Because of anecdotal reports of CO2-related symptoms onboard the International Space Station (ISS), the relationship between CO2 and in-flight headaches was analyzed.
Headache reports and CO2 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 CO2, headache probability, and covariates.
CO2 level, age at launch, time in-flight, and data source were significantly associated with headache. For each 1-mm Hg increase in CO2, the odds of a crew member reporting a headache doubled. To keep the risk of headache below 1%, average 7-day CO2 would need to be maintained below 2.5 mm Hg (current ISS range: 1 to 9 mm Hg).
Although headache incidence was not high, results suggest an increased susceptibility to physiological effects of CO2 in-flight.