1.   But do not put the drain directly into the sanitary sewer line, because this could cause sewer gases to come into the basement.

2.   If it were sewer gas coming up from an empty trap, the odor would be continuous.

3.   If natural gas or even sewer gas is present, the sparking can touch off an explosion.

4.   It is not exhausting sewer gases but rather stale air, so you can keep the foundation planting a few feet from the outlet.

5.   Q. I sometimes get a small whiff of sewer gas in the basement of the old house I just bought.

6.   Sewer gases show up in the back yard from the soil stack that goes through the roof.

7.   This could result in sucking water out of the trap, thus allowing noxious sewer gases to enter through the sink drain opening.

8.   Turning on the tap might aggravate this, but water from the sink quickly fills the trap, cutting off the sewer gas.

9.   When the fan is not on, sewer gases can downdraft into the bathroom.

10.   Without the water in the trap, smelly sewer gases would come up through your sink drains, making your house unlivable.