Looking at the association between Apollo astronauts and cardiovascular disease- is it a good thing?

Posted on February 1, 2017

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Space is awesome. Going to Mars is awesome. It’s important to do. I’m not going to make an argument why, but you can see one here.

A problem for human spaceflight is countering the negative impacts of moving in an environment where gravity is less than Earth’s. This is the predominant impetus for some space posts I’ll have, as there is a ton here which can be applied to everyday people on Earth, and vice versa.

I want to stress, even if you’re not interested in space, this can be valuable to learn about. This post will show how much exercise can decrease your risk of death.

The below study got a good amount of publicity in summer 2016-

Apollo Lunar Astronauts Show Higher Cardiovascular Disease Mortality: Possible Deep Space Radiation Effects on the Vascular Endothelium

When looking at astronauts who have flown out of low Earth orbit, only a few Apollo ones have, compared to astronauts who have not, the far flying Apollo ones were found much more likely to die of cardiovascular disease. Theory being the further from Earth we go, the less protection we have from radiation, bad stuff may be more likely.

An animal study backed up the potential harmful effects of radiation on the vascular endothelium.

Inner lining of a vessel.

Inner lining of a vessel.

After going through the full text and various press releases, some points I haven’t seen addressed:

Sample size schmample size

Many press releases were quick to point out the small sample size, as the study’s authors did, but small sample size is what every space study is. So it’s worth considering. You go with the best information you have. We can’t just dismiss this. If you’re involved in human spaceflight planning, what do you do? “The sample size wasn’t very big. Let’s ignore it and hope for the best.” or “This is a possible risk factor. What can we do to mitigate it?”

Granted, spaceflight has seemed significantly hampered by these ifs. At some point we need to be like Nike.* Not every contingency can be known or planned for. Which is why we’ll get to a mitigation technique we should already be implementing.

*Just do it baby

The big bugaboo wasn’t an issue? (the positive of the study)

Any time deep space radiation is mentioned as a hazard The Big C is not far. But Apollo astronauts, and low Earth orbit ones, did not suffer greater cancer deaths:

cardiovascular-cancer-deaths-astronautrs-low-earth-orbit-apollo

If we’re going to pay attention to the increase in cardiovascular issues then we need to pay attention to the lack of increase in cancer deaths, which is what everybody has been worried about! We’re talking a five percent lesser cancer death rate compared to the average population. Low Earth orbit flyers, with a much bigger sample size, even experienced a 3% lesser rate.

One reason this is a potential positive is cancer is more likely to hit at any time in life. We’ve all heard of the 32 year old woman, in-shape, healthy lifestyle, who gets breast cancer. Testicular cancer is known to hit younger males. Meanwhile, I’m not sure any of us have heard of the 32 year old in-shape, healthy lifestyle, heart attack due to cardiovascular disease / blocked arteries. (The marathoner who drops dead is different.) We have a whole category of cancer called pediatric oncology. While we have pediatric cardiology, how worried are we about 15 year olds dying of e.g. high blood pressure?

It’s not until 65 heart disease surpasses cancer. 

If we’re trying to minimize the risk of traveling astronauts but have to deal with some risk increase, you’d rather have an increase in a disease that tends to hit older people rather than one that’s more likely to hit younger people. If you’re going to increase the risk of getting the flu, you’d prefer to do that in younger people. They’re more resilient to it and recover from it better. Younger people are significantly more resilient to heart disease than they are cancer. We’re talking 1.5 – 4x differences in the charts above. If you have a 3% increase in disease risk, 3% of the cancer numbers is bigger than 3% of the heart disease numbers.

Another aspect is cardiovascular issues are, at least up to a point, easier to treat. Change the diet, pop some pills, survive a heart attack with no intervention (another advantage of youth), those are much easier to do than remove a tumor, chemotherapy, radiation, survive cancer with no intervention (age doesn’t seem to care here). If nothing else there is a lot less equipment to bring => less mass => less $$$. We can do some heart examinations with a phone nowadays. We can’t look for a tumor with a MRI on a phone though.

Positive #2

I’ve written how women would be cheaper to send into space than men. Perhaps this has caught on someway, as now people are asking noted NASA employees about it. The pushback being women don’t handle radiation as well as men, so they are at increased risk of cancer, thus they can’t fly on as many missions and may prove more expensive over the longterm.

While women vs men was not delineated in this study, it gently pushes back against the notion of increased cancer risk, as the men, what astronauts mainly are (Apollo was only men), did not have any increased risk. ALL the astronauts -at least in aggregate- experienced lesser cancer risk.

