Your toilet may smell when you flush because of a broken airtight seal. Sewer gas is blocked from entering homes by the drain venting system. A U–shape in pipe holds a designated amount of water in the bowl after each flush. The water acts as an airtight seal to block sewage gases. If too much negative pressure occurs, however, the water level can lower and break the seal, causing a smell after you flush. Or the seal between the floor and the base of the toilet has come away and the toilet might need to be re-set.