Relationship Between Carbon Dioxide Levels and Reported Headaches on the International Space Station

OBJECTIVE: Because of anecdotal reports of CO(2)-related symptoms onboard the International Space Station (ISS), the relationship between CO(2) and in-flight headaches was analyzed. METHODS: Headache reports and CO(2) measurements were obtained, and arithmetic means and single-point maxima were determined for 24-hour and 7-day periods. Multiple imputation addressed missing data, and logistic regression modeled the relationship between CO(2), headache probability, and covariates. RESULTS: CO(2) level, age at launch, time in-flight, and data source were significantly associated with headache. For each 1-mm Hg increase in CO(2), the odds of a crew member reporting a headache doubled. To keep the risk of headache below 1%, average 7-day CO(2) would need to be maintained below 2.5 mm Hg (current ISS range: 1 to 9 mm Hg). CONCLUSIONS: Although headache incidence was not high, results suggest an increased susceptibility to physiological effects of CO(2) in-flight.