The introduction of the first water-retaining traps or seals in the 1850s, and the common use of "U"-shaped traps in the 1870s, were major steps forward. The design of the trap ensured that enough water remained in it to effectively block the passage of sewer gas back through the plumbing fixtures. During the 1870s vent pipes were introduced. They not only carried sewer gas out of the system, but also broke the suction created by movement of liquid in the pipes and eliminated the possibility of siphoning water that sealed the traps.
Those who attempted to bring plumbing indoors faced technical as well as attitudinal challenges. Decisions on how wastewater was removed required as much concern as those made to ensure an adequate water supply. But equally vexing was the prevailing miasma theory of disease, which held that illnesses stemmed from "bad air" that was readily identifiable by its offensive odor. This led to a distrust of early indoor plumbing that tended to leak and a deadly fear of the sewer gas that accompanied the leaks. It is no wonder then that many individuals maintained a strong belief that elimination was best taken care of out of doors.