to reduce variability and improve the integrity of measurements and data.
Alternatively, labs will often use an open hood with laminar air wash to provide clean air without pressurized isolation. While this can work well when the system is properly designed, maintained and operated, it doesn’t provide the fail safe protection of a 
pressurized barrier isolation approach. This approach uses slow moving (laminar) air that has been filtered to provide protection and consistent conditions.
When configured for positive flow air (flow outward) this system can be used to provide clean and sterile conditions. When work is conducted in a negative or vacuum hood (flow inward) then operator protection is provided from noxious or unhealthy fumes, but the experiment or test is exposed to moving air from the room, so sterile or clean conditions are unlikely. Unfortunately, the vacuum hoods are common in labs, so they are often the cause of contamination and operational noise. This often results in quality and integrity problems with test data.
*NOTE- Some items in this post are provided by sponsors who support this website – see our sponsorship page for details.
Disposable glove bag isolation enclosures can provide full pressurized barrier isolation at a modest cost (some under $50USD). *Glove bags come with gloves bonded into them, a sealable opening for insertion of materials and instruments and a port for connection of a gas line for inflation. These 
The unit shown is available for *order: Amazon glove bag link
There is an economical, low cost glove box now available that is more costly than the glove bag, but fully reusable, rigid
and stable. This unit is described in more detail in a post on flexible lab glove box isolation. It provides basic positive pressure isolation and delivers atmosphere quality at or below that provided in the pressurized gas used to fill it.
When very extreme atmosphere purity is needed, there are large, heavy wall controlled atmosphere systems available that provide chemical atmosphere processing and circulation to purify chamber gasses beyond quality levels of gas initially injected. These units are more expensive and complex to operate, but are sometimes needed for operations that require oxygen or water vapor levels maintained below 50 parts per billion.
Positive pressure isolation works
The important thing to remember is that when you must avoid contamination of the interior of a house, a room, a building or a work-space, positive pressure isolation makes that possible and practical. Whether the ventilation system is a fan and filter, a pressure tank or a pump, if it is properly controlled and applied, it will keep outside contaminants from leaking into the inside.
Have you used positive pressure isolation, or observed it at work? Have you encountered it in buildings where you had to push hard to get an outside door to open inward (pushing against the positive pressure)? Do you have a question, a story or an idea to share? Please use the form below to leave us a comment – we’d love to hear from you.
This is really an interesting article. The positive pressure ventilation is very well explained and understandable for a layperson in this field, like me. The well chosen pictures are a great addition to this article, makes it even easier to read and understand.
I did work as a Flight Attendant for several years and it always fascinated me how it’s possible to fly in high altitudes and keep the pressure inside the cabin. Well, it still fascinates me how such big vehicles can take off and fly in the air and keep us passengers safe and keep the pressure upright so we can still breathe even at the heights above 12’500 feet above sea level.
Steve, how about radioactive radiation? In case of a nuclear crises it is recommended to stay inside a house. I know there are different sorts of radiation: natural radiation and those caused by artificial sources. I can’t believe we can stay safe inside a normal house if there would be a nuclear disaster. Or would there be some sort of protection having a positive pressure ventilation in the house in this case?
I’m eagerly awaiting your answer!
Pernilla
Thanks for your great observations and question, Pernilla. An airliner is a good example of positive pressure isolation at work. Even short flights at relatively low altitude would be very uncomfortable, even painful for passengers, and normal high altitudes could be lethal without the pressure control in the aircraft keeping a healthy positive pressure. A few years ago I learned this the hard way when I was air-shipping equipment in packages that used air-bags to cushion the delicate products. These plastic air bags help keep products safe as the package may be bumped or bounced around during manual handling. I noticed that customers received some equipment in poor condition after air shipping, and found that the air bags were not working, as they had burst while at high altitude. The pressure in air cargo compartments is often much lower than in the passenger compartment because it is not protected with strong positive pressure control there; this allows air bags to expand more and pop, so other ways must be used for protective packaging for air shipment. We now use crumpled paper, which doesn’t depend on pressure.