-> The authors bring up some good points why this might be. Astronauts are better educated, make more money and are in better shape than the average population. When we try to deduce astronaut cancer risk, have we been taking these mitigating factors into consideration? It would appear no.

It also seems worth being sensitive to how convenient this argument is for what has been a male dominated endeavor. For instance,

“In comparison to NASA limits, the US nuclear industry has adopted age-specific limits that neglect any gender dependence.”

NASA’s Radiation Risk Acceptability And Limitations.

The Europeans, Russians and International Commission for Radiological Protection don’t differentiate between gender either. Maybe NASA is truly looking out for the welfare of women. Or maybe they’re making an unnecessary distinction that could hamper them?

We’d have to be in spaceflight enough for these limits to matter anyways, which is where the argument becomes bogus. “could be more expensive over the longterm.” Shouldn’t this be calculated then?? You can’t read a comment from NASA without hearing what they could do if only funding were better, yet our most prized asset -astronauts- and we don’t know what minimizes their cost?

Apollo 14, a moon landing, experienced

1,400 mrem for the skin over nine days.

This is by far the most radiation absorbed Apollo mission, despite not the most time on the lunar surface (for reasons unbeknownst to me, but I’d guess something with proximity to the sun at time of landing)-

Three missions had more than double the time on the lunar surface, yet less than half the radiation dose.

Three missions had more than double the time on the lunar surface, yet less than half the radiation dose.

So we’re overestimating here. That’s not a bad thing to do in these situations. Anyways,

1,400 mrem = 0.014 sieverts

0.014 Sv / 9 days = 0.0016 Sv per day for deeper space travel

The lifetime limit for women and men differs by roughly 27.5%.

lifetime-radiation-limit-sv-men-women-nasa-astronauts

33% difference on left down to 25% difference on right.

A limit of 6 Sv for the skin is listed here, presumably for men. 27.5% less would be 4.35.

6 Sv / 0.0016 sv per day = 3,750 days for men

4.35 Sv / 0.0016 sv per day = 2,719 days for women

Like this is at all relevant to contemporary space travel. That’s almost seven and half years deep space travel vs just over ten. We can’t even get to the moon anymore, we currently can’t do any deep space travel with humans, but we’re worried about being there for seven or ten years? For a presumed 3% increase in cancer mortality risk, which is based on ground workers, doesn’t take into consideration the superlatives of the demographic we’re looking at (astronauts), not backed up by the current study we’re looking at, and for a demarcation no other radiologically concerned entity appears to make.

Even if we look at effective dose, where non-skin organs have a ~30% lesser maximum radiation dosage allowed,

nasa-radiation-dosage-limits-by-organ

Using a blood forming organ limit of 4 and 6 for the skin => 4/6 = 0.66

lessening the days by 30% and we’re still talking thousands of days.

If we go strict 1 Sv limit: whatever hits the skin looks to hit the blood forming organs with less intensity. (Presumably particles have to travel further to hit the marrow than the skin, so not as many get there.) The skin is at 4.5, but bone marrow is at 3.4:

organ-radiation-exposure

Space Radiation Organ Doses for Astronauts on Past and Future Missions (https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070010704.pdf)

At 25% less for a blood forming organ, even with a 1 Sv career limit, using our high Apollo 14 reference,

0.0016 Sv per day * 0.75 = 0.0012 Sv per day

1 Sv / 0.0012 Sv per day = 833 days

About 2.3 years in deep space. Again, completely irrelevant for what we’ve done, what we’re currently doing, and the foreseeable future. Apollo astronauts spent an average of ten days in deep space. When we start doing 100 moon trips in one person’s adult years, this’ll be relevant. If you’re e.g. a NASA astronaut, if you go to Mars once, that’s probably it. Nobody went to the moon twice, and that’s years less traveling than Mars. If you’re a regular person, like SpaceX is trying to do, then you’re paying for it yourself.

Speaking of Mars, if we look at a three year round trip, the radiation dosage would be 1.2 Sv. Close enough to our 1 Sv limit (guess). Maybe dying of cancer years after landing on Mars is going to be the least of concerns. One many are already volunteering for.

Eye concerns are currently a big priority. Vision issues are happening with six month journeys on the International Space Station. Women are less susceptible than men.

women vs men space travel

What’s more important? Picking people who are known to have less risk of eye issues or worrying about an unknown small increase in cancer risk years later?

