How Much Seawater Can You Safely Drink?
July 6, 2010 5:37 AM Subscribe
What percentage of sea water can you safely drink?
Drinking straight sea water will dehydrate you. But your body can get rid of some salts, and in fact needs some salts. If you were in a survival situation, is there is a ratio of fresh water to sea water that would extend your fresh water supplies and keep your body replenished with salts? What is that ratio? Or should you just never drink any percentage of sea water whatsoever?
Drinking straight sea water will dehydrate you. But your body can get rid of some salts, and in fact needs some salts. If you were in a survival situation, is there is a ratio of fresh water to sea water that would extend your fresh water supplies and keep your body replenished with salts? What is that ratio? Or should you just never drink any percentage of sea water whatsoever?
The salt in seawater isn't just sodium chloride. There's potassium salts, epsom salts, iodine and magnesium salts, all kinds of other stuff.
You can, in principle, safely have a few gulps of seawater; happens all the time, you choke a bit and don't die, but you'll also need enough fresh water handy to flush that salt out of your system, which in a survival situation you likely won't have. Or it wouldn't really be a survival situation, you know? If you're in that situation, the safe amount of seawater to drink is zero; the salt makes you more dehydrated, requiring you to drink more of the fresh water you presumably don't have.
posted by mhoye at 5:50 AM on July 6, 2010
You can, in principle, safely have a few gulps of seawater; happens all the time, you choke a bit and don't die, but you'll also need enough fresh water handy to flush that salt out of your system, which in a survival situation you likely won't have. Or it wouldn't really be a survival situation, you know? If you're in that situation, the safe amount of seawater to drink is zero; the salt makes you more dehydrated, requiring you to drink more of the fresh water you presumably don't have.
posted by mhoye at 5:50 AM on July 6, 2010
Interestingly, one man conducted just such an experiment, although another tried to repeat it and failed.
In short, the ratio will be quite low. Because the moment you exceed tolerable levels your kidneys will excrete some of the salt in your urine. It needs water to do that. Go further and your kidneys can't cope.
In addition, it is likely you'll be getting some salt from your food too. So one factor that determines the ratio is both how much salt and how much water you are getting from your food.
posted by MuffinMan at 5:53 AM on July 6, 2010
In short, the ratio will be quite low. Because the moment you exceed tolerable levels your kidneys will excrete some of the salt in your urine. It needs water to do that. Go further and your kidneys can't cope.
In addition, it is likely you'll be getting some salt from your food too. So one factor that determines the ratio is both how much salt and how much water you are getting from your food.
posted by MuffinMan at 5:53 AM on July 6, 2010
An old blurb (not even an article) in JAMA says no mixing at all.
An abstract from the NIH talks of experiments done on rats and suggests "that when a man is stranded at sea it is not advisable to drink all the fresh water and then be compelled to drink sea-water when dehydrated. It is better to slowly increase the sea-water uptake." Apparently when the ratio of fresh to seawater gets below 1:1, things get bad.
I know freshwater and seawater mixed 3:1 gives you a fluid slightly less salty than blood so that might be acceptable for drinking.
posted by 5ean at 5:56 AM on July 6, 2010
An abstract from the NIH talks of experiments done on rats and suggests "that when a man is stranded at sea it is not advisable to drink all the fresh water and then be compelled to drink sea-water when dehydrated. It is better to slowly increase the sea-water uptake." Apparently when the ratio of fresh to seawater gets below 1:1, things get bad.
I know freshwater and seawater mixed 3:1 gives you a fluid slightly less salty than blood so that might be acceptable for drinking.
posted by 5ean at 5:56 AM on July 6, 2010
According to Wikipedia, "fresh water" is classified as water with water of less than 500ppm of dissolved salts, and seawater has on average 3,500ppm of dissolved salts. So the question is then "what ppm of dissolved salts can a human tolerate for prolonged period without ill-effects?"
This Australian government page says that a salinity of 800 - 2,500ppm "Can be consumed by humans although most would prefer water in the lower half of this range if available".
Wikipedia's seawater article also has:
Although it is clear that a human cannot survive on seawater alone, some people[who?] claim that one can drink up to two cups a day, mixed with fresh water in a 2:3 ratio, without ill effect. The French physician Alain Bombard claimed to have survived an ocean crossing in a small raft using only seawater and other provisions harvested from the ocean, but the veracity of his findings was challenged. In Kon-Tiki, Thor Heyerdahl reported drinking seawater mixed with fresh in a 40/60% ratio. A few years later another adventurer named William Willis claimed to have drunk two cups of seawater and one cup of fresh per day for 70 days without ill effect when he lost part of his water supply.
There seems to be little hard evidence for those claimed "safe" ratios, but if we take the above measurements of 500ppm for freshwater and 3,500ppm for seawater, then a 2:1 mix gives us a salinity of 1,500ppm which is well in the "can be consumed by humans" zone specified above.
