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There are several questions related to effects of different gravity levels on the human body, but none adequately answer a fundamental question: What is the maximum and minimum gravity that nearly all humans can survive under and perform all functions without side effects?

This answer says that many humans would receive significant harm in a 1.5G environment over a six month period and this answer claims that 1.5G would not harm healthy humans over a seven day period. A third answer claims that Earth's gravity is the only gravity level that humans can tolerate over a long term period, yet I would imagine that 0.99G or 1.01G would not be much different than 1G; I can't find an answer citing the gravity levels that all humans can tolerate without any side effects over a long term period.

To satisfy the requirements, the following terms must be met:

  • At least 95% of humans would not have any severe long-term effects that they would not experience on Earth caused by the different gravity level over a period of 5 years
  • All humans can survive in the different gravity level over a period of 5 years
  • Tasks essential for living in a society should not be interrupted by the different gravity
  • All other environmental factors are the same

This does not require that machines operate correctly in the different gravity level, only humans.

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    $\begingroup$ I'm not sure any symptoms is reasonable. A heavier world, for example, may lead to many sore evenings for a while as humans adapted. A lighter world might lead to bumped heads for a while; darn those low ceilings! Consider instead a standard like 'no additional long-term effects', though that would seem to keep your window of habitability fairly narrow. Some authors postulated survival-of-the-fittest, where most of the original colonists don't live long, but (some of) their children survive and thrive. $\endgroup$ – user535733 Mar 31 at 2:13
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    $\begingroup$ I'm pretty sure the answer is that we simply do not know all that well. Our proper experience with different gravity levels is measured in days, and while we largely know what happens in zero gravity given experiences on the ISS(nothing good and plenty of bad), we don't really know what happens with a different amount of gravity long term. $\endgroup$ – Adam Reynolds Mar 31 at 2:15
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    $\begingroup$ @user535733, I meant that nearly everyone would survive, not just a few people. It would be similar to a human moving from one place on Earth to another place with the most severe effects being no worse than altitude sickness. $\endgroup$ – Galactic Mar 31 at 2:25
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    $\begingroup$ There is definitely a range... otherwise you could not survive moving from somewhere below sea level (e.g. Holland, or living under the sea) to, say, the Himalayas. 9.80665 m/s² is a "standard", but just like STP, isn't actually found everywhere (note). $\endgroup$ – Matthew Mar 31 at 14:33
  • $\begingroup$ If you would like plausible guesses to something on which there can be no actual science, you’ll need to remove the science-based tag. $\endgroup$ – Mike Scott Apr 2 at 10:57
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We don't know.

You are asking to know something which is similar to the medial lethal dose $LD_{50}$.

The value of $LD_{50}$ for a substance is the dose required to kill half the members of a tested population after a specified test duration.

In this case you are asking for the $LD_5$ at 5 years.

Why we don't know?

To make this kind of determination we would need to test various level of gravity on a lot of people to have a statistically significative sample, with different gravity levels, and have these people live in those conditions for 5 years.

We don't have the facilities to perform such extensive studies, and those we have in place have been used to test subjects akin to astronauts, which are nowhere representative of the 95% of the population. If ever, they represent the 5% of the population.

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    $\begingroup$ +1, we have some educated guesses about the minimum levels (it was studied for developing spin gravity systems) but all values for the maximum are AFAIK just arbitrary numbers that sounded right. Other than the usual "If Stapp didn't do it, it will kill you" clause. $\endgroup$ – Ville Niemi Mar 31 at 5:18
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    $\begingroup$ Also, most (all?) of the current data involves grown adults. For obvious reasons there's been little incentive and even less opportunity to look at how children live and develop in a low- or high-G environment. $\endgroup$ – Cadence Mar 31 at 6:39
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    $\begingroup$ @Cadence Inner kinder grow better down the well, sa sa que? No need for to try growing on the float full of pharma. $\endgroup$ – Joe Bloggs Mar 31 at 8:03
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    $\begingroup$ Agreeing with this answer. We know that 0g is bad. We know that 4g is bad if prolonged for more than a few minutes. Beyond that, we have no idea. $\endgroup$ – CAE Jones Mar 31 at 8:58
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    $\begingroup$ Not a single 5-year sample for microgravity exists, even among those who have been stationed in the ISS. $\endgroup$ – Mad Physicist Mar 31 at 21:43
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We know that anyone with osteoporosis is prone to bone breakage even in a 1G field, but unless we're dealing with a very heavily aged population (due to reduced birth rate for population control?) that won't encompass anything like 5% of the population. We also know that beyond about 3G even normal folks in good condition will have similar problems -- back injuries just from walking around, for instance, and rapid knee degeneration.

