Pick up a jar of vick's vapo rub. Put some under your nose and in the mask if you wear one again.

I have zero tips for you, but I love this post. Why does his house smell so bad? Has anyone mentioned it to him? So many questions.

There are specialist face masks with active charcoal filters that filter out smells.

Sensitivity to smells will dissipate over a short time. Your senses will pay more attention to novel stimuli, so your nose will eventually get "bored" of it and stop even noticing it when you are acclimated enough.

Tell him his house smells like ass

Fun fact: one of the symptoms I get of migraine is the inability to ignore smells. At all. Cat owners? Your house smells like ammonia. Cologne wearers? You still stink underneath the cloying odor. Cleaning products? Smell forever. I get to choose between smelling my own halitosis or the unbearable mint odor for hours on end.

Can you not just visit with him outside on some lawn chairs? Or does the outside smell as well?

Is it just his place or the entire retirement community place? It might be something property management is doing or not doing

In my experience with proper stinks, you wait and you'll get used to it.

I once entered into a underground sewage treatement plant and the smell was almost physical. However after sometime your nose starts to become dissensitised to it.

Ditto when we had to work with SO2 gas (which smells like rotten eggs) in our Chemistry classes: pretty sickening to begin with, then eventually you get used to it (which was funny because we could hear the students from other, unrelated classes in the rooms nearby, complaining loudly about it whilst we didn't really care about it anymore ;)).

Haven't done it myself but people who deal with corpses supposedly take Vicks vapor rub and rub it under their nose

Help your grandfather fix the smell problem.

You could try keeping your mask inside a container with a few spoons of coffee grounds. (Use a paper towel to avoid dirtying the mask.)

If it's any consolation, the smell will probably become less unbearable in the following days, as you get used to it.

Also, depending on how close you are to your grandpa, you could find some tactful way to mention it. People don't often realise how much their houses stink.

Foam earplugs, shoved gently up your nose, but not so far you can’t get them back again, and a face mask to hide them.

This could be difficult to apply for such an extended period of time, but I generally have good results by just breathing exclusively through my mouth. Pretend you're underwater and breathing through a snorkel.

This technique has gotten me through many a fart and temporary sewage/fertilizer exposure.

Breathe through your mouth, not your nose.

What a great description 😆

Honestly, I don't think there's much you can do but I have a relative who is a nurse and they mentioned they double up on face masks and basically overdose on strongly smelling stuff. Mint drops, gum, toothpaste between the masks, essential oils, etc. I know you're not looking for ways to deal with the smell but opening some windows will help as well.

Fart. You will smell it but it won't be offensive.

Offer to do some cleaning?

Snort a bunch of drugs 1 or 2 days before you go lol. When I party on saturday I only start to smell things on wednesday

Get a mask, maybe a scented one.