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Humans can tolerate a maximum pressure of 100atm, but it becomes uncomfortable above 30 atms. Deep divers usually wear protective equipment like goggles among other things and our eyes seem to be more fragile than the rest of our body.

So, without using goggles how deep can a human open their eyes under water before suffering from an injury?

The injury, I believe, would be the eye getting crushed by external pressure.

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    $\begingroup$ I could be wrong but I can imagine that it doesn't matter much whether you have your eyes open or closed in terms of protecting your eyes against pressure, so it's merely a question of how deep can you go without wearing goggles/mask. $\endgroup$
    – Ivo
    Feb 14, 2023 at 8:01
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    $\begingroup$ A mask doesn't protect a scuba diver from water pressure, the surface air in a mask will shrink to a 1mm bubble at 300m, and scuba divers often fill their mask with water at any depth to get rid of condensation, to adjust the straps, to fix the red lines and have a scratch. So mask or no mask it doesn't matte. A mask doesnt provide any pressure protection whatsoever. youtu.be/PuVXxj70s1w?t=38 $\endgroup$ Feb 14, 2023 at 19:30
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    $\begingroup$ @Ivo My eyelids do nothing! $\endgroup$
    – DKNguyen
    Feb 14, 2023 at 19:33
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    $\begingroup$ I wonder: What makes you think the heart or ears are not crushed by the pressure? Also "naked eyes": What effect do you think goggles have? Obviously the pressure is the same. $\endgroup$
    – U. Windl
    Feb 16, 2023 at 0:00
  • $\begingroup$ @U.Windl Yes, the pressure would also affect the heart or ears, but my question was directed at the eyes in this case. By goggles, I was referring to protective equipment used by divers. Although now I have realised wearing goggles/masks will not have any effect. $\endgroup$ Feb 16, 2023 at 2:06

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Crushing damage from pressure occurs due to pressure differences. Imagine you have a rigid container. If you have equal gas pressure inside and outside, the pressure acts on just the walls and the container will survive unless the pressure is high enough to heat/melt the walls to failure. However, if you have high pressure outside and low pressure inside, you can expect the vessel to at some point violently collapse due to the pressure difference.

When diving with a mask, it's important to equalize the pressure inside and outside the mask. E.g. from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4943308/

Diving can affect the IOP [intraocular pressure] in a number of ways; firstly from the change in the internal mask air pressure at different underwater depths, secondly from the inhaling or exhaling from the nose, and thirdly through the diving mask itself. The first and second factors can be controlled by the diver by exhaling from the nose during descent, also known as equalizing the mask pressure. Senn et al. [5] found that diving mask pressure spikes were less common in experienced divers compared to beginners. Failing to equalize the diving mask pressure can lead to ocular barotrauma, which ranges from lid edema, ecchymosis and subconjunctival hemorrhage to more serious injuries such as hyphema and subperiosteal orbital hemorrhage [6, 7]. The diving mask itself may also affect the IOP. Swimming goggles are known to increase the IOP, with larger-frame goggles having less effect than smaller-frame ones [8,9,10].

Or from https://www.ncbi.nlm.nih.gov/books/NBK545224/

Mask squeeze is a type of facial barotrauma injury that occurs most commonly while self-contained underwater breathing apparatus (SCUBA) diving or freediving. This condition occurs when divers fail to equalize pressure in the face mask to the surrounding water pressure as they descend. The result is a negative pressure (relative to the surrounding water pressure). The difference in pressure inside and outside the mask can lead to injury of blood vessels and tissue of the eyes and tissue covered by the mask, including forehead, nasal areas, and periorbital regions. The main risk factor for a mask squeeze is the inexperience of the diver. Masks with high internal volume or the use of snorkeling masks or swim masks that do not cover the nose can also increase the risk of this type of injury.

Or, in simpler phrasing, if you have low pressure inside a mask and high pressure inside the rest of the body, the pressure difference is going to push/suck the eyes outward and crush the blood vessels or damage the eye itself. You want to have the same pressure pushing on the eyes from the outside and inside. It's really no different if you 1) have high pressure outside the mask and 'normal' pressure inside the mask, or 2) have low pressure (vacuum) inside the mask and 'normal' pressure outside: both situations have a pressure difference, and it's the difference that does the damage.

So, to answer your title question, a mask is more likely to increase pressure risks to the eyes rather than to decrease it.

