We moved into our 35 year old house 3 years ago. Ever since we moved in, we have experienced and suffered from a rotten egg type smell intermittently. Sometimes we can go a month without smelling it, other times we smell it for three days in a row. We can't locate the source as it quickly spreads through most of the house (basement, main floor Living and dining rooms, upstairs bedrooms). It does not appear to be coming from either of the bathrooms. The only thing we can determine is it seems to be mostly at the front of the house on all levels. All bath, toilets and sinks are used on a regular basis. We have tried dumping bleach down all the drains and letting it sit overnight. That didn't works as we smelled "the smell" the next night. We had a plumber come in and do a smoke test from the outside cleanout by the sidewalk. I thought that would find something. The test did not reveal any leaks. My wife and I are pulling our hair out trying to resolve this. We don't know if this smell is harmful. We have a newborn and we are already paranoid. So far, no professional has been able to help us. Any enlightenment would greatly be appreciated.

"intermittently" is a word a plumber hates to hear. If something's working at the time we check then it's unfixable as long as it keeps operating. Having said that, any thing I say will be a guess.
Has anyone considered backdrafts that can send sewer gas BACK INTO your system instead of taking it out? They make a back draft preventer that fits over the roof vent,(see image) that just might solve your problem. Good luck, Tom

Thanks for your input. Where can you get these backdraft preventers. (The image is very small, I can't make it out.)

Thanks