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We know that there is air pressure on top of us all the time. But is the same amount of pressure on us inside a building as outside?
Question Date: 2005-05-06
Answer 1:

That's a good question! I'd never thought of it before. Because most buildings are not built air-tight, air pressure can equalize inside the building, so yes, you are generally under the same air pressure inside a building as you are outside.

Pressure works horizontally as well as vertically, and air can be forced into or out of a building through cracks in windows, ventilation systems, open doors and windows, etc.

The difference in air pressure inside and outside a building due to the difference in the weight of the air is probably small compared to the difference in air pressure inside and outside a building due to other effects. As wind blows past a building, it will cause a large difference in air pressure inside versus outside the building, as will differences in air temperature (if the building is heated or cooled, for example) and humidity.

According to the EPA, even an elevator moving inside a building will create air pressure differences great enough to generate air flow inside the building. The EPA and other regulatory agencies, as well as architects and building contractors, study the flow air through large buildings for health (ventilation) and comfort reasons.

I guess the real question is: If you could somehow make a building completely airtight and never open any doors or windows and never cause any changes in air flow inside the building (no fans, elevators, etc), would you be able to measure a small difference in air pressure inside the building versus outside the building?

Answer 2:

Actually, it is perhaps not correct to describe the pressure as being "on top of us" because the pressure is everywhere. It is within us as well. That is why we don't feel it. The technical term for such a pressure is "isotropic" (meaning it is the same from all directions) or "hydrostatic" meaning that there is a pressure-transferring medium (air in this case) that allows the pressure to be the same everywhere: inside us, in a car, in buildings ...

The way this pressure would change is if we move further from the center of the earth, because the pressure is caused by the gravitational attraction of the earth's atmosphere to the earth. So in Mexico City (which is at 8000 feet) the pressure is less than in Los Angeles (which is at sea level). It could also change if we either remove the pressure transfer medium (in a vacuum) or we fill lots of it in a small volume (a car tire).

Answer 3:

The air pressure is all around us, no matter if you are inside or outside of a building. In order to change the pressure, you have to change the amount of molecules of air around you. This is possible either using a pressure chamber or moving upwards or downwards from the sea level.

If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in order to balance the pressure between the outside and inside of your ear.

Answer 4:

Usually there is the same pressure inside a building as outside because there are openings such as vents and windows which equalize the pressure. One way to tell if the pressure is not the same on two sides of a door is if a door is difficult to open for "no reason." This usually means that the room you are trying to push the door open towards has higher air pressure.

Answer 5:

The same air pressure is on us inside a building as outside - as long as the building has air connections to the outside (which it normally does). If you were in an airtight room and extra air was pumped in or out, the pressure would change -- that is the only time the inside of a building is different than the outside.

Answer 6:

Generally, yes. Sometimes, though, the pressure might be kept a little higher inside a building by using the equipment that circulates air. The pressure difference would be rather small but you can tell if the pressure is higher or lower inside a building by noticing if there is a breeze when you open the door (notice direction of the breeze as well). Why do you think you might like to have the pressure inside a building a little higher than outside?

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