I've done a video on exactly this point which may help:
http://www.youtube.com/watch?v=lCUld-qLTMA
Just to warn anyone reading this: i'm going to be frank and graphic.
Besides that, i have thought about this a lot. I also added safety advice to the Wikipedia article which was sodding deleted, so that was helpful of them wasn't it?
Anyway, my view on this is as follows. Ultimately, of course it isn't safe because you would burst! The risks are probably mainly peritonitis, rupture, perforation of internal organs, haemorrhage and air embolism. By mouth there's an additional risk of aspiration of stomach contents. Two of those risks could cause sudden death. However, you can do various things to minimise the danger and as i say on YouTube, i've been doing it regularly for nearly thirty years with no ill-effects.
The answer, i think, is to go slowly. I usually take about twenty minutes to do it. Also, be aware of the internal capacities of the organs involved. You can't mess around with your lungs because they're more delicate than the digestive organs and you won't be able to go past maximum inspiration with them without doing something horrible like giving yourself emphysema or tension pneumothorax. Another risk by mouth is perforating your eardrums, which isn't fatal of course but you don't really want to do it either.
Also, i avoid using liquids because of the problem of diluting body fluids, which again can easily be fatal. Also, liquids are almost incompressible so their volume before they go in is the same as their volume afterwards. Five litres of water will fill your stomach. Six litres, unless it finds another way out, will injure or even rupture it. Six litres of air will be compressed by your stomach, so the risk is smaller. Also, you can get rid of gases faster than water and you won't choke on air (though you might choke on your own vomit).
If you introduce air via your mouth you have the advantage that there are reflexes which are "designed" to draw food and water into your body for digestion. Stomach filling triggers stomach emptying and bowel emptying and increases the movement of food through the digestive system and something in the small intesting causes relaxation "below" it, so a bubble of gas will be drawn through it. If you do it via an enema, which i've never managed, you're working against the natural direction of your digestive system.
There seems to be some kind of adjustment if you do it over a long period. That is, you can train yourself to being able to take more air in as time goes by.
The capacities of the main internal organs involved are: stomach - 5 litres (1 gallon) (literature says much less but believe me, this is the capacity of my stomach, maybe because it's adjusted); colon - 9 litres (2 gallons, but this is measured in a colon outside the abdominal cavity after death, so in life it'll be a lot less except maybe just after giving birth); lungs - 5 litres. However, the air gets compressed after you take it in.
There are some other risks, notably toxic megacolon, which is like crush syndrome. Pressure squeezes the blood out of the colon, causing the tissue of the colon to start dying and releasing toxins into the bloodstream. Also, the bacteria in the colon can get into the blood, which is seriously not good.
So no, it's not safe, but on the other hand you can adjust and get used to it.
So, after playing around with roleplay with my partner for a while she and I have begun actually inflating me. It's been exciting and slightly scary (like any good experience along those lines should be), though I am wondering what possible ill effect I could be wreaking on my system.
I don't have health insurance, so I don't have a doctor, let alone a "kink friendly" one as certain advice columnists would recommend talking to. I'd certainly go to the ER should it be necessary, but as to talking about the risks, I could scarcely mention it to my girlfriend last year, and I doubt I can bring it up in a roundabout way in a clinical setting.
*cough* That's a bit off the mark, though. So, a bit of backstory... Initially, she would blow into me while I held my throat open, but we've since graduated to things like actual pumps. They work a bit better, and as such, I'm a little concerned about possible ill effects.
Earlier this week she inflated me to at least 2.5x my usual width? I'm not skinny per se, but I'm also not fat, so the effect was pretty goddamned striking - I looked rather incredibly pregnant, my midsection nearly a perfect, taut dome. :V
It didn't hurt at any point, and we paused a few times to let the air spread through my system. Aside from any medical knowledge, I think we were playing it as safe as could be. I do want to find out if I'm setting myself up to become an exceptionally early corpse or something, but the other night was so amazingly fun, I'd really like to do it again.
So, um. TL;DR: I've been getting filled up until I'm incredibly full and tight. It hasn't hurt, but I'm worried I could be doing damage to myself. Am I doing damage to myself? :V
Myself in three words? Killer alien vagina.