What do traps primarily prevent from entering a structure?

Traps are an essential component in plumbing systems, specifically designed to prevent the backflow of sewage, offensive odors, and vermin into a structure. They create a seal with a small amount of water that sits in the trap, which blocks harmful gases and smells generated in the sewage system from escaping into occupied spaces. This trap mechanism ensures a sanitary and safe environment within the building, contributing to both health and comfort.

The other choices do not accurately represent the primary function of traps. Fresh air is necessary for ventilation and is generally encouraged to enter a structure. While traps do play a role in managing temperature within plumbing systems, their primary design does not focus on heat or cold air. Similarly, although they contribute to preventing leaks by maintaining proper water levels in drain systems, the primary intention is to contain sewage and uphold sanitary conditions.