This is regarding the question about pressure. I mean if the pressure rapidly decreased in a normal building. Would that cause an explosion? Can an explosion come from pressure decreasing?
This is really more of a physics ask, but what the hell. Let’s talk about pressure.
Pressure is defined as force per unit area. We are talking here about air pressure, which can basically be thought of as air molecules bouncing around, exerting force onto the things–you, buildings, other objects–around them. Air pressure is affected by
the number of molecules. More molecules pinging around means more pressure.
the temperature. As temperature increases, gases expand and the molecules have more kinetic energy.
the volume. The bigger the space is, the less frequently the molecules hit the walls of its vessel.
altitude. The higher you go, the less gravitational force affects the air molecules. For the sake of this answer I’m assuming constant altitude.
Let’s call the area of interest the system and everything else the surroundings.
When you change the pressure of a system in relation to its surroundings, you create a pressure differential. If you have an airtight, closed system in which matter may not enter or exit, you maintain this pressure differential until the closed system is opened (whether that is by your choosing or by failure of the vessel). Gases (air) are highly mobile and will rush from an area of higher pressure to an area of lower pressure. Dramatic effects, if any, occur as the pressure equilibriates between your system and its surroundings; the degree of drama is dependent on how large the pressure differential is and how quickly the system and surroundings equilibrate. (If the vessel containing your system remains closed and impervious to the surroundings, and does not fail in any way even after you increase or decrease its pressure, then nothing really happens. Carry on.)
So what happens when you rapidly decrease the pressure in a system? Consider the following options:
1) You have increased the pressure of your system, then opened the system to equilibrate with its surroundings. This can be subdivided up into the change from 1a) normal atmospheric pressure (1 atm) to something below that, or 1b) from very high pressure (many atm) to atmospheric pressure (1 atm). In both cases the pressurized air from your system will vent outwards to the lower-pressure surroundings, and the air pressure of your system will decrease until the system and its surroundings reach equilibrium.
1a) A change from 1 atm to something below. The most extreme example in this category is the change from atmospheric pressure to a vacuum (pressure difference: 1 atm), which is encountered in space; less extreme examples include depressurized airplane cabins in mid-flight. Aside from the outward rush of air from your system (space shuttle, plane cabin, etc) to the surroundings, there will be some adverse effects to humans from the decompression of dissolved gases in our bodies. Effects range from swelling of tissue, hypoxia, unconsciousness, and lung barotrauma. But human skin is fairly stretchy and humans will not explode from a change of even 1 atm, never mind less than that.
1b) A change from many atm to something approaching 1 atm. This usually doesn’t occur except in very specialized environments. The most notable example of this is the Byford Dolphin incident where there was a dramatic change in the system’s pressure from 9 atm to 1 atm (pressure difference: 8 atm) in a fraction of a second (note: DO NOT google this if you are squeamish. There are pictures). Such a drastic decompression will result in an explosive blast of air outwards toward the surroundings; unsecured objects (including human bodies) become projectiles, gases (including gases within the body) rapidly expand, leading to boiling blood, ruptured organs, violent dismemberment (when forced through too-small spaces), and death.
2) You have decreased the pressure of your system, then opened the system to equilibriate with its surroundings. The higher-pressure air from your surroundings will rush into your system, and the pressure of your system will increase–essentially, the reverse of the previous scenario. You will get something resembling an implosion.
The severity of the implosion will depend on the pressure differential between your system and surroundings. On Earth, the air pressure at sea level is one atm, and thus the worst you can do is drop your system’s pressure to 0 atm, i.e. a vacuum (pressure difference: 1 atm). At a pressure differential of 1 atm or less, humans in the lower-pressure system are unlikely to be strongly harmed by the incoming rush of air, absent physical trauma or other projectiles (though the same cannot be said of structures; even with a small pressure differential, you’ll very likely shatter some windows at least). However, even at equilibrium with the system, humans will suffer adverse effects from decreased atmospheric pressure. An example of this is found in high-altitude mountaineering: 0.5 atm (5000 m above sea level) is about the lowest air pressure humans can tolerate without supplementation, and you begin to run into various symptoms of altitude sickness. The risk of pulmonary or cerebral edema rises as surrounding pressure drops. At 0.35 atm (8000 m above sea level), the amount of oxygen dissolving into the blood is insufficient to sustain life. At 0.03 atm (0.38 psi), water will boil at room temperature, though nobody will be alive to appreciate it.
It also goes without saying that if your story is not set on Earth and the atmospheric pressure of the surroundings is much higher, you could have a much more dramatic implosion, in addition to the effects of low atmospheric pressure on humans.
Final thought: you need to consider how you are going to create and maintain a pressure differential in your system (in this case, your building) in order to have a dramatic effect.
Recall that gas (air) pressure is typically changed by manipulating temperature, volume, or number of molecules. Those are what you have to work with if you simply want to manipulate the air pressure on its own, without introducing a chemical explosive (which provides an explosive effect on its own), magic, or other external factor.
Your regular building isn’t going to be airtight and is not designed to be able to build a significant pressure differential. Air flow is a good thing within buildings, and the movement of air is inevitable, what with air gaps, void spaces, hollow partitions, service chases, fans, ducts, AC systems, and the like. There is also a large volume of air within a building. The air pressure won’t be perfectly uniform; you will have slight variations (this is why your Fitbit’s altimeter can detect when you climb a flight of stairs). But overall, whether you choose to dramatically crank up the heat (increase temperature), pump a lot of compressed gas into your building (increase the number of molecules), or fire Mr Freeze’s freeze gun around (decrease temperature), you will be hard pressed to produce a significant pressure differential over so large an area.
Also, most buildings are not built to withstand large overpressures. Since your average building can’t build up that kind of pressure differential the point is moot, but should you decide to handwave the pressure differential into your story anyway, you will expect a lot of structural damage when the pressure differential is high. Per J’s recent post, at an overpressure of 20 psi concrete buildings are severely demolished; if you did handwave the 8 atm (117.6 psi) pressure differential into a building in a fraction of a second with the resulting pressure wave, I have no hope of your average building remaining standing afterward.
Edited to correct my typo: the pressure at which water boils at room temperature is about 0.03 atm, not 0.06 atm. Sorry!