If you wanted to get it down below 800ppm (classified as "Good drinking water for humans" on the Australian gov page), then you need to dilute a hell of a lot more, with a 10:1 ratio of fresh to sea giving you a salinity of 773ppm.
posted by EndsOfInvention at 5:57 AM on July 6, 2010 [3 favorites]
This Australian government page says that a salinity of 800 - 2,500ppm "Can be consumed by humans although most would prefer water in the lower half of this range if available".
Wikipedia's seawater article also has:
Although it is clear that a human cannot survive on seawater alone, some people[who?] claim that one can drink up to two cups a day, mixed with fresh water in a 2:3 ratio, without ill effect. The French physician Alain Bombard claimed to have survived an ocean crossing in a small raft using only seawater and other provisions harvested from the ocean, but the veracity of his findings was challenged. In Kon-Tiki, Thor Heyerdahl reported drinking seawater mixed with fresh in a 40/60% ratio. A few years later another adventurer named William Willis claimed to have drunk two cups of seawater and one cup of fresh per day for 70 days without ill effect when he lost part of his water supply.
There seems to be little hard evidence for those claimed "safe" ratios, but if we take the above measurements of 500ppm for freshwater and 3,500ppm for seawater, then a 2:1 mix gives us a salinity of 1,500ppm which is well in the "can be consumed by humans" zone specified above.
If you wanted to get it down below 800ppm (classified as "Good drinking water for humans" on the Australian gov page), then you need to dilute a hell of a lot more, with a 10:1 ratio of fresh to sea giving you a salinity of 773ppm.
posted by EndsOfInvention at 5:57 AM on July 6, 2010 [3 favorites]
The Australian page linked by EndsOfInvention actually measures salinity in units of electrical conductivity (EC) in micro-Siemens per centimeter instead of parts per million. One EC unit at 25 degrees C equals 0.6 ppm. So the upper limit for humans is 2500 EC or 1500 ppm. The sources I've found (here and here) give ocean salinity of roughly 40,000 to 60,000 EC, or 24,000 to 36,000 ppm. That's consistent with Wikipedia's 3.5% sodium chloride (35,000 ppm). I don't think you want much seawater at all in your drink.
posted by drdanger at 12:55 PM on July 6, 2010
posted by drdanger at 12:55 PM on July 6, 2010
Best answer: Whoops, missed the different units. Let's have another go, converting from micro-Siemens per centimeter to ppm.
1EC = 0.6ppm
This Australian government page says that a salinity of 800 - 2,500EC, or 480 - 1,500ppm "Can be consumed by humans although most would prefer water in the lower half of this range if available".
If we take the above measurements of 500ppm for freshwater and 3,500ppm for seawater, then a 2:1 mix gives us a salinity of 1,500ppm which is at the maximum upper limit for the "can be consumed by humans" zone specified above.
If you wanted to get it classified as "Good drinking water for humans" on the Australian gov page), then you need to dilute it down to 500ppm again, which is effectively impossible. Putting any amount of seawater into freshwater will raise it above 500ppm and it will no longer be classified as "Good drinking water for humans".
So in conclusion, a 2:1 ratio works to make the seawater just bearable, but probably not healthy to drink for a prolonged period.
posted by EndsOfInvention at 1:50 AM on July 7, 2010
1EC = 0.6ppm
This Australian government page says that a salinity of 800 - 2,500EC, or 480 - 1,500ppm "Can be consumed by humans although most would prefer water in the lower half of this range if available".
If we take the above measurements of 500ppm for freshwater and 3,500ppm for seawater, then a 2:1 mix gives us a salinity of 1,500ppm which is at the maximum upper limit for the "can be consumed by humans" zone specified above.
If you wanted to get it classified as "Good drinking water for humans" on the Australian gov page), then you need to dilute it down to 500ppm again, which is effectively impossible. Putting any amount of seawater into freshwater will raise it above 500ppm and it will no longer be classified as "Good drinking water for humans".
So in conclusion, a 2:1 ratio works to make the seawater just bearable, but probably not healthy to drink for a prolonged period.
posted by EndsOfInvention at 1:50 AM on July 7, 2010
Wait, I think I messed up the % to ppm conversion as well. 3.5% salinity is 35,000ppm, not 3,500ppm! Doh!
So a 2:1 ratio of fresh (500ppm) to sea (35,000ppm) would only give 12,000ppm. To get the salinity down to 1,500ppm you'd need a ratio of over 100:1.
Basically it's not a great idea.
posted by EndsOfInvention at 1:58 AM on July 7, 2010
So a 2:1 ratio of fresh (500ppm) to sea (35,000ppm) would only give 12,000ppm. To get the salinity down to 1,500ppm you'd need a ratio of over 100:1.
Basically it's not a great idea.
posted by EndsOfInvention at 1:58 AM on July 7, 2010
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posted by cronholio at 5:44 AM on July 6, 2010