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Can Sharks Smell Period Blood and Will They Eat You Because of It?

If you're on the rag in Hawaii, are you on the menu for deadly sea creatures?
Illustration by Brandon Bird

Tracy Jordan once famously said, "Live every week like it's Shark Week," but what if that week falls on that time of the month when our uteruses bless us with cramps and increase the odds that we weep hot tears while watching The Bachelor? In other words, if sharks can smell blood from a mile away, will sharks be drawn to you if you swim in the ocean whilst menstruating? And then will they find you and gobble you up like so much chum-flavored chocolate?


"The real test is behavioral," says Dr. Steve Kajiura of Florida Atlantic University's Shark Lab.

Do sharks attack the menstruating more than other humans? "We don't have evidence of that," Kajiura asserts.

For starters, the notion that a shark can smell blood "from a mile way" is not true. Plus, though sharks can detect blood from a quarter of a mile away, period blood is not blood. Not just blood, anyway.

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According to The New Our Bodies, Ourselves (90s edition, but don't worry—periods haven't changed much since the 90s), menstrual fluid contains "cervical mucus, vaginal secretions, mucus and cells and endometrial particles as well as blood (sometimes clotted)." If it were true that your period could attract sharks, a shark would need to be able to sniff out blood that was mixed (or possibly masked) by non-aquatic mucus and have to be able to sense endometrial particles.

Perhaps this is not out of the realm of possibility. Sharks are popularly considered to be the best smellers in the business because, anatomically, the amount of surface area in their snouts devoted to smell receptors is major. But does size actually matter?

We asked Dr. Tricia Meredith, who literally wrote the book on the olfactory response of sharks. For her dissertation, she hooked sharks up to a device that introduced controlled amounts of prey odors (smells associated with a shark's next meal) into a shark's nose, then measured the electrical impulses in their nasal cavity. She weakened the concentration of these prey odors to determine how diffuse an odor a shark could still pick up. Dr. Meredith found that sharks can detect prey odors as minute as one part per billion—still superhuman, but not better than other fish with similar schnozzes. One part per billion is roughly the background scent level of the ocean. If a shark's sense of smell was any better they would be flooded with stimulus, the olfactory equivalent of those people who can't deal with the sound of chewing.


If it were true that your period could attract sharks, a shark would need to be able to sniff out blood that was mixed (or possibly masked) by non-aquatic mucus.

There is one sensory arena where sharks excel, but it isn't smell. Sharks are incredibly electroreceptive, meaning they can detect teeny tiny electromagnetic fields in water. Sharks possess a science fiction-y and awesomely-named organ called the ampullae of Lorenzini, which are pores, located on the snout, that end in jelly-filled bulbs. These bulbs contain nerves that detect electric fields in the water as small as five millionths of a volt per centimeter. Sharks use the ampullae of Lorenzini to navigate the ocean and detect prey. All ocean-dwelling animals emit an electrical field: Muscle contractions release bioelectricity, and, as Dr. Kajiura says, "any animal in the ocean with a thin, leaky mucus membrane acts as a battery in seawater," because of the differing pH levels inside and outside the animal. Dr. Kajiura was talking about gills, but "thin, leaky mucus membrane" could also double as the least sexy description of a vagina ever (and that's including Martin Lawrence's infamous SNL monologue).

So now we have to determine the electric conductivity of blood (and mucus and endometrial particles). If blood conducts electricity, it would carry the signal of our natural electric field further and thus advertise our tender flesh to more sharks. Red blood cells are not especially conductive, but the plasma they float in is highly conductive. The conductivity of blood is also dependent on blood flow. The higher the velocity, the more conductive. Period "blood" (again, not really blood) isn't flowing—it's being sloughed off the inside of your uterus. It's also low on plasma, which significantly reduces its conductivity.


The main electrical threat your period poses goes back to that "leaky mucus membrane." Humans have a biolectric field like any other animal, but it's essentially insulated by our skin. But when the differing electrical charges of your body and seawater are connected by your Aunt Flo, you become a floating battery. Not a very good battery, mind you—you couldn't charge your iPhone off your period, but a shark could still detect you.

Dr. Kajiura says that even though this scenario is scientifically plausible, it's still nothing to worry about. Sure, you're a battery, but the ocean would ground the charge pretty quickly. It's not going to cue in a shark from miles away. "Sharks only use their electroreceptors when they're half a meter away, a meter on the outside. If a shark is that close, you've got more concerns than whether you're electrically leaky," Dr. Kajiura says. He adds that a tampon would be sufficient to short the electrical circuit between you and the water.

All this science is very well and good, but it ignores the main truth about humans and sharks: Sharks are just not that into you. Dr. Meredith says that human meat is not a delicacy in shark circles. She didn't study human blood's effect on sharks because it's not relevant. "I never investigated shark's sensitivity to blood. I used amino acids instead because they are prey-related odors, and human blood is not. I know people are interested in sharks sniffing out our bodily fluids so they can find us and bite us, but that's just not how it works."

A shark might be able to smell you, but that doesn't mean it will equate that smell with food. Or, as Dr. Kajiura put it, "You can smell a landfill, but it won't make you want to eat it."

Ask-Hole is a regular column in which Broadly investigates questions you probably already knew the answers to, but we didn't, so here it is. Do you have a question about honestly anything at all? Ask us about it.