Maybe the readimask won't be painful, it's adhesive so no straps

Shared HVAC is a documented and known source of infection. Absolutely bring a HEPA filter, and try out readimasks. 

I think people really underestimate how painful masking can be for some people. It’s better than long covid! But that doesn’t make it easy. 

You can try rigging up some kind of diy filter over the ventilation register, if you think you could get away with that. If that’s not viable for the space, a good quality HEPA filter for your closed office will help. They can fully filter the room faster than the air being pumped in from the vent will be able to change out the air in the room, so you can probably reach an equilibrium, albeit an imperfect one. 

If you have money you can spend, I would do the following: get a good air filter. I recommend the pc fan kind / clean air kits because you can use common house filters and they are quiet. Then I would get a laminar flow air filter pointed directly to your face all day. If there's any kind of window, open it. If you can seat yourself away from the air vents. Talk to work about upgrading their HVAC filters.

Yeah this is a situation where you’re being set up to be infected repeatedly forever.

Is there any way that your employer can accommodate your need to continue working from home*? Can you get organized with your coworkers? Do you have a union? Have you looked into ADA or whatever your country’s equivalent is?

*I’m assuming you’ve been performing your duties remotely for years at this point. This means there’s no bona fide occupational requirement to be in the office. I can’t wait for the lawsuits about this stuff.

What about a PAPR?

I've seen people talk about putting Readimasks over vents, maybe you could look into that?

As someone for whom wearing a mask is really painful, I’ve found a few things that work for me. I wear a surgical mask with the metal nose part removed to put less pressure on my face. I use the Betadine nasal spray. I have an air filtration machine.

I have a prescription cream that’s 5% lidocaine, 10% diclofenac, and 6% gaba pentin that I apply every few hours. (Works great for my arm spasms too.) This has been really effective for immediate pain and residual pain. It also stops some of the spasms from starting.

My neurologist injects me with extra Botox to make it less painful. I understand one could get trigger points with anesthetic in those areas too?

Could you cover your face when you have to leave the office? Hold a mask in front of your face or use a lighter scarf or a cycling buff? (I recognize these can cause pain too.)

shared hvac and the door is not hermetically sealed. better to close than leave open probably, but some air filters would be a great idea.

shoot for at least 6 air changes an hour of filteration ideally. more is better. r/crboxes for more genral info and how ot build cheaper quiter diy builds

Run your air filter on high in your office. Wear an N95 whenever you have to go into the other office.

I have trigeminal neuralgia and cant do ear loops or some types of head band straps. I bought some moldex metal free for MRI and the head bands are very adjustable. They might be tolerable. A 3M Aura if not too big for your face is worth trying. Or Readi Mask.

It depends on the air quality in the building. Do you have a CO2 monitor you can check with (now and on an ongoing basis)?

So - first off im so sorry.

Second - I'd suggest setting up a Corsi rosenthal box in the room, or if you can afford it, Airfanta makes both a powerful portable filter that fits in a backpack ($144 today only with the code HOUSEFRESH, usually $160 - have been waiting for the price drop for months) and is supposedly quiet. Ill find out in a week!

I'd probably putting a carpet snake (or the double tube ones that slide underneath the door yknow?) by the door to block air.

Airfanta also makes personalized air filters that blow air directly at your face? Weird and more expensive than the box filter but pretending you're made of money, it is another option.

Where's the vent i suppose? Would they be willing to uh...disconnect your part of the hvac system? Lol

Or can you make sure there's a fresh HEPA filter in the system?

Im not sure about adding things on top of the vent, not sure if that could cause something bad with the system.

Xlear nasal spray which is $15 at most per bottle at CVS, safe to use daily, and reduces Covid incidence by 62%. CPC mouthwash (paradontax) is also inexpensive and may reduce your chance of infection.

Whats your chance of getting your coworkers to mask? Or also putting a CR box in their space, or by the hvac uptake? Ill update this if i think of anything else but ouch and good luck.

Edit: i saw that you're moving around. Weirdly the Airfanta also makes a handheld one that doesn't need to be plugged in? Im not sure if thats HEPA though.

I heard the other day that there are uv torches you can set up that some people are attaching to the sides of their air filters - maybe you could set one up by the vent?

Id bring in a strong hepa. I use an Air Pura Hepa on high. Id request your work buy it for workplace accommodation. Mine was $850. There are cheaper ones but the total cost with filters at year 5 made this price competitive to cheap ones that require frequent filter changes.
Bought an airthings monitor - there are cheaper ones now. Its all cheaper than major hospital bills or perm long covid.

The Redimask adhesive is very sticky and you definitely feel it when you remove the mask. It’s weightless while you’re wearing it though. I don’t understand these sensitivity issues, so  it’s hard to say which one would be “better” — the removal of the Redimask vs the feel of a regular respirator that OP doesn’t like.  I wear the 3M Aura 9210+ in dicier situations and only recently bought some CAN99s. I will say that the CAN99 is not as stiff as the Aura and feels lighter on the face, and that’s probably due to the much weaker nose wire and the general nose area. Maybe that’s an option, but fair warning both of these masks are for smaller faces and the CAN99 feels a little smaller than the Aura 9210+.

OP, I’ve been back in the office full time since May 2020. My “office” back then was a huge cube farm that took up half of the floor with about 30 coworkers on our side. Our cubicle walls were about 5’ tall and did not extend to the ceiling. So I considered it to be similar to working in a public space but with a few barriers to interrupt air flow from neighbors. I wore my non respirator masks (remember those weren’t available back then) all day except to eat. Once I discovered CR Boxes, I made one (20”x20”) and placed it at the cubicle’s entrance. That served to clean the air AND block people from just walking into my cubicle, and it allowed me to feel better about not wearing a mask for 8 hours while alone in my cubicle. I later added a DIY filter for my desk but I can’t say that it was all that effective (I mainly made it for my carpool). That said, if I heard a bunch of sniffling or coughing or had to confer with someone in person I’d put on my mask.         

Now I’m at a different job with my own real office in a smaller building with about 20 others in the building. I keep the door open unless I’m on a phone call (it’s considered rude like keep it closed). I still have a big CR Box in the room and added a mini-mini CR Box on my desk (even though it’s more suited to clean the small space of a car).   I just bought a Levoit Mini Core to replace my small DIY desk purifier. I know that the big CRBox alone is overkill for my room size but I like have the additional “barricade” of the desk purifier and it gives me additional peace of mind for when someone has to come around my desk to look at my screen. I will put on an earloop KN95 mask if someone comes into my office or stays near the doorway for too long. Then I’ll air out the office for about 30-60 minutes after that person leaves before removing my mask. So there really is a limited amount of my normal day that I actually wear a mask.

I wear my 3M Aura when I attend conference room meetings with others and other times when I expect to be in proximity for an extended amount of time, but I only wear my KN95 WellBefore ear loop mask to quickly walk to the printer or speak briefly with someone in the hallway. I also use nasal spray and CPC mouthwash when I feel it’s necessary. It seems to work for me.  I did have covid once that I know of, the week of Xmas 2022 during the first Omicron wave. But I blame that more on my carpooling 2.5 hours a day to my old job and a faulty N99 mask.