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    $\begingroup$ @anongoodnurse The difference between a submarine and a diving human is the difference -- in pressure: There is no pressure difference between the "inside" and the outside in the case of the diving human, so there are no forces (or, if you want, there are huge forces, but they cancel each other out). By contrast, the submarine (typically) retains its internal pressure of one atmosphere when diving and thus has to withstand a huge pressure difference resulting in huge forces that try to compress it. $\endgroup$ Feb 14, 2023 at 13:19
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    $\begingroup$ @anongoodnurse To elaborate: Because the body is mostly water and solids it is largely incompressible. It's essentially like submerging a water balloon with a few bones in it: No forces whatsoever. The exception are the gas-filled lungs and the changed solubility of gases in fluids (blood) under these pressures. The lungs need to be filled with equally pressurized oxygen mixtures (I'm sure the bottles with the required mix and pressure are an example of the custom-fitted equipment). High partial oxygen pressures are toxic, nitrogen is a no-no etc. $\endgroup$ Feb 14, 2023 at 13:35
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    $\begingroup$ @anongoodnurse, if you're filling your chest with gasses that are at the same pressure as the water outside, there's no differential to fight. $\endgroup$ Feb 14, 2023 at 16:47
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    $\begingroup$ @Michael Gases in cavities within the body are really only a problem during decompression. As you increase pressure, any existing gas bubbles will get smaller. Bubbles getting smaller don't do any damage. However, if you have a bubble under pressure (dissolved or not) and then decrease the pressure, the bubble gets bigger, and if it gets bigger than the cavity holding it, you're going to have problems. The main problem is with dissolved gases coming out of solution in the bloodstream and occluding vessels, because the surface tension at the blood-gas interface is stronger than blood pressure. $\endgroup$
    – Bryan Krause
    Feb 14, 2023 at 17:10
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    $\begingroup$ @anongoodnurse: A SCUBA regulator is a key part of the setup: its input is high-pressure air from the tank, its output is air at about the same pressure as the water, so you can breathe from it without needing to fight the water pressure or having your lungs over-pressurized and pop. en.wikipedia.org/wiki/Diving_regulator - they have some mechanical valves that are controlled by ambient pressure (of the water). $\endgroup$ Feb 14, 2023 at 21:48
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Goggles do not, cannot, protect eyes from pressure. The pressure on both sides of goggles should be nominally the same. In fact, divers tend to use a mask, which communicates with the nose, to be able to equalise the internal pressure with ambient easily, rather than goggles which can damage the eyes if a descent is made without awkardly adding air by exhaling bubbles while lifting the lower edge of the goggles. The only thing that can isolate you from pressure at depth is a rigid suit, or a diving bell.

So why do people wear goggles/mask underwater?

The main reason professionals wear them is to keep a layer of air in front of the eyes, so that the eye lens is working between the correct refractive indexes and giving you normal vision. Replace that air with water and everything becomes very blurred. Basically the front face of the eye lens stops bending light, so you become extremely long-sighted.

The main reason amateurs wear them is to avoid the yuk factor of opening eyes under water, or irritation from biocides in the swimming pool.

There are multiple things that limit the depths divers can reach, but none involve the eyes.

Oxygen toxicity and nitrogen narcosis limit compressed air or nitrox sport dives to a few 10s of metres. To go deeper one needs extensive decompression time and/or expensive helium, and deeper still an addition of hydrogen. Regular commercial dives rarely exceed 300 m, with only the military accessing anything approaching 500 m. Studies (usually in pressure chambers on land) have pushed lower, but seem to run out of human tolerance for the effect of high pressure gases on the central nervous system in the 650 to 700 m range.

I did a short diving course, and the exam started with them throwing all the gear into the pool, mask included. The idea is that you need to be able to recover from an accident where everything is knocked off your face, and then some. So, open your eyes, swim to the bottom, locate blurry tank and take a breath, grab your blurry weight belt, locate blurry mask and put it on, breathe air into mask so now you can see properly, and it gets easier from there.

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    $\begingroup$ So the air inside the googles / mask is effectively another lens element between the eyes and the water. Importantly the rear of the air is shaped with a curve that conforms to the eye, and the front of the air is shaped with a flat or near-flat surface that conforms to the plastic surface. $\endgroup$
    – bdsl
    Feb 14, 2023 at 18:11
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    $\begingroup$ @bdsl The air inside the mask is the dielectric constant that the eye has evolved to work with. Well, approximately at least. I'd imagine the refractive index of the gas increases slightly with pressure, but not to the extent that increases by replacing it with water. $\endgroup$
    – Neil_UK
    Feb 14, 2023 at 18:16
  • $\begingroup$ @bdsl Yup, mask/goggles are basically corrective eyewear that since everyone needs the same prescription for and thus are stock rather than from an optometrist. $\endgroup$ Feb 16, 2023 at 2:55
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Currently the record for the deepest dive wearing scuba equipment is 332.35m. The descent took only 15 minutes while the ascent lasted 13 hours 35 minutes.

Liquids do not compress, so the sinuses, inner ear, lungs and circulation are in far more danger from pressure than the eyeballs.

The main danger is not the pressure at depth, it is the readjustment to the expanding gases as the diver rises, decompression sickness, which rarely affects the eyes.

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    $\begingroup$ This doesn't answer the question at all. $\endgroup$
    – Ivo
    Feb 14, 2023 at 7:52
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    $\begingroup$ The question explicitly states "without using goggles". Goggles/masks are included in scuba equipment. $\endgroup$
    – Mast
    Feb 14, 2023 at 10:14
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    $\begingroup$ Yes, it does answer the question. Face and mask of the diver could never withstand such a pressure difference, meaning that the diver needed to equalize the pressure, which is normally done by taking of the mask and blowing (high-pressure) air inside. Descending so quickly, there is a chance that you can't get the mask off your face if you wait too long, meaning that pressure would build up until implosion, which would pose an enormous health risk. For this reason, such records are generally done without wearing a mask, while descending. The answer is that eyes are not the bottleneck. $\endgroup$
    – KaPy3141
    Feb 14, 2023 at 11:24
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    $\begingroup$ @GoalaSestant that's with scuba equipment $\endgroup$
    – Ivo
    Feb 14, 2023 at 14:06
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    $\begingroup$ @Ivo did you know that scuba equipment doesn't protect the eyes from water and water pressure, and scuba divers regularly take off the mask underwater and fill it with water to get rid of condensation and other reasons? youtu.be/PuVXxj70s1w?t=38 $\endgroup$ Feb 14, 2023 at 19:24

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