I have a toilet on the ground floor of a 3 story 100 year-old building which was converted to condos in the 80's. Recently large bubbles of air splash up into the bowl, often repetitively, sometimes for a minute or more, without my flushing the toilet or using any of the plumbing in my unit. If I flush my toilet soon after this happens, it backs up nearly to the rim, then slowly drains over the next several minutes. Flushing the toilet when these loud, large bubbles haven't occurred for awhile seems to be perfectly normal, but I'm afraid this is only going to get worse with time. No one else in the building reports any plumbing problems. Does anyone have any idea what the problem might be, and what I can do about it? A friend's husband is a plumber, and he says he's stumped, so I'm a little reluctant to call a plumber...

A friend's husband is a plumber and he says he's stumpedSend your husbands friend back to plumbing school 101.
Those big bubblers you see aren't air they're sewer gas and sewer gas is a health hazard to your family.
You have a partial blockage downstream from your toilet that must be snaked. This can be done by pulling your toilet and snaking from there.
Because this happens when another unit flushes or drains this puts the problem in a common area and the expense should be picked up by the condo association. Talk to them about having 6this problem taken care of. Good luck, Tom

Tom, thanks for your speedy reply. I have a fairly sensitive nose, and haven't noticed any sewage odors, even when sniffing for them over the bowl after a "gas attack". Can the bubbles in this case just be air in the pipes from the vent(s)?

Can the bubbles in this case just be air in the pipes from the vent(s)? No! Think about where they came from. What happens is this.
You flush and the discharge hits a partial blockage and bounces back sending a bubble of sewer gas ahead of it. It doesn't make any difference if it smells or not. It still come back up from the sewer pipe. Does that make things a little clearer?

Yes, that makes a lot of sense. I read what you said about all the different toxic gasses in sewer gas in another post. Should I keep the bathroom fan running until a plumber fixes this problem?

If I understand you, then, if the blockage were in my units' plumbing, then there would be no problems when my own fixtures were not being used, and probably a different set of problems (like no bubbles, but consistent back up when flushed) whenever I did use my fixtures. Is this correct? I'm asking in case there is some dispute over this by the home owners' ***'n.

Trust me! The blockage is in common area pipes used by other units. The fact that your toilet bubbles when a upper unit flushes speaks loud and clear to me. There should be no dispute over this. Since the clog is in a pipe that everyone uses it falls on the Condo Board to have it repaired. Let me know if they give you a hassle over this.

Should I keep the bathroom fan running until a plumber fixes this problem?] I don't know about all the time but I would keep the door shut band turn the fan on for a few minutes before I went in and used it. Please keep me up to date. Cheers, and bTom