The effects of high gravity would be similar in many ways to morbid obesity; in fact, you could make a case that non-endocrine pathologies related to obesity can serve as models for a high G environment -- that is, living in 2.5 or 3 G won't make you diabetic (seemingly), but it's likely to cause most of the same other problems as if you weighed two and a half or three times what the health community believes you should.

We also know that (nearly?) everyone will suffer bone and muscle loss in prolonged microgravity -- even with daily vigorous exercise. There are other effects from microgravity, as well, even less amenable to countering without artificial "gravity". Here, prolonged bed rest has been used experimentally as a stand-in for microgravity, because of observations that astronauts suffer some of the same issues as bed-bound medical patients.

What level of reduced gravity is modeled by lying in bed is in question, but the studies I've seen via headlines seem to suggest that it's below 0.5 G, possibly as low as 0.2 G equivalent. Don't take this as gospel, however; I haven't specifically researched this and I don't work in any related field. Prolonged bed rest promotes loss of bone mass and muscle strength, as does prolonged microgravity, in virtually all test subjects.

So, based on the limited data in hand, we can reasonably claim that any G field above 2 G is likely to cause problems over the long term for nearly all subjects, and fields below about 0.2 G as a lower limit are likely to cause problems for a significant fraction.

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As L.Dutch stated, we don't know a whole lot. We know that for top physical specimens, 0 g has serious effects, but nothing lethal so far. But we can hardly expect that to be entirely representative for the entire population.

So what do we know? Well, gravity on earth isn't exactly the same everywhere. You may have been taught that g = 9.81 m/s2, 9.8 m/s2 or 9.79 m/s2. None of these are wrong, but except for 9.8 m/s2, they are overly specific. Gravity may differ by as much as 0.02 m/s2 across the surface of the earth.

So there's what we do know. $$LD_5 \geq \pm0.02 m/s^2 $$

This remains true when changing 'Lethal' to 'Serious effects', though there are probably obese people who would do better in lower gravity.

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    $\begingroup$ A fair number of people are killed by gravity; I wouldn’t be surprised if more people survived in total at 0.9g. $\endgroup$ – Tim Apr 1 at 1:57
  • $\begingroup$ no effects on adults in zero G, but reproduction would be impossible. Zygotes need gravity to orient before dividing properly . $\endgroup$ – John Apr 3 at 3:48
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    $\begingroup$ "No effects" is a significant understatement. Our bodies are very much adapted for upright living in gravity. Taking away that gravity causes loss of bone and muscle mass, changes the shape of your eyeballs and causes fluids to build up in the upper body (because your cardiovascular system is essentially built for pumping them up all the time). None of these are necessarily a dealbreaker, but they do need to be dealt with. As for zygotes, do you have a source for that? I wouldn't expect cells floating in liquid to notice gravity at all. $\endgroup$ – AI0867 Apr 4 at 11:00
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The question is difficult (impossible) to answer both because no scientific data exists, and because of how it is worded.

There exists some data which could be extrapolated, but it has a small sample size, was collected from a set of carefully chosen individuals, and the effect is intermixed with other effects (e.g. radiation, stress, air composition, nutrition). Thus, it is not possible to generalize to "nearly all humans", nor to blame any observed effect on gravity without the possibility of other factors also having an influence (unlikely as it may seem, but truth is we simply don't know, we can only assume what's likely).

