I once had a used 2012 iMac which I bought from a home with pets. When I had to have the hard drive changed, the person who did it said that it was full of dust inside and the fan was really covered in it.
I’m about to buy a used iMac M1 and have been offered one from a home with dogs and cats. Are the fan systems loads better these days, or is it still better - if possible - to avoid buying electrical items with fans from homes which are dusty?
Thoughts?
I don’t think this will ever practically be an issue for most people. Unless you’re talking about a computer being used in a concrete sculpting workshop, these machines are designed to handle household dust.
Not a technician, though, and perhaps other folks have higher standards than I…
This is one of the reasons I love my M1 MacBook Air. Just like an iPad, it has no fan! I can only assume it collects much less dust internally than a system that must circulate outside air to keep itself cool.
I have serviced a lot of portable (and non portable) Macs, and this have almost never been a problem. If the fans and air intake is clogged with dust/hair, it’s easily solved with some compressed air.
If you get a good price, don’t hesitate due to this, is my advice!
I don’t think the problem would be as bad with the M1, unless the person games or analyzes data all the time. The fan on mine hardly ever runs.
As a former technician, I would purchase a system from a home with pets over a smoker anytime. Fur is much easier to remove than a smoke/tar/nicotine buildup. Compressed air and an electronics vacuum make quick work of a fur buildup. The smokers buildup is equal spray paint IMHO. You can’t clean those parts. Replacing parts is the only option available.
I would always explain to customers that Computers with fans are the same as an Air Filtration system… just without a filter. Their Home or Office are always dirtier than they think they are.
My favorite repair story was a guy who owned a Deli and brought in an iMac that was overheating and shutting down. Once I opened it up, I found that the interior was coated in a yellow/tan substance and smelled like French Fries. Turns out the office was in a loft above the deep fryers. We suggested purchasing a new iMac and a new location away from the fryers!
If it was a 10 year old machine it could be more of a concern, but at most you’re only looking at ~18 months, so even if there was some dust it would be fairly minimal/typical (assuming they didn’t have it in the dog bed or have the cat crawling all over it). 
If it was used on the sets of Mad Max: Fury Road, I may skip
Most people will have a hard time for an M1 iMac fan to kick in. Will certainly not happen with casual use. Doubt therefore the fan will have a lot of mileage, let alone have been able to suck in lots of dust. Safe buy imho.
To go sure do use a gas duster to spin the fan properly and see if any lint shows.
I have no experience with cat hair, but for sure with dog hair. Short answer: I never have found any hair clogging ventilation, I never have had any issues. Not all pet owners are equal, but I have found that some pet owners may even keep their house more clean because dog hair and other dirt means that the need to vacuum or to wipe the floor more often intensifies with a dog. And that again means that there might be even less dust in a home with pets. Yes, it sounds strange. I get that.
If the Mac looks clean, you definitely should be fine.
Just a note of caution, if using canned air, don’t over-rev the fans, as that can lead to bearing/bushing failure.
I checked my January 2021 M1 and didn’t see any hair or lint around the vents. Most of that time we had a dog that shedded a lot, and a cat.
I was going to take the bottom cover off, but it was being a pain. You’ll also need a pentalobe screwdriver if you’re going to do that. I recommend Wiha tools. Excellent quality at a reasonable price.
Old time story here: about 35 years ago my dad worked in a concrete plant and he took me to visit a small cabin on the top of a silo where they loaded the agitator trucks. The operator up there of course was a heavy smoker and everything was dusty as hell. There was this IBM XT loaded with the software that controlled some parts of the process. “It has more keys than a piano”, the friendly operator would say. The surprise was when this guy smiled at me, ejected the 5 1/4’’ floppy from the disk drive, with the magnetic surface totally yellow from dust and who knows what else, and said: “The IBM people came here and said they don’t know how this thing is still working in this environment”.
