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Astronauts need to add space headache to their list of occupational hazards, say researchers.
After quizzing 17 seasoned astronauts they found more than two-thirds suffered from headaches on missions yet were headache free back on earth.
The disabling headaches appeared unique - described by the crew as "exploding" - and were generally unrelated to common space motion sickness.
The Dutch investigators work is published in the journal Cephalalgia.
They propose space headache should be classified as a separate entity in its own right.
In the past, experts have thought all headaches in space are a symptom of motion sickness, which is caused by disorientation due to the absence of gravity and plagued the Apollo program.
More than three-quarters of the 21 headaches documented by 12 of the astronauts in the latest study had none of the associated main symptoms of space motion sickness, such as nausea, vomiting or vertigo.
Nine of the headaches were triggered during launch, nine during the stay at the space station, one during activities outside the space station and two during landing.
The researchers believe there are a number of reasons why space travel could cause headaches, the root cause being microgravity.
Microgravity is known to cause lower oxygen levels in the blood and this may be the trigger for space headache.
It can also cause a shift in the body's fluid towards the brain which would raise intracranial pressure - another possible trigger.
Most of the headaches were of moderate intensity, but were bothersome to the astronauts and many needed to take headache pills.
Lead researcher Dr Alla Vein from Professor Michel Ferrari's Headache Research team at the Department of Neurology, Leiden University Medical Centre, said: "Although headaches in space are not generally considered to be a major issue, our study demonstrated that disabling headaches frequently occur during space missions in astronauts who do not normally suffer from headaches on earth.
"Previous research has shown that astronauts can be reluctant to reveal all the physical complaints they experience in space, so the actual incidence could be even higher than our study suggests."