I saw that there was a discussion about a rotten eggs smell. I get that when it rains, and the toilet has not been used for a few hours. Why does the rain make it worse?

Get the smell from where? Is it only when it rains that it happens, or can it be reproduced other times?

The smell is in the bathroom,it is very strong but only there intermittently, and I noticed it yesterday night. I cannot locate it. There is a bath, a toilet and a handwash basin. I saw this topic on this website, which said that it could be caused by gases trapped. The toilet had not been used about 6 hours, and it was raining yesterday night.
However, it has totally vanished this morning.

This topic (though slightly different) was discussed here. The toilet,however, in my case, is used everyday.
[www.plbg.com]



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It could be a problem that is completely unrelated to plumbing and more to do with moisture in general, or you have a sewer gas leak coming from a bad seal under the toilet. Sometimes with atmospheric changes sewer gas may come out when it normally doesn't. Replacing the wax ring would be the easiest way to eliminate the possibility and possibly stop the smell.

Only happens in the rain. That would make perfect sense if rainwater was entering the sanitary sewer. As the water height increases in a drain pipe the air that flows opposite the drainage gets compressed a bit. If your wax seal is weak , then, yes, sewer gas can enter the home. Storm pipes are designed to flow full and sewer lines are sized to flow half full. Decreasing the air space has a huge effect on all the vents on the sewer. Without vents the traps would siphon and vents cut off. The system needs to breath (yes, has bad breath). I would guess you have a combined sewer in the street or a flagerant cross connection somewhere. Try replacing the wax seal first. Use 2 of them.

Remember, the gas company adds "rotten egg" smell the the gas so you can smell it. But gas leaks "leak" rain or shine. So I do hope it IS hydrogen sulfide or methane you are smelling.