I read that a passenger of MH17 was found with an oxygen mask.
Is it possible to manually release just a single mask in case someone is having a medical condition like an asthma attack?
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Is it possible?
Yes it is, in most airplane models a pointed object (a pen or so) to be inserted in the small manual release opening in the oxygen mask compartment will release that specific mask.
Are they released manually for sick passengers?
No they are not, the seat oxygen masks are not meant for medical reasons, only for emergency reasons, namely decompression. Most of modern passengers oxygen mask systems generate oxygen chemically, each unit is separate and generates oxygen independently from others by an attached chemical device for ~10-20 minutes, which is enough time for planes to reach below 14,000 feet in case of decompression. In older airplanes, a master oxygen tank distributes oxygen to passenger masks.
What about passengers who need oxygen during the flight?
For sick passengers who need oxygen due to sudden sickness such as asthma, portable oxygen bottles are used upon request from cabin crew. These bottles have compressed oxygen and can produce oxygen for long times, usually more than one hour. There are more than one bottle of these portable oxygen bottles in each airplane, the number of these bottles usually is as much as the number of the crew for that plane model, so do not worry there is always enough oxygen.
For people with chronic problems and who are in need of oxygen at all times, they will be provided with a larger oxygen bottle, they are usually seated behind bulkheads where oxygen bottles can be installed in advance for that specific passenger, you can usually request this from the airline during reservation.
Regarding the MH17 passenger with the mask
Out of personal experience, a severe turbulence can open the oxygen mask compartment, I guess in the case of MH17 which was something much more stronger than turbulence it is highly possible that the oxygen mask opened due to the missile impact and somehow he/she managed to put it on.