I have a half bathroom on the 1st floor of my 2 story Cape home, when we turn on our attic fan on the second floor a sewer smell comes from the 1st floor half bath. I have another bathroom on the first floor and it never smells. Only this half bath. Occassionally I can smell something in the basement, but primarily the smell exists in this bathroom. We are connected to public sewer, so it's not a septic issue. Any suggestions what this is caused from or why it's only smelly when we put on the attic fan? Please help!