Positive vs. negative pressure specifically to hospital rooms.



On surgical floors Operating Rooms(ORs) have positive pressure checks daily to ensure air handlers are working effectively. Why is positive pressure more desirable for surgery vs. negative pressure? Can either be compared to the exhaust fan in my bathroom on a smaller scale? Thanks!

In: Engineering

I once spent time in a negative pressure isolation room. The idea was to make sure that my nasty germs did not leak out in an uncontrolled way. As long as the room I was in had a lower pressure than the rest of the hospital, my germs did not get out through the door. The way they made sure to keep my room at negative pressure was to have an exhaust fan pulling air out of the room. That exhaust fan the put the air it pulled in through a filter system that made sure no germs got through.

Operating rooms want to make sure that no germs get *in* and infect the poor guy with an open incision. So they keep positive pressure by using a fan to blow air into the operating room. And, like the exhaust fan, there is a filter system on the *intake* side of the fan to make sure they don’t have any nasty germs in the air that the fan blows into the operating room.

The basic idea is that it is easy to filter air at one point: the inlet of a pressure fan, or the outlet of an exhaust fan. Trying to filter all the air that creeps in at every crack is impossible.

Positive pressure means that air will flow from the high pressure area (surgical room) to the low pressure area (the corridors). This means airborne bacteria from the less sterile outside does not get carried into the more sterile surgical side by airflow.

Presumably the exhaust fan in your bathroom is designed to vent humid air from the bathroom to the outside – not quite the same intent.

Your example of the exhaust fan in your bathroom or your kitchen is excellent examples. The problem is that walls and doors are not completely airtight. There is very often some crack between panels or a hole left by an electrician or something to allow the air through. Especially doors as they may warp, the seals may wear if they are installed in the first place that is or the door may not be closed all the way. The problem in your home is that there is bad odors and humidity in your bathroom and kitchen. If nothing is being done about it then the air will creep out through the doors and walls into the rest of your house. To prevent this we install extraction fans which creates a negative pressure in those rooms. And because air moves from high pressure to low pressure the air moves from the rest of your house to your bathroom and kitchen but never the other way. If you have a balanced air ventilation system in your house you will have inlets in for example your bedrooms and living room to ensure a positive pressure.

This is also used at hospitals to make sure that airborne pathogens go the right way thorough the hospital. So a place that needs to be very clean such as an operating room will get fed with filtered clean air to create a positive pressure so that air will not leak through the walls from other parts of the hospital. It is not a lot of pressure difference needed. Just enough to ensure the air is moving the right way. If you have enough pressure difference between rooms that you can feel a breeze or doors start closing or opening on their own then you have too much pressure difference.

(Not medical a professional) air flows from positive to negative pressure. having positive pressure in an OR is a good thing given that when you open the door the air flows out of the room instead of into the OR. (potentially dragging bacteria whit it )

regarding your bathroom fan, not directly it’s main purpose is to create air circulation or more directly to drag the moist air out

I’m an architectural designer who designs hospitals.

ICUs will have negative pressure rooms and/or wings. They create a seal around the entrance so that infectious particles are pushed back into the room if they try to escape.

For surgery, you have a person who is likely opened up to outside air. In this case, you want any infectious particles to be pushed *back out* if they try to enter the room.

Positive pressure is for protecting whoever is in the room. Negative pressure is for protecting whoever is not in the room.

Houses do not create seals around doors. The fan in your bathroom is merely used to circulate air which pushes humidity and odor out. This air is replenished by other air coming in through an intake register and cracks around the door. Bathroom fans are not comparable to pos/neg pressure rooms since they don’t create any difference in measurable pressure on a whole between outside the bathroom and inside the bathroom.

If you are planning on isolating yourself in your bathroom for either of these reasons, it isn’t going to work.

Positive pressure: Pressure is higher in the room, so that when the door is opened, air flows out. You put an immunocompromised person in here, so that nasties don’t get in.

Negative pressure: Pressure is slightly lower in this room, and air flows in when the door is opened. You put your Ebola patient in here, so that nasties don’t blow out into the hallway.

To put it simply: If you don’t want things from outside from coming in, you have high pressure in the room. With this you can keep contaminants and dust outside of an area, and all air that comes in is filter, like operating theatre. If you want to make sure nothing escapes the room without being filtered, you have negative pressure.

This is used in clean room and contamination room universally.

And in a way… yes your exhaust fan works the same way. It tries to keep humidity and odours from going to other parts of the building, by pushing it out and creating a draft. Now… if you wanted your bathroom’s humidity and smells to spread, you could reverse the airflow.

I design hospital renovations for a living. Air, and everything that’s in the air, moves from areas of high pressure to low pressures. Certain parts of the hospital need germs to move towards them, or away from, depending on the function. The operating room needs germs to move away from the operating table, so air supplies are positioned directly above the table and force the clean air to hit the table/patient and move away from the table and then return vents are positioned near the floor at the edge of the theater. Positive air pressures are also used in areas where immuno-compromised patients are, such as infusion therapy areas. Negative air pressures are used where we want to keep germs from spreading, such as infectious disease patient rooms. Because of COVID-19, the hospitals I work with are converting normal ICU rooms into infectious disease rooms by adding exhaust systems to those rooms that dump the air outside. Construction sites in the hospital are also negative pressure areas, they use exhaust fans to force air out of the construction area and dump it outside, forcing clean air into the construction zone and then takes the dust out of the building.

So yes, they are very similar to the exhaust fan in your bathroom, in fact the exhaust for your bathroom is often on the ceiling because the warm humid air rises so the fan creates air movement from the floor to the ceiling, which is the natural movement of the warm air. The difference is that your house probably doesn’t have controlled ‘makeup air’. Basically, if you are pulling air out of an area, then air will find its way into the area to balance out. For a house, the air is pulled out by the exhaust fan, and because there usually isn’t mechanical equipment to pull air from the outside, clean/condition it, and put it inside the home in a controlled manner, outside air seeps through the cracks in your house. Hospitals have dedicated makeup air units that will draw air through filters and condition the air before putting inside the hospital. This allows the hospital to control the air from the moment it enters until the time it is exhausted.

Think of a room submerged under water, with a hole in the side. If it has negative pressure in the room, that means water is getting sucked in. If it has positive pressure, it is pushing air out.

Now replace that water with air. Some rooms don’t want air coming inside, as it could infect things inside the room. Some rooms don’t want air being pushed out, as it could infect everything outside it.