TLDR: Work outdoors, expose to sunlight, wear mask, disassemble, clean circuit boards with 90% isopropyl alcohol, clean covers with soap and water, dry completely.
I will give an answer to my question as to what I am doing with an Atari 800 that I just received.
Firstly... it was an untested unit, no power supply. First thing I did was open up the box outside and put all the cardboard and packing in a recycle bin outdoors. Next I did take the chance of bringing the unit into my little workshop.
I noted some insect parts in the serial connector.. oh boy... and I plugged it in to a monitor and a power supply... it did not boot up but the light came on.
I then wrestled with the top cover to get it off.. it was held down with screws and aluminum tabs, and the top seemed a bit jammed on... got it off, reseated all the cards, applied power and i did hear in the audio the typical boot up buzzing sound.. no video.
By that point I had a strong dust reaction to it, so I put it in the garage, inside a plastic garbage bag zip tied shut. to leave until I was up to it.
Today I took it out of the bag and set it up on a tray table in direct sunlight, clear day, and let it just sit in the sun for 30 minutes, with the cards exposed. took the cards out, tipped it over so the bottom was up and let it get exposed for another 20 minutes, while I washed the cards down with a spray of 91% isopropyl alcohol.
Oh, all this with an n95 mask on. Then took the screws out of the bottom cover, removed it and found lots of loose dust in there, got a handheld vacuum cleaner that I don't mind contaminating and started vacuuming.. I noted two large "dust balls" about an inch and half inch in diameter in the case... Later I realize since there were insect parts in the dust balls, that these were insect nests, but I saw no living insects... ( I did ask chat gpt about this, it's probably some spider lived in there at some point.
Done for the day, and since my vacuum battery quit before I could finish the main unit. I put it and the keyboard back sealed in the plastic bag.
After doing this I have no allergic reaction to the activity...
Next steps.. again outdoors. take the main unit apart to pieces. Wash the aluminum, top and bottom cases in soap and water, spray down the cpu board , mother board and power board in isopropyl alcohol...
Oh... I have to remove every keycap and wash them in soap and water -- they have dirt on them -- probably this machine was kept in a dusty, but humid environment.
And I'll decide if I need to take the keyboard apart to pieces to clean it's components.
Then put it all back together, and on a day when I have no allergy problems, give it a test.