Time spent in space appears to have a dose-response relationship with acute and chronic reductions in hemoglobin (Hb), also referred to as “space anemia,” according to study results published in the American Journal of Hematology.

The study took advantage of accumulated data on human space flight from all American and Canadian astronauts (301 individuals) and matched controls (964 individuals) between 1968 and 2015. Missions were grouped by duration: short (fewer than 8 days), medium (8-21 days), and long (more than 21 days). Most astronauts had a blood sample collected within 3 days of returning to Earth. These samples were compared with the most recent premission blood draw for each astronaut. Additional blood samples were collected during routine medical care over time but had no consistent timing pattern.

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In total, 17,336 Hb concentration measures from controls and astronauts from 721 space missions were used to study acute and long-term effects of duration of exposure to space on Hb concentration.

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After space missions, 12.9% of astronauts had anemia in at least 1 postflight blood draw, including 7%, 10%, and 48% of astronauts returning from short, medium, and long missions, respectively.

Hb decrements were dependent on duration of exposure to space. In the short duration group, preflight Hb was reduced by 4.1% (-0.61 g/dL) after a mean exposure to space of 5.4 days. In the medium duration group, preflight Hb was reduced by 5.4% (-0.82g/dL) after 11.5 days in space, and in the long duration group, preflight Hb was reduced by 10.9% (-1.66g/dL) after 145 days in space.

After returning from space, Hb concentrations for astronauts in the short duration group took 24 days to return to preflight levels; for astronauts who spent between 11.5 to 145 days in space, Hb concentrations took 49 days to return to preflight levels.

After space travel ceased, statistical models predicted that reduction in Hb concentration persisted throughout astronauts’ Earthbound lives for each day spent in space (women, -0.001 mg/dL per day in space; male, -0.002 mg/dl per day in space; P <.05 for both).

“The effects of space travel have renewed importance with space tourism and plans for long-term missions to the moon and Mars,” wrote the investigators. “This epidemiological analysis of 5 decades of human space data overcame limitations of small sample sizes of past experimental studies. Whether acute space anemia will turn into chronic anemia depends critically on the duration of exposure to space.”


1.     Trudel G, Shafer J, Laneuville O, Ramsay T. Characterizing the effect of exposure to microgravity on anemia, more space is worse [published online December 9, 2019]. Am J Hematol. doi:10.1002/ajh.25699