A S trap is not considered legal in most state plumbing codes due to which of the following factors?

The primary reason an S trap is not considered legal in most state plumbing codes is due to the lack of venting. Venting is essential in plumbing systems as it allows air to enter the drainage system, helping to maintain proper pressure and aiding in the smooth flow of wastewater through the pipes. When a trap is not vented, it can create a vacuum that siphons water out of the trap, allowing sewer gases to enter the building and compromising indoor air quality.

Without proper venting, S traps risk causing plumbing issues such as slow drainage and foul odors, as the water seal in the trap can be lost. This potential for sewer gas intrusion and the related health hazards is a critical concern that plumbing codes address. Thus, the absence of venting is a significant reason for the prohibition of S traps in most jurisdictions.