I have a foul sewer smell in my bathroom, it get stronger when water is ran anywhere in the house. But the smell stays in the bathroom. We have checked the sink pipes nothing in there. We used rid-x but nothing change. I tried bleach down the drain still nothing changed. The smell is not in any other sink, just the one. I need to know if this sewer smell is bad on health? I have my 80 year old mother here, and don't want her or anyone to get sick until we find out where it's coming from. What can I do to get this smell out of my bathroom? What could possibly be the problem?
First I'd want to confirm that it's really from the waste system (as I would think). When there's no smell, plug another sink and run water to put some in the basin. Stop and verify still no smell. Then drain the basin and see if the smell arises. If that happens, you can be sure it's related to the waste system.
Can you plug the sink temporarily (both the drain and the overflow)? If you do so, does that prevent the smell? It would be nice to isolate it to a single location. Is there a tub/shower? Could it be coming from there?
If it's from the sink, I'd think there's either improper venting or incorrect p-trap in the bathroom. Can you show a picture of the pipes under the sink? If it's not set up correctly, then water running down the pipes from elsewhere can siphon the water from the p-trap, allowing sewer gases to escape the drain.
I would bet it's "sewer gas" aka methane. And, yes, it's not good for your health, though it takes a fair amount, in high concentration, to cause an explosion or other health concerns.
Your problem is most likely caused by one (or more) of the following:
drain is partially clogged by really smelly black "gunk" which is decomposing slowly
your P-trap isn't retaining water that prevents methane from getting into the room (leaking, not vented properly, water velocity too high...P-trap too low or horizontal section into the wall is too long...these things would be noted when work was completed)
not vented correctly (not likely unless work has been done recently) or vent no longer working correctly (clogged where it exits the roof by leaves, bird nest, etc. or cracked...did you hang any pictures recently in which the nail hit the vent pipe?)
Note: the above assumes the smell is coming from one bathroom sink drain, though a clogged vent stack, through roof, would cause smell anywhere.