You know, since sharks can smell blood and all…
Hmmm, on the menu of forums, all I could see of your title was “should menstruating women” and immediately I knew it would be about sharks 
I always heard they should avoid areas with bears.
You hear that Ed? Bears! Now she’s puttin’ the whole station in jeopardy!
–FCOD
The way I always heard it from my lady relatives was: Girls should not go swimming while menstrating. Period.
Err, well, you know what I mean.
I suppose I can understand the reasoning behind the thoughts in the OP, but I’m not sure about this toted advice my many aunts ladeled on me when Aunt Flo first came a-knockin’. Except that perhaps that other swimmers might find it icky to go swimming in water that might–eeewww!–have menstrual blood in it.
As opposed to 3 billion years worth of fish crap? 
At least fish don’t leave a “warm spot.”
First of all, there’s very little actual blood in menstrual fluid. We just call it blood because it looks somewhat (not very) like blood and “blood” has less syllables than “menstrual fluid”. It’s actually mostly water, and what isn’t water is cells-other-than-blood that lined the uterus to try and catch the egg and form the placenta. We don’t have free flowing blood just hanging out on the uterine wall. IF a fertilized egg implants, then blood vessels form to contain the blood that flows from mother to placenta and from fetus to placenta (the two blood streams should never mix - nutrients and oxygen pass through the blood vessel walls, but blood cells don’t.)
Anyway, it’s not blood. But more importantly, menstrual fluid just doesn’t come out that fast and tends to not flow while you’re in water. My best WAG is that the water pressure at the opening of the vagina* keeping the fluid in is greater than the force of gravity pulling the fluid out. But I can assure you that girls on their period can go swimming in the ocean or even a swimming pool without a cloud of bloody water giving them away.
*which is not a rigid open tube, but smooshed flat and mostly closed like the top of a sock in your sock drawer
Probably not an issue for sharks, sez one expert: Menstruation and Sharks – International Shark Attack File
Nor for bears, according to another study: Home - AC-Tafk
as a learning scuba diver at the time, we were told by our instructor that diving during a full-blown period was not the best idea in all the world, as sharks do smell blood in a seriously diluted parts per billion percentage in salt water, but whether or not you are bleeding is not the biggest thing you need to be concerned about when you’re breathing under water.
however, WhyNot is exactly right. there isn’t a huge amount of blood in menstrual fluid to begin with. nontheless, i’d say that the basic rule of thumb applies: if the woman is on birth control, as many of us are these days, the dosages can be manipulated so that when you’re planning that kind of vacation, your period need not be an issue.
that was how i handled it at the time. seawater plus that time of month? why tempt fate? fiddle with your dosage timetable. no big deal.
as a rule, sharks won’t bother you at depth. you’re a non-issue - another piece of aquatic fauna - and they really don’t care. i’ve been within 10 feet of a silver-tipped reek shark at 20 feet down - literally right under the dive boat - that was supremely unconcerned. he looked me over, i looked him over - and we just swam away from one another.
it’s the folks splashing around at the ***surface * ** that need to worry. i don’t tarry at surface. i stay about 10 feet down, watching both for sharks and to see when the ladder is clear for me.
I suspect that advice comes from a pre-tampon era. Remember, tampons have only been sold in the US since the 1930s and are still unacceptable to some religious and cultural groups (and some women just don’t like using them). If a tampon isn’t an option, you’d probably get at least a little blood on your swimsuit if you went swimming during your period, which would be kind of gross (and possibly hard to clean).
Also, tampons still work when the user is immersed in water. If all the menstrual fluid is being absorbed by a tampon in the interior of the vagina, and the tampon isn’t being soaked with water at the same time (due to the pressure-keeping-vagina-closed thingy), then no blood is getting into the water.
I was told that they shouldn’t join in the hunt least they alert the saber toothed tigers or scare off the mastadons.
While menstrual flood has very little blood, unless my health class, and more importantly my past amorous adventures, have been extremely mistaken, there is a fair amount of bleeding during a period.
As for the OP, the water pressure and/or tampon would keep most blood at bay, but dont sharks detect blood at something like 1 part in a million? I’m not convinced it’s totally safe.
Gee, if I had one of them in my sock drawer, I’d never leave the house! :dubious:
Okay, now here is a topic I can discuss from personal experience. In the late 1970s, I was on a SCUBA diving trip off Isla Mujeres, Mexico. It was right after Jacque Cousteau had visited and photographed (for National Geographic) the mysterious “sleeping sharks” that sat motionless beside shallow caves at about 90-100 ft. I met the Mayan fisherman who discovered the sharks and had led Cousteau there. His nickname was “Valvula” because he apparently had developed incredible lung power, allowing him to dive to great depths - spearfish and pick up lobsters, etc. - without tanks. (He’d jump in the water carrying a big rock to reach the bottom fast.)
Valvula took my two buddies and me out on his boat, looking for the sleeping sharks. About 75% of the time, Valvula said, the sharks wouldn’t be there, but we got lucky and saw, photographed and even touched them. Large sharks sitting very still (I don’t remember what kind they were, but I don’t think they were nurse sharks, that are known to be able to bask like that). Because we were so deep, we couldn’t stay long or we’d have to decompress on the way up. As we started to head up from the bottom, I saw a look of alarm on my friends’ faces. Then I spotted a large tail sail around behind me. My friends (against all “buddy rules”) headed straight for the boat. I continued to swim along the bottom (as unlike a wounded fish as I could) while this huge shark followed me, his nose close to my swimming fins. My Mexican friend identified it as a gigantic tiger shark, an aggressive species that is known to attack humans.
Later it occurred to me that, as well as having cuts on my legs from being washed up on fire coral the day before, I was also on my period.
No proof of anything, but at least I got to tell this story.
Good story! 
Well, the answer is- in most areas sharks aren’t any significant danger. But I suppose, in areas where there are shark attacks, warning and such, it might be better to be careful. Generally, wading is safe even in shark areas.
Here on the West Coast I think the only real shark danger area is off the Golden Gate in areas where few swim anyway. (too fucking cold, and the rip currents are murder).
As a scuba diver and a surfer, this question is of interest to me.
The US Navy studied a variety of substances to see what effect they had on sharks (as part of looking for a shark repellent). They divided the responses into four groups - those that attracted the sharks (some), those that repelled them (hardly any), those they didn’t pay any attention to (plenty), and those that they noticed and sniffed at, but didn’t follow (some).
Human blood, surprisingly, was in the last group. When sharks smell it, they notice it, sniff around, and then keep going.
The greatest attractant, surprisingly, was not fish blood. It was fish gastric juices … which I guess makes sense, any little scratch puts fish blood in the water, but when the gastric juices are coming out, the fish is badly wounded …
Sorry, no cite, but it was in an authoritative book I read a while ago.
w.
Very cool story. I woulda soiled my wet suit.
jillcat i hate you for having the mexican deep diver experience instead of me, but at the same time i don’t envy you the tiger shark experience.
the most aggressive shark in the water is the bull, because of the high testosterone levels they have, but the tiger can rank right up there on the ‘pissed off for no real reason’ scale. blood is blood - parts per million or billion be damned - and i’m not planning any time soon to actively tempt fate, as i mentioned earlier.
and your ‘friends’ oughta be hung up by the short hairs for doing what they did. you don’t leave your buddy in a pinch like that - unless there’s an equipment malfunction or you’re malfunctiong for some reason.
good on you for keeping your head. many divers haven’t in the past, and that can get you dead in a real big hurry.