Which piping material is NOT valid for all plastic vent pipes used in mitigation systems?

The choice of piping material for use in radon mitigation systems is crucial for ensuring effectiveness and safety. Steel is not considered valid for all plastic vent pipes used in these systems. Plastic materials, such as Schedule 40 PVC and ABS, are commonly used because they are lightweight, resistant to corrosion, and simple to install. They also ensure a good seal and are effective in maintaining the necessary airflow for venting radon effectively away from buildings.

In contrast, while steel can technically be used in some applications, it is not classified as a "plastic" vent pipe material, nor is it commonly used in radon mitigation systems for maintaining airflow effectively. Steel may suffer from rust and corrosion, particularly in environments where moisture is present, which can compromise the integrity and functionality of the venting system over time. Therefore, its use is not suitable for all plastic vent pipe applications in radon mitigation, solidifying why it is the correct answer in this context.