Depending if the cap was missing from the house side of the running trap or the street side (sewer) would determine the type of gas you have been inhaling.
PLUS other variables that cant be diagnosed sight unseen and lenth of exposure etc
There is also some documented cases of several strands of hepatitis that are in the sewer waste
Certain sewer gases have been liked to some cancers like methane, nitrous oxide, hydrogen sulfide etc.
a flammable poisonous gas that has an odor suggestive of rotten eggs and is found esp. in many mineral waters and in putrefying matter
Hepatitis caused by an RNA virus that does not persist in the blood serum and is transmitted esp. in food and water contaminated with infected fecal matter — called also infectious hepatitis.