Without trying to sound unduly condescending, I feel that the reactions to decompression on this page are rather ill informed.
I have been flying for 35 years and the only time I have depressurised was on purpose and that was in a military transport to allow people to get out (parachutists). I spent some six weeks in my youth seconded to the Royal Aerospace Establishment at Farnborough England where I was involved in the testing of the high altitude oxygen regulators for the (then) new Nimrod Maritime Aircraft (Can I say this now ? <g> ).
This involved peddling a aerobic cycling machine and wearing various combinations of masks / no masks and being decompressed from 4,000ft to 40,000ft in very short order indeed, many times a day. Well even without the hindsight of practise it isn't that bad. There can be some toothache if you have air under a filling and a sensation rather like when a train comes out of a tunnel - but it goes on for longer. But the worse thing by far is the GAS. In an enclosed space like a decompression chamber this is a very significant reminder to put your mask on <g>. Seriously though, if you have some gas in your gut (and who hasn't most of the time), it will expand and have to get out. If a depressurisation ever happens to you (which is so unlikely that all this typing is probably a waste of time) - the one thing you will remember is the gas episodes. This is why the Guiness Book of Records specifically forbids the use of decompression chambers in the record attempts for flatulence records.
When the decompression is spectacularly rapid then there may be a lot of "fog" since less dense air cannot hold the water vapour as well, and there will be a good deal of noise as the air gets out. However in a large airliner there is so much volume that unless there is a large structural failure (such as the Aloha 737) things will happen very gently. The main point here is that if there is a large structural failure the chances are that the lack of air will be a minor problem by comparison.
The issue of the learjets has been stated. However these aircraft have a double jeopardy in that they have a very small volume and fly very high. However the real problem is the slow leak / failure that is insidious and gets the cabin altitude up with anyone noticing. Or worse still Carbon Monoxide induction.
I would like to address those two points with regards to modern airliners.
The slow problem : All modern airliners have several consecutive safety features to watch out for this problem. They consist of progressive warning states and finally at 13500ft (or so) cabin altitude - and automatic drop down of the masks (the "rubber jungle" in the trade). This is then accompanied by such a racket in the flight deck that it is quite impossible to ignore. As soon as the systems detect a loss of pressure quite a few things happen, and it may be constructive to explain the how and the why of this :
========= Lots of Techie Stuff - can be boring ==========
The B777, as is typical of modern twins, has two airconditioning units (packs). These live in the forward wing roots and are driven (note: driven not fed) by bleed air from either the engines or the APU. The packs take outside air and condition it (depending upon altitude) to a suitable temperature and density to be fed into the cabin. The details of zoning and temperature control are not really relevant here. What is significant is how that air is moved around and controlled to achieve pressurisation and flow through the length of the cabin.
In Pressurisation 101 we learn that the cabin is a closed tube which we then pressurise to a nominal 8 to 9 PSI differential (between the inside and outside air). This equates roughly to a cabin environment equivalent to 8000 ft AMSL (Above Mean Sea Level) when the aircraft is at its' maximum certified altitude of 43100ft. Since the 8 PSI diff is maintained throughout the flight envelope, proportionally lower cabin altitudes are experienced when the aircraft is flying below 43000 (which is most of the time - except perhaps when very empty and at the end of a flight). This constant relationship between cabin and outside altitudes explains why the aircraft can descend from 40000 ft in 20 mins (2000 ft/min) and the cabin comes down very gently at around 300 ft/min (6000 / 20).
As was mentioned before - 8000 AMSL sounds a lot, but for a normal person, particularly at rest, it is a non event. If you were not used to 8000ft and you started exercising then you would puff sooner than usual. However as anyone who has been skiing will know it is not a problem, in fact it is probably good for you. The maximum cabin that passengers can be subjected to by law is 10,000 ft (local mileage may vary). As an aside, the "Rubber Jungle" (drop down masks) deploy automatically at around 13,500 ft cabin altitude.
But I digress (as usual) - back to the mechanics of it all. This 8 PSI differential in the fuselage is achieved by a constant INflow of air from the packs and a controlled OUTflow of air through Outflow Valve(s). The 777 has two, the 767 / 757 have 1. When there is one OV it is in the aft body floor and the 777 has the extra one just below the front door on the left hand side. Why has the 777 got two OVs - well it has multiples of nearly everything else - why not. Seriously though it is a large cabin and flow patterns are improved with a pair. It is also not totally unknown for an OV to jam - so there is an added bonus for control. The normal settings are that the aft valve is 80% and the forward valve is 20% of the required discharge. (Doesn't this drag on ...)
