They’re on Mars. A whole different planet. There will be a difference in pressure regardless of the Earth-like conditions within the colony. She’s also never been exposed to other humans, so her immune system isn’t equipped to deal with it yet.
It’s the same thing as Europeans bringing diseases over to the New World that wiped out a huge % of the Natives who had zero immunity to them.
Could just be a future sports team jacket.
Because our air is polluted with microplastics
She grew up on Mars and her microbiome has adapted to Mars, not to Earth. She would definitely get really sick in the best case scenario, worst case is she dies.
To Infinity and Beyond, As They Say! 
my guess is her bone density is way off as well she doesn’t have an immune system s other suit probably filters the air a lot.
it probably actually the illnesses her being from mars earth illnesses that just take us out for a little bit would devastate her immune system making them fatal
Only if you buy the battle pass. 
Allergens, if your born and raised on Mars you would never adapt to pollen and other things like animal dandruff in the air.
Juno is Overwatch version of Tali from Mass Effect
Imagine the allergies she would get from breathing all the pollen and dust here
The helmet dampens the lethal effects of yodeling she may encounter on Earth.
Look!!! Real martian girl!!! She is so cute!!!
idk if she has a voiceline saying she HAS to, but I imagine its a comfort thing from home lol
And you, are the winner.
She’s acclimated to Mars, a sterile environment. Coming to earth, meeting up with all the nasty crap here she’s never been inoculated for? Yeah, she’d have coronaherpegonnasyphlaids in about 2 seconds without the helmet.
Too much bacteria and germs in the air.
It’s reasonable to think that living in the relative sterility of a space station her whole life might leave her more suspectible to stuff like the regular flu and it’s variants for example.
Or maybe the sudden exposure to an atmosphere filled with pollen triggers a terrible autoimmune response and gives her deadly asthma?
It’s her security blanket. She’s on a strange new planet, feels uncomfortable and lacks confidence in her own abilities.
she is the Sandy Cheeks of Overwatch
I mean omitting the fact that her character is defined by her anxiety, I guess it’d be more so to avoid spreading viruses into this planet as opposed to protecting herself but maybe both
If she grew up on Mars, maybe she was never exposed to all the viral infections most people has to go through and build resistance to it. This is commonly an issue for tribes that never got exposed to viral infections because of their isolation, and integrating them with modern society can often be deadly for them. Maybe the helmet helps with that.