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I discussed with somebody about the fact that I would like to buy a cat. The answer was that it can be dangerous, but in a different way than I expected.

Their argument was that inhaling or ingesting the cat's hairs can cause the (human) body to react, causing local bleeding, complications, possibly encouraging cancer growth. Also, their statement was that the human body does not have the ability to remove those hairs - similar to a cat vomiting hair balls.

While it makes sense in a way, it is still contrary to the overwhelming experience: people own cats and do not get sick - at least, not in this way.

Does anyone have reliable information about this?

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    Not a qualified answer, but considering that the only source of such a claim I ever came across before your post was my great-grandma (born in the early 1900s in a Danube Swabian enclave in rural Yugoslavia) who was also a staunch advocate of such solid medical advice as treating burns with butter and flour and never to drink too much water with a meal or a pill because “it dilutes the digestive juices”, I am very much leaning towards “urban legend”. – Stephie Aug 28 at 18:34
  • Human hair can kill you: en.wikipedia.org/wiki/Rapunzel_syndrome , "Because the human gastrointestinal tract is unable to digest human hair" – dandavis Sep 3 at 6:00
  • @dandavis: thanks for the hint, very interesting. It is not very explicit in those articles, but my understanding is that the hairball is deadly only because it is a "cork" in the digestive system, preventing normal transit of the "materials". Is my understanding correct? – virolino Sep 3 at 6:29
  • Yes, it's a physical blockage problem. And you have to eat LOTS of hair, far far more than even cats shed, which is about 1 hair every 5 seconds. Perhaps if you shaved a cat and gobbled it all down at once, it could kill you, though cat hairs might be too short to form a strong enough tangle, but at any rate we're off in the weeds here about realistic concerns, I just wanted to document an objective clarification to some of the speculation here. – dandavis Sep 3 at 6:31
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That sounds like made up panic. Now, I will freely admit this anecdotal, but having grown around cats, with several friends who also grew up with cats, this "ingesting cats hair is dangerous" doesn't really hold water. It is about as likely as ingesting your own hair, and about as dangerous.

Now, matter is different if you have allergies, in which case it doesn't really matter. Although I would share anecdote of my brother, who does have pet allergies, but they work rather interestingly. Without a cat or other pet, his body becomes sensitive and will react to various animals, but if he has pets or regularly interacts with animals, his body "gets used to it" (highly scientific term used by doctor) and stops overreacting.

In order to have enough hair (human or cat. Human body treats two as the same, hence my "as dangerous" comment earlier) to cause trouble you need to be actively eating it. I do not mean "swallows one strand every week", I mean actively picking it up and eating clumps of them.

TL;DR

No, it is not dangerous unless you are actively trying to eat as much at hair as possible.

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    Not to mention the human body is actually designed to get rid of foreign substances. The mucus and hairs inside the nose will trap the rare cat hair you might breathe in, as nature intended, and a cat hair accidentally eaten will just go on through the digestive tract no problem. A human hair is honestly more possibly dangerous for being potentially longer and stronger than cat hair. It could get tangled up in there. I wouldn't expect even a long cat hair to be able to do that. – Kai Aug 28 at 15:09
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    The real scientific name for your "highly scientific getting used to it" is Desensitization and exactly what allergy therapy does. Small amounts of allergen are regularily introduced to the body and over time the allergic reaction to those substances lessens. – Elmy Aug 28 at 16:52
  • My cat likes to jump on the counter when I'm cooking, so I can say that I've definitely eaten my fair share of cat hairs over the years and have never had any ill effects. – bta Aug 28 at 22:29
  • @bta Our cats were very similar and despite our attempts to teach them not to, they still preferred to steal their food, rather than eat what was put in front of them. Even if it was exactly the same thing. So I would not be suprised that we indegested quite a few cat hairs too. – Mandemon Sep 2 at 6:16
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Your friend probably either was the victim of a scam or heard of an anecdotal story and misunderstood.

Regular cell damage due to bruising, local bleeding and other processes like inflammation can sometimes lead to cancer if the irritation is chronic. There are also a few viruses (like HPV) that can lead to changes in cell structure and eventually cancer.

Now, how would cat hair cause such a chronic irritation? My honest oppinion is that it can't. The usual hairs in a cat's coat are about as thick as human hair and our bodies are well equiped to deal with those if not ingested in vast amounts.

Whiskers are much thicker, though, and the scaly outer structure gives them interesting physical properties. If you let a hair glide through two of your fingers, you'll notice that it glides very smoothly from root to tip, but with much more resistance from tip to root (the same applies to human hair, but to a lesser extent). This means that if you ingest a piece of whisker, if it survives all the way into the intestine and if it happens to get caught root first in your intestine (or you manage to poke it into your skin somehow), it can only move in one direction. It migrates very slowly through the tissue, causing constant irritation and microscoping tissue damage, and the body will eventually encapsulate it in a dense ball of cells (commonly known as "tumor") as a protective measure. This process is known from awns or "mean seeds" (the tough hairs at the tips of grass seeds) as documented here.

To be honest, I have no idea if this can actually happen with hairs. The human body is able to digest hairs in the stomach and might be able to desintegrate cat hairs outside of the digestive system. But I couldn't find any evidence that any human ever developed cancer due to cat hair.

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