A decrease in air pressure is generally accompanied by a decrease in the partial pressure of oxygen in the air (i.e. the absolute amount of oxygen in air), which leads to acute mountain sickness. I suppose it would be perfectly alright to breath pure oxygen at one fifth the normal atomspheric pressure (that’s about how much oxygen is in normal air), but you really can’t get any lower than that, because it won’t exactly be low pressure that will kill you, it’s a lack of oxygen. Low pressures just sets an upper cap on how much oxygen there can be, and when that cap gets below what your body needs, you’re going down no matter what. There may be other debilitating effects that occur when exposing the body to general low pressure, but I can’t recall any off hand…
There are a couple of issues about air bubbles in the bloodstream that need to be clarified.
Yes, air can form an embolism from infused bubbles injected into a vein, but, as many fo you have said, it needs to be a significant quantity. Embolism do cause events such as strokes, heart attacks, deep vein thrombosis, pulmonary obstructions etc, but they are far more likely to arise from fatty globules or clot fragments than air. Such events are a potential threat to patients as a result of severe trauma, where these typs of particles can easily enter systemic circulation.
Air in veins is a much bigger problem when the patient is subjected to changing external pressure, as in air evacuations. Many of these occur in unpressurised aircraft, so EMT’s etc are mindful of the need to avoid infusing air with drips. The gas laws that effect divers ascending to the surface are continued when a person who is equalised at ground level gains altitude. Gas that is dissolved into liquid at sea level pressure may be liable to come out of suspension and form bubbles that coalesce to make an “air lock”. If this happens inn the brain the patient is in dire straits.
The Diver’s Emergency Service based at the Royal Adelaide Hospital (South Australia) classes air embolisms is a grave threat to diver safety. In their lectures and handouts the term Cerebral Gas Arterial Embolism (CAGE) is given prominence as the biggest lifethreat in diving medicine. They have some very graphic slides taken during autopsies of brain tissue with some remarkably transparent blood vessels.
Hopefully this explains the original question.
Cecil makes mention in More of the Straight Dope that blowing air into the vagina can cause death and states that there have been, as of 1983, some 10 cases of this and another case where severe brain damage occured. Although this is mostly a risk during pregnancy, the fact is that air was taken into the veins and caused an embolism. I would imagine that using a needle to inject air into the veins carries the same risks.
When I was getting my tonsils out a few years ago (at the age of 22!) I was a bit nervous. This was my first major surgery, and I tend to avoid pain like the plague, so I was scared. The anethesiologist came up to me and explained what all was going to happen, then said he needed to hook me up to an IV of electrolyte fluid. The nurse put the little vein thingy in my arm, hooked up the IV bag, then stuck the tubing into my vein thingy. The first thing I saw was a good 12 to 16 inch section of AIR running down the tube from the bag into my arm.
I freaked. I grabbed the tubing and pinched it in half and glared at the doctor with eyes that my friend described as “bigger than plates.” He slowly patted my hands and looked at me with a smirk on his face and said “Only in the movies. Only in the movies.” I let go and watched this whole big bubble of air go right on into my arm. The doc commented that I seemed a little stressed and gave me a shot of something that made me…fffeeel…jjust…ffine.
I asked Cecil about this awhile back, but (surprise!), didn’t hear anything about it.
My experience- no problems with a fairly large bubble.