What will happen if you land on the moon without any spacesuit but only with an oxygen cylinder attached with an inhaling mask?

“Your death would be fairly horrifying, but probably quick.

In vacuum, you cannot draw a breath from an oxygen cylinder, no matter what kind of regulator you use. The minimum pressure needed to keep you alive is about 4 PSI of pure oxygen.

Your first problem is that even with a low profile full face mask, the air pressure is going to force the mask away with a total force approaching the weight of a grown man. Adjusting straps tight enough to keep the air from leaking out would be challenging

to say the least, and will likely cut off blood flow beneath the straps.

But let’s pretend you planned for that, and while you didn’t bring a spacesuit, you did bring a full face mask, tank, and regulator specially designed to keep you breathing. That was probably a mistake.

Without a spacesuit:

• Your body will start to swell (not as dramatically as in some movies, but painfully).

• You may experience explosive diarrhoea, depending on what you last ate and when.

Depending on your health, genetics, and the particular air pressure you are breathing, you might rupture a lung. At 4 PSI, you will be in fairly serious pain, but your lung might still function. At atmospheric pressure, your lungs will rupture and you’ll die in agony.

Upper body strength, transmitted through the rib cage, might offer some additional protection.

• The capillaries in your skin will swell and burst, causing a generalized moulting and bruising, which will quickly harden.

• If you were breathing normal air before you fell out of the window, you will probably get the bends as nitrogen bubbles form in the bloodstream, then catch and accumulate in the joints causing excruciating pain.

• If one of these bubbles doesn’t grow large enough to block blood flow through a major organ, you may notice that you are standing in soil far, far too hot for human tolerance, and now have second degree burns on your feet. But don’t fall, that will make it worse.

• Alternatively, if you are standing in the shade, you will no doubt notice that that pain in your feet has subsided because they are now well on their way to freezing solid. But don’t fall, that will make it worse.

• If you grab a handle, ladder, tool, hatch, or any other metal surface in order to seek rescue, you will find your hand quickly crippled by pizza-oven heat or cryo-freeze cold depending on whether what you’ve grabbed has been sitting in the sun or shade.

• If you are lucky, perhaps the hatch you fell through hasn’t had time to heat up or cool down much and you can crawl through it before your feet will have to be amputated, or maybe someone will come to your rescue and you’ll only have to face lifetime disability and ignominy.

• If not … now might be a good time to break the seal of your mask so you can pass out before the pain gets any worse. You really should have worn a spacesuit.”
-C Stuart Hardwick via Quora