Reduced gravity most probably predominantly leads to bone demineralization and muscle atrophy (and possibly other things). That's because this is what people who are (among some other effects) exposed to reduced or zero gravity demonstrably develop. There may of course be other reasons for that effect, but gravity is the likely factor. Exercise can at least partially counter these effects, but exercise is not a premise of the question.

If, what's very likely, gravity is indeed the deciding factor (and not e.g. NASA's bad nutriention scheme) simply because less "pull" means less stimulus, then the effects will likely occur at every significant reduction. Whether or not it counts as "not tolerable" is up to your decision, so it's fundamentally impossible to provide a number. Is it tolerable to have 5% less strenght and bone density? What about 10%? Who can tell. For a boxing champion or a rugby player it sure wouldn't be. For a computer geek? Why not.

Seeing how "return to normal environment" is not a premise of the question (only "compared to" is), though, one might as well say "zero gravity is perfectly sustainable" since you do not need (shouldn't need?) strong muscles and hard bones in zero-G. Basically, it would mean: Yes, there will definitively be serious changes, but they aren't a problem.

The same goes on the other end. Something like 4-6G will without doubt cause noticeable discomfort, and rather soon evolve into problems. However, 1G or 1.5G is not really much of a difference, nor is 2G. Only just, everything is a tidbit (tidbit, eh?!) heavier. You'll grow more muscle and denser bone, tripping and falling will hurt considerably more, and you'll need more energy. It'll be generally harder, your heart and your circulatory system needs to support higher pressure, but all in all, there's not really that much difference. Sure, more load on your spine, your knees, and on your inguinal canal. All in all, that's not precisely enhancing durability or longeviety.
But there is no real reason why you couldn't live in 2G for 5 years, if you are reasonably healthy. Mind you, there's people who have 150-180 kilograms of body weight (or more?!), and some of these carry that around for 30-50 years. Do they eventually have problems? Well sure, but not after 5 years.

However, "nearly all" includes infants and people who are 95 years old and suffer from heart insufficiency and COPD, so... it may be troublesome to make a claim!

A baby in zero-G will be "fun" when gooey, stinky stuff comes shooting out of it at both ends. Which tends to happen regularly. A "typical" 95 year old will not be very happy in 2G, at least not for long. Someone with vein insufficiency will not be very happy either -- them thick legs getting twice as thick now. Allergic and going to zero-G? Well guess what, sneezing all day long can be a lot of fun!

Speaking of infants, it is not certain in any way inhowfar growth in children (or animals in general) is influenced by low gravity.
Generally, data about animals in space is relatively sparse, and in my opinion the conclusions that are made are dangerously naive. For example it is not appropriate to extrapolate from mouse experiments based on "they have short life spans, so this is a long-time mission for them" as has been done on the ISS. While it is true that their lives are shorter, it does not mean that they automatically experience long-term effects faster because of that. Sometimes, this "logic" holds, but sometimes it doesn't.
Medical research is full with examples of this fallacy. Such as the famous longeviety experiments where a few years ago someone got "immortality, soon!" for humans because he managed to extend the lifespan of some bug from one day to two weeks or so. Which is great, but it's also entirely meaningless.

Acceleration on Earth is constant within what's reasonable to claim. There's 5 Gal difference between the absolute lowest and highest, that's ~0.5%. The difference between what "nearly all" people will ever experience during their lives from mountains or denser ground is in the two-digit milli-Gal range. That is "constant" for all practical purposes.
So, all we really know for sure is that most humans stay mostly functional most of the time at around 980 Gal (i.e. pretty much 1G +/- 0).

And even under these conditions, 5 years are demonstrably not sustainable for some individuals.

Tasks essential for living in a society should not be interrupted by the different gravity. Well, what does that mean!
What tasks are essential for living in a society? As we are presently being forced to experience, going out and meeting people is not essential. Nor is working. Although both would in principle be perfectly possible in either a zero-G or a 2G environment.

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