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NASA's Steve Swanson works on the Carbon Dioxide Removal Assembly on the International Space Station in April 2014. (NASA)

Why Living in Space Can be a Pain in the Head

A new study says carbon dioxide levels on board the station should be lowered.

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NASA has been studying the health of astronauts living on the International Space Station for more than a decade. Now one commonly reported ailment -- headaches -- has been linked to high carbon dioxide levels on the station, according to a paper just published in the Journal of Occupational and Environmental Medicine.     

NASA flight surgeon Jennifer Law and her co-authors scoured reports from astronauts’ private video conferences with crew surgeons, as well as medical team meetings. They found that headaches are commonly reported early in a mission, when astronauts are adapting to spaceflight. But headaches reported after the first week are assumed to have other causes. The team was able to study more than 1,700 reported and non-reported observations of headaches since the first carbon dioxide monitors were activated on the station in February 2001.

Due to the difficulties in “scrubbing” an artificial atmosphere, carbon dioxide levels on the station have always been higher than they are on the ground, but NASA always assumed the levels were within the non-harmful range. Between 2001 and 2012, carbon dioxide levels on the ISS typically stayed between 2.3 and 5.3 mm Hg (a measure of concentration); those levels, however, sometimes change fairly quickly over hours or days.

On Earth, carbon dioxide is a trace molecule in our atmosphere, at a low 0.3 mm Hg. In space, exhaled CO2 builds up in the cabin, and would even linger in a cloud around the astronauts’ heads were it not for fans that circulate the onboard air. Tests on the ground have shown that healthy males have no ill effects when breathing a CO2 concentration up to 7.5 mm Hg, and can handle as much as 11 mm Hg for up to eight hours with no acute effects, like headaches.

Law explains, though, that being in space can complicate things:

In microgravity, blood does not drain from the head as well as what we’re used to in 1 G, and this in itself causes an increase in intracranial pressure.  (Picture what happens when you stand on your head.)  We know that CO2 is a potent vasodilator (which dilates the blood vessels), so it also can increase intracranial pressure. Thus, microgravity and CO2 may be working in synergy to cause headaches.

Astronauts have reported headaches and other carbon dioxide-related symptoms since the earliest days of the space station. One report stated:

Last week, the crew reported that they were not aware of the high CO2 levels. After further discussion, however, they reported that they noticed a big difference when the [carbon dioxide remover] was turned back on and the levels went down. [Two crew members] stated that levels as high as 6.0 [mm Hg] were incompatible with performing on-board activities. The crew members reported sleep difficulties and increased irritability, which was noticed by ground controllers.

There may be even subtler effects. In our 2011 Air & Space Special Collector’s edition on the Space Shuttle, Expedition 6 commander Ken Bowersox recalled watching the memorial service for Columbia’s crew while on the station: “For me, emotions were more visceral in orbit. I don’t know why. I don’t know if it was the composition of the air, the carbon dioxide levels, or the isolation. If I was happy I could feel it just bubbling out of my chest. And if I was sad, it was crushing.” Law says she hasn’t heard of elevated emotions being associated with elevated carbon dioxide, “but I suppose it’d be on the same spectrum as irritability and cognitive effects.”

The researchers found that the incidence of headaches was the same for men and women, and, interestingly, that the incidence went down as an astronaut’s age went up. “As we get older, our blood vessels get stiffer, so CO2 does not dilate the blood vessels as much in an older person,” says Law.

When flight surgeons began to see the connection between elevated carbon dioxide and headaches, they asked that carbon dioxide levels be lowered temporarily -- to 5 mm Hg in 2006, and to 4 mm Hg in 2010. (Law says that work is ongoing to update NASA flight rules to 4 mm Hg permanently). That apparently has worked; with each lowering, the number of crew headaches goes down.  

Law’s paper is the first serious look into the subject, and her team’s recommendation is to go even lower, to 2.5 mm Hg. They found that “for each 1-mm Hg increase in CO2, the odds of a crew member reporting a headache almost doubled.” Their recommended level of 2.5 mm would, according to the paper, “keep the risk of headache to below 1%, a standard threshold used in toxicology and aerospace medicine.”

But that’s easier said than done. As Law says, “Today CO2 on the ISS is generally controlled to 4 mm Hg or less. While crew surgeons would love to keep the CO2 levels lower to protect our crewmembers, the reality is we are limited by the hardware we have onboard.  Back when the ISS was designed, it was thought that higher CO2 levels were no problem, so the hardware was designed for higher levels. Trying to bring the levels lower would require running the scrubbers more, which would wear them out faster and require more maintenance and consumables.”

She also says that doctors need to learn more about other possible long-term effects of breathing elevated carbon dioxide, such as changes in vision. “We don’t know yet what CO2’s role plays in the Visual Impairment/Intracranial Pressure (VIIP) syndrome, which has been documented in a number of long-duration crewmembers and, in some cases, resulted in ocular changes that seem to persist for years,” says Law. “Headaches may just be the tip of the iceberg – while headaches are relatively easy symptoms to identify, there are likely subclinical changes to physiology that we can’t measure.”

“We are concerned about their effects on astronaut health and possible implications for exploration missions in the future,” says Law. So reports like this should improve the health of future astronauts wherever they’re going, whether to Earth orbit or beyond.

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