A foul odor every time you flush your toilet is often due to a broken airtight seal. Normally, the drain venting system prevents sewer gas from entering your home. A U-shaped pipe, known as a trap, holds a specific amount of water after each flush. This water acts as an airtight seal to block sewage gases. However, if excessive negative pressure occurs, the water level can drop, breaking the seal and allowing unpleasant odors to escape into your bathroom. Regular maintenance and ensuring proper venting can help prevent this issue.