Final answer:
Schedule 40 plastic DWV systems must not be tested using air because of the safety risks; water or vacuum tests are preferred.
Step-by-step explanation:
Schedule 40 plastic DWV (Drain, Waste, and Vent) systems shall not be tested by air. Testing these systems with air can be dangerous due to the risk of the plastic piping shattering if it fails under pressure, