As you probably have noticed from your existence on the planet as a human, there's dust everywhere. It's probably around you right now, and you might have heard that it is made of human skin cells. But that's not quite true.  Some studies have found that more than 60% of household dust comes from outdoors, like pollen and dirt and sand and even tiny particles of meteorites!  But the rest IS mostly dead bits of you floating around, along with lots and lots of feces from dust mites.  Like, uh, like lots.  And, fair warning, after learning about this, you may never want to, like, get in your bed again.    [Intro]   Dust mites are nearly microscopic arachnids that hover on the border between visible and invisible, with an average length of about 0.3 millimeters.  Despite their tiny size, dust mites are all over.  There are two main species: the American dust mite and the European dust mite, and in the U.S., one study found that 84% of households had a detectable dust mite population.  The good news is, they aren't technically parasites because they don't feed off of the live parts of humans. But I specify "live parts" because they do eat human tissue. Our dead skin cells are their favorite meal.  The average human sheds about one million skin cells a day, and some of those inevitably come off in the tossing and turning of sleep. So mattresses and pillows tend to be full of those human skin cells, making them an ideal dust mite home.  And since a female dust mite will usually lay up to a hundred eggs in her lifetime, the mite population tends to grow pretty fast.  There are, on average, a hundred thousand dust mites living in the typical mattress. Now, that may be kind of gross, but eating dead skin cells doesn't do any humans harm, I mean, it's just dead skin! Just let it go.  It's when the dust mites, you know, excrete it, that's the problem. Mites use digestive enzymes, called proteases, that can cause allergic reactions in humans.  And there's plenty to go around - your average mite excretes about 20 fecal pellets per day. Multiply that by the hundred thousand dust mites in your mattress, and that's - that's a lot of fecal pellets. They're very small, though!  Those little pellets end up in your pillows and your mattress and get spread through the air, and when they're breathed in by someone who's sensitive to the enzymes, they can cause wheezing and sneezing and itching and all sorts of not-fun asthma and allergy symptoms.  So killing dust mites isn't easy, but they do have a weakness: humidity, or the lack of it. Dust mites don't drink water; they have to suck it out of the air, so they need at least 50% humidity to survive. If the humidity drops below that, they go into a sort of panic mode.  When this happens, a lot of mites will gather  together in one spot to try and decrease the surface area of their bodies that's exposed to air, so they lose less water. They basically try to form one giant mite-ball, like a Megazord mite.  And if the humidity gets too low for even that to work, the young mites, called nymphs, will latch on to whatever material's available, like carpet fibers, and basically hibernate until the humidity increases again.  That's why some people find that their dust allergies are seasonal. The dust mite population decreases in colder months when the humidity is lower and comes back when the nymphs emerge from hibernation and resume... crapping all over your house.  This survival method is so effective that there's really no way to completely get rid of a dust mite colony. There are some ways to decrease their numbers, though, like frequently washing sheets and pillows in very hot water, using polyester pillow cases and mattress covers to create a barrier, getting rid of carpet, and using vacuums with special filters.  But most of us will continue to play host to these mostly harmless, but probably unwanted, house-guests. At least now you know exactly who - or what - you're sharing your bed with.  Thanks for watching this episode of SciShow, especially to our Subbable subscribers who make this whole channel possible for themselves and for everyone else. To learn how you can help keep SciShow going, just go to Subbable.com/scishow, and don't forget to go to youtube.com/scishow and subscribe if you want to keep learning about all the ways in which the world is terrifying.