One theory women are less susceptible: they have healthier cardiovascular systems. Bringing us back to our first positive- if we’re going to increase the risk of a disease, we’d prefer to do it in the population which is more resilient to it. Women are more resilient to heart disease than men. Women are more resilient to aging and death to begin with.

NASA already knows radiation isn’t an issue right now,

“Because space missions have been relatively short in the past requiring minimal mitigation consideration, the impact of dose limits when space programs actually approach such boundaries […] has been unexplored.”

Perhaps when we’re living on Mars we can talk more about this. But in that case women are going to be a necessity, and we would want, if not need, more of them than men anyways. One man can impregnate endless amounts of women. One woman can’t handle endless amounts of births. You want to start a colony? Those who can give birth take priority. (One of the premises of Seveneves.)

But radiation exposure was similar?

The authors go into detail how the low Earth orbit astronauts actually had comparable radiation exposure to the far flying Apollo ones. They make up for the lack of intensity (they’re closer to the Earth) with longer durations in-flight. Yet there is still the discrepancy in cardiovascular deaths. They give some plausible reasons, such as maybe radiation exposure isn’t what we think it was on e.g. the moon. However, there is one other factor they did not mention.

To get an appreciation for this we’ll look at the animal study. Mice were made to simulate a space trip. They were either exposed to radiation, hindlimb unloaded (~weightlessness), or both.

TBI + HU is radiation + hindlimb unloaded

While not statistically significant -another study with a small sample size- in nearly every instance being hindlimb unloaded exacerbated the effects of the radiation. (The mice study was only two weeks long too. Maybe not long enough to hit significance?)

-> In a related study it’s discussed how the combination does impair recovery from bone / muscle pathology, changes the mechanism of vasodilation, and the type of radiation may be a factor too.

Effects of Hindlimb Unloading and Ionizing Radiation on Skeletal Muscle Resistance Artery Vasodilation and Its Relation to Cancellous Bone in Mice

The far flying Apollo’s were compared to Mercury, Gemini, Apollo-Soyuz, Skylab, and Shuttle astronauts. For Gemini, Skylab and the Shuttle, either exercise was done or space walks happened, which are more or less exercise themselves. Is one reason the non-far flying astronauts did better cardiovascularly because they were more skewed towards exercise?

Apollo of course had space walks too, but what we’re saying here is the ratio of exercise to radiation exposure was greater on other missions. Say Gemini and Apollo had roughly one hour of space walking for every day of spaceflight. Because Gemini wasn’t as far away from the Earth as Apollo, the ratio of exercise to radiation would be greater.

Then for Skylab exercise was done in the station, and for the shuttle (a quick look says) there was mainly either more spacewalking for each day of flight, or it was back to the ~hour per day of flight, but in low Earth orbit, which would still enhance the ratio compared to Apollo. OR the astronauts stayed on the Mir station, where exercise was also done.

Said another way, relative to total exposure, did Apollo astronauts partake in a radiation countermeasure the least amount? It would appear we don’t need a thorough analysis to say yes. Skylab and Mir -space stations where exercise was done- comprise 21 years of space flight missions. They’re a big part of the sample.

As far as being a countermeasure, there are innumerable papers on the benefits of exercise with vascular endothelial dysfunction (or maybe it’s 194,000):

Mitigating radiation exposure always reverts to shielding, needing to live underground once on Mars, and so on. That’s fine, and will probably be done in the longterm, but they’re expensive and difficult measures. And useless for those who end up back on Earth. You’re not going to spend the rest of your Earth days underground, but you can spend them exercising.

The research isn’t clear, but it’s trending a certain direction, where exercise should be more talked about -we know the better in shape you are the less likely you acquire cardiovascular issues- as should potential for mitigating the effects of radiation exposure. What if mitigating exposure isn’t needed like we think? It’s like when Thanksgiving dinner rolls around. Focusing on mitigating the calories in the meal isn’t very tenable. Mitigating the effects of the meal are though. You exercise more / eat less that week as one example.

Note the mice in this study were hindlimb unloaded. That’s not a full weightlessness analog, because the mice were still getting some loaded movement with their upper body. Would they have been worse off if they were fully weightless?

Exercise is an easier method to assess cardiovascular function. And perhaps a better one, as the American Heart Association is proposing it should be part of a checkupand a vital sign.

Somebody’s 5k time has been going up? Or they aren’t where their age group dictates? Perhaps that can supplant something like an angiogram while in space or on Mars. We’re not going to be able to give these people the same level of technology as on Earth; do we even need to?

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