============ End of Techie Stuff ==========
All that said was that there is lots of redundancy in the system and even a pax window going out (and I cannot recall an incident of that) will not cause any drastic problems. The outflow valve(s) and packs will most likely take up the slack and at worst produce a gentle cabin climb. Sure the crew will have to do a descent (and these can feel quite steep - but that is the idea after all), and you may end up having a short holiday in a place you have never heard of at the airline's expense - but that is about the worst of it.
Carbon Monoxide : The only place that excessive carbon monoxide can come from is the engines. All but the smallest airlines DO NOT use engine air for cabin conditioning. The air from the engines is used to power the conditioners, but that engine air does not get into the cabin - what you get is what is outside, smells and all. As an aside - if you have a good nose you can smell turbulence. Well not the turbulence as such, but the clouds that often cause it. It goes like this. In clear air at altitude the air is very dry and the filters on the air conditioner packs dry out. As you enter cloud the moisture content increases and some of the dust on the filter screens releases a "dusty" smell. It's not unlike turning on an A/C at home that hasn't been used for a while - and that is very close to the smell. It's subtle, but see if you can spot it next time you fly. Now that there is no smoking in the aircraft (hooray) it is a lot more obvious even above the body odors, cooking smells, deodorants and what have you.
I have droned on for so long - I have forgotten where I am or where I was going with this <g>.
The penultimate issue is the one about time of "useful consciousness" that is often misquoted. Let's try for a few facts. There are lots of tables published for different altitudes and conditions, but who commits this to memory or has time to look it up if it happens. The bottom line is 20 seconds would be a good figure to remember - and that is if everything is in the worse case. The aircraft is very high, the depressurisation is total etc etc. 20 seconds to put on your mask. Just sit there and count out twenty potatoes and imagine taking hold of the mask, pulling to activate and putting it over your face. It may not be wonderfully adjusted, but that is not a problem at all. All you need is a little supplemental oxygen for a few minutes - not a space suit. Your blood is not going to boil or your lungs explode or anything drastic like that. If you have a bad head cold then you are in for an earache - but more likely on the way down (see below) and you will certainly be well on the way for a record breaking "wind" event - but then so will everyone else, so no prizes. There may well be some noise and mist in the cabin. The aircraft will also be starting down on it's descent. All this is pretty expected - almost normal - but it's too easy to say that. There will be much alarm and even some panic which is just what isn't needed, but that is human nature and nothing can change that much.
Lastly then, the descent. When it's all over the pilots have to get the aircraft down to the ground again. This will involve bringing the cabin down much further than normal (6000ft) and there are greater than normal pressure changes. They will do this as gently as possible to avoid giving everyone (themselves included) earache. However the crew are well aware of these problems and will have a remedy. In the worse case the aircraft can be leveled to allow everyone's ears to equalize. The big thing though (should this ever happen) is to speak up early. Young children can be a problem here also, but they will cry when they get uncomfortable and a suckling will nearly always do the trick.
But enough already.
Oh, when you are going flying, worry about the important things. Did you get ripped off on the Duty Free ? Did you put the cat out ? that sort of thing. Certainly don't worry about a depressurisation. The worst that might happen is that you might need a change of underwear <g>.
Finally my tips for a good long range flight :
1. Don't drink alcohol, drink lots of water instead - the cabin air is dry enough to turn your throat to leather and your kidneys to golf balls. Don't make it worse.
2. Always keep your seatbelt loosely fastened, even if the sign is off. Double so for the kids.
3. Don't over eat and stay away from heavy or spicy food.
4. Always carry a wash bag (teethy cleans and a shave as required) and a change of underwear. (See above spicy food and gas).
5. Same as tip #2
6. Sleep with a moist towel near your face (see #1)
7. Loosen shoes, ties and girdles (as required)
8. Take a good book, since the most common "in flight emergency" in my experience is the total loss of the "in flight entertainment system". Or worse I have seen all the movies.
9. Same as tip #5
Relax and enjoy your trip ....
MG.