This article is from the Space FAQ, by Jon Leech leech@cs.unc.edu and Mark Bradford tla@surly.org with numerous contributions by others.
If you *don't* try to hold your breath, exposure to space for half a
minute or so is unlikely to produce permanent injury. Holding your
breath is likely to damage your lungs, something scuba divers have to
watch out for when ascending, and you'll have eardrum trouble if your
Eustachian tubes are badly plugged up, but theory predicts -- and animal
experiments confirm -- that otherwise, exposure to vacuum causes no
immediate injury. You do not explode. Your blood does not boil. You do
not freeze. You do not instantly lose consciousness.
Various minor problems (sunburn, possibly "the bends", certainly some
[mild, reversible, painless] swelling of skin and underlying tissue)
start after ten seconds or so. At some point you lose consciousness from
lack of oxygen. Injuries accumulate. After perhaps one or two minutes,
you're dying. The limits are not really known.
An expanded discussion of this issue, citing several case studies, may
be found at
http://www.jsc.nasa.gov/sa/sd/intro/vacuum.html
References:
"The Effect on the Chimpanzee of Rapid Decompression to a Near Vacuum",
Alfred G. Koestler ed., NASA CR-329 (Nov 1965).
"Experimental Animal Decompression to a Near Vacuum Environment", R.W.
Bancroft, J.E. Dunn, eds, Report SAM-TR-65-48 (June 1965), USAF School
of Aerospace Medicine, Brooks AFB, Texas.
"Survival Under Near-Vacuum Conditions" in the article "Barometric
Pressure," by C.E. Billings, Chapter 1 of "Bioastronautics Data Book",
Second edition, NASA SP-3006, edited by James F. Parker Jr. and Vita R.
West, 1973.
 
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