Credit: STS-114 Crew, NASA
International Space Station

Headaches in astronauts on the International Space Station (ISS) are attributed to elevated levels of carbon dioxide (CO2), reports a study in the May Journal of Occupational and Environmental Medicine, official publication of the American College of Occupational and Environmental Medicine (ACOEM).

Led by Dr. Jennifer Law, Flight Surgeon at NASA Johnson Space Center, the researchers analyzed data on CO2 levels and headaches reported by ISS crew members between 2001 and 2012. The results suggested that headaches were more frequent during periods of higher CO2 levels.

For each 1 mm Hg (millimeter of mercury) increase in CO2, the odds of headache reported by crew members doubled. Headaches were less frequent in older crew members and those with more days in flight.

Higher CO2 levels also appeared related to other symptoms, such as sleep problems, fatigue, and irritability. After steps were taken to keep CO2 levels on the ISS at 4 mm Hg or less, the risk of headache decreased from about 3.3 to 1.6 percent per week.

To keep the risk of headache below one percent, the average CO2 level on the ISS would need to be less than 2 mm Hg — which isn’t practical using current systems. The CO2 level on Earth at standard pressure is 0.3 mm Hg, although healthy people can tolerate higher levels of CO2 without adverse effects.

The new results suggest that headache can occur at CO2 levels well below those historically regarded as safe for space flight. The study supports current efforts to maintain CO2 levels on the ISS at or below 4 mm Hg. Meanwhile, NASA researchers are performing further studies to assess the health effects of short- and long-term CO2 exposure, as well as to set new safety limits and hardware requirements.

Citation — Law J, Van Baalen M, Foy M, et al. Relationship between carbon dioxide levels and reported headaches on the International Space Station. J Occup Environ Med. 2014; 56(5):477-83.