The home we just bought had an addition put on. The addition bathroom waste pipe goes into a "y" pipe with one end tying into the main pipe and the other part of the "y" is uncapped. Besides fumes which we rarely seem to get from it, what dangers does this pose and what should we do with it?

The home we just bought had an addition put on. The addition bathroom waste pipe goes into a "y" pipe with one end tying into the main pipe and the other part of the "y" is uncapped. Besides fumes which we rarely seem to get from it, what dangers does this pose and what should we do with it?

This is not good! You house has a direct open connection with the city sewer allowing sewer gas to enter your home. Sewer gas is a health hazard to your family and the methane content makes it explosive.
That wye should be capped off with a clean out, (see image) Don't let this slide, get it done soonest. Good luck, Tom