Your question about protection in a nuclear disaster is a good one, and you are wise to show concern for this case. Normal homes are designed to provide many air changes per day so that air is fresh and healthy for people inside. What is typical is 2 complete air changes per hour for a house when there is no extra ventilation from bathroom fans, range hoods in the kitchen, dryer vents in the laundry, or other exhaust fans in the house. If the home has a roaring fire in a fireplace, that can drive air changes to 20 times per hour or more. That means that the radiation-free air in the house would normally be replaced with contaminated air from outside in about 30 minutes or far less when fireplaces or clothes dryers are in operation. That isn’t much help if there is dangerous contaminated air outside the home, and you hope to stay healthy.
Positive pressure ventilation can be a life saver in case of a nuclear crisis or release that threatens residents with exposure to nuclear radiation. This depends on the kind of radiation and the way positive pressure in the home is put in place. We published an article on protecting a home from air pollution, which you can read here, and it explains how to prepare a home for pollution events with unhealthy smoke or other particles. This protection from dust, or smoke will generally protect from most radiation threats, including nuclear fallout.
In a positive pressure home, prepared for pollution protection, a ventilation system uses filtered outside air to pressurize the home by forcing the air through a filter to remove dust, and into the house. Since most pollution, including most nuclear hazards are particulate, this will catch them and leave them in a filter, and not in our lungs. The exceptions to this protection are toxic or radioactive gases like carbon monoxide or radon, which are very difficult to filter out. In fact, one of the reasons why it is important for homes to have outside air ventilation is because of radon gas that can leak into homes from basements or foundations and can impact our health if we breathe them too much.
If you are warned about a nuclear release, chemical spill or other environmental crisis and can retreat to your home, I recommend the following steps to protect your family:
1 – Be informed: find out as much as you can about the nature of the threat. Is it gas or dust? Is it nuclear, chemical or biological? Is it likely to last for minutes, hours, days or longer?
2 – Get inside and close windows and doors, and shut down the dryer, exhaust fans and fireplace (close the flue damper)
3 – If you have a working system for filtered, positive pressure ventilation, and the problem is not a gas, make sure it is on and operating.
4 – If the problem is a gas or you don’t have a positive pressure system, turn off all central heating, air conditioning or other air circulation equipment and seal off the family as best you can from outside air. This may require choosing a part of the home with no or very few windows and using duct tape and plastic sheeting to seal this room or section from outside air. This may not provide absolute protection, but it will reduce exposure risk.
If you live near a nuclear facility or are concerned about a possible war or accident, I do recommend preparation including setting up your home to use positive pressure ventilation as we describe in the article above. Another helpful article you may want to review is this one on preparing to survive a disaster.
Thanks again for your great question, Pernilla. I hope we will here from you again.
Steve, thank you so much for this excellent answer of my question, it’s obvious that you are an expert in this field. I do appreciate that you took the time to answer it so thoroughly.
I’m grateful for the recommendations on what actions we can take in order to protect ourselves in case of a nuclear release, chemical spill or an environmental crisis. I will print out these 4 steps and put it at a handy place.
I’m certainly going to read more of your informative and useful articles.
Pernilla
Thanks for contributing to our conversation, Pernilla. We are here to help, so it’s rewarding to know when our advice makes a difference. I hope we hear more from you soon.
Excellent and informative article. I’ve been struggling with this concept for an earthbag home that I am planning to build and this gives me some very good ideas of how to solve my problems. So GLAD to find this article. Thanks for writing, and keep up the great work of sharing!
Glad you find it helpful, William. Earthbag construction should be a good application for this approach if you want to carefully control the interior environment. This post on pressurizing homes to prevent pollution might be helpful, as it has more details on how to apply this to your dwelling.
Of course, if control of interior conditions and exclusion of pollution, microbes, etc. are priorities, it will be important to seal the gaps, and the earthbag shell to minimize airflow through walls and roof. Positive pressure ventilation can control the direction of airflow and contamination, but only if the structure is sealed well enough so that pressurizing it is feasible. This will probably require plastering, stucco or other treatments to enhance the gas flow barrier provided by the soil bags.
Thanks for joining the conversation.
I hope you will come back and share your learnings as your project unfolds.
Steve