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Under the current limits, set by NASA in 1989, the effective dose limit for an astronaut's career is based on a maximum 3% lifetime excess risk of cancer mortality. That risk is evaluated with a sliding scale based on age and sex, ranging from a lower career limit of 180 millisieverts (mSv) of radiation for a 30-year-old woman to an upper career limit of 700 mSv for a 60-year-old man.
So why is there a lower career limit for radiation exposure for female astronauts than for male astronauts?
According to R. Julian Preston, a special government employee with the U.S. Environmental Protection Agency's Radiation Protection division, NASA's lower radiation threshold for female astronauts was based on the following finding: When women and men were exposed to high levels of radiation for similar periods of time, women had more than twice the risk that men did of developing lung cancer. //
"It has been generally considered — based largely on the survivors from the atomic bombs in Japan — that, particularly for lung cancer, that women were more sensitive" to ionizing radiation than men were, Preston, who serves on committees for the National Council on Radiological Protection and Measurements, told Live Science. //
However, NASA's radiation thresholds are expected to change in the near future. In 2021, NASA asked a panel of experts convened by the National Academies of Sciences, Engineering and Medicine to assess the space agency's plan to change its career radiation limit to 600 mSv for all astronauts of all ages. NASA determined that limit by applying the agency's cancer risk model to the most susceptible individuals: early-career women. NASA calculated the average risk of exposure-induced death for this group and converted that risk, which allows for a much larger margin of error than previously, to a dose. That 600-mSv dose translates to the exposure an astronaut would receive during four six-month expeditions on the ISS. For comparison, the average annual dose of radiation received by a person on Earth is about 3.6 mSv, according to NASA, versus 300 mSv per year on the ISS.
