#since it describes tthe quirks physically affecting them

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my personal bnha academia headcanon/au: quirk repression

basically, along with the whole negative side affects of using your quirk too much, repressing your quirk and purposely not using it can also cause issues

this wouldn't apply to people with physical quirks, like spinner, shoji, or tokoyami's bird head (but not dark shadow)

its like if you force yourself to not use your quirk, it'll start to build up, and the chances of someone accidentally using it increases

like a teapot

the water will start boiling and as it gets hotter pressures increases and the steam goes out, releases the pressure into the air

so someone who just doesn't use their quirk very often or doesnt use their quirk for a little bit for some reason might have times where the steam goes out of the pot and they accidentally use their quirk a little bit

like someone with a fire quirk getting startled and the stress combined with the quirk not being used ends up breathing out a few sparks

the quirk only gets released in small ways though, and it helps keep that balance so the pressure inside the kettle stays stable, even if it is kinda high

so while it's kinda inconvenient sometimes, and unless the person is already in a dangerous position, like if they're working with gas lines, they won't be a danger to themselves or others

but long term quirk repression, like don't even let the steam out of the kettle at all, and the pressure just keeps building

goes into dangerous territory

at that point, the quirk starts physically affecting others and even if they do use their quirk a little bit, it's harder to control and reign in, since it's like opening a needle sized hole in the kettle, letting an exact amount of steam get out, and seal the hole, all without letting the kettle explode

some examples with some ua students-

uraraka: she'd probably start getting lightheaded often, and she starts getting altitude sickness, even though she hasn't left the school or dorms,

kirishima: instead of the outside of his body hardening, his internal organs, muscles, tendons, etc...start stiffening up, and eventually he'd stiffen to the point where his organs can't function properly, like his lungs not being to expand enough to properly breathe, and blood clots

jirou: her hearing starts becoming more sensative, and sounds start sounding louder than they actually are even if she's not using her quirk, and hearing damage becomes more likely

mina: acid reflux, acid starts seeping into the rest of her body, and worse case scenario, the lyosomes (parts of the cells that have acid in them and help destroy bacteria and ciruses with it) start to go haywire and releasing acid even if there isn't anything to attack

bakugou: really high blood pressure, the pressure literally builds up in his body, and also, since it's not being used, he stops producing as much glycerin, so if he does try using the quirk again, it's not as effective plus higher chance of burns

iida: any of you ever drive a manual car and put in too much gas without taking the clutch out/the cars in neutral, so the car doesn't move forward and it revs really loudly? that basically happens to iida, and it ends up causing long term damage to the engines and legs

deku: the pressure will literally build up too, but with no release, it'll physically start breaking his body appart and as his body breaks apart, he can't contain the built up power anymore, and it's basically like a star going supernova and collapsing in on itself, oh yeah, there's also the voices of the past afo users starting to get louder, to the point where he can't even hear other people

todoroki: he's got an advantage when it comes to the rest of the class, because if he starts to gets a fever from not using his fire quirk or gets hypothermia from not using the ice one, he can just counteract it, but, if it goes on long enough, he'll have to heavily rely on his other quirk to keep him alive, and that leaves him unable to defend himself or use his quirk for other things, until his energy runs out and he can't sustain the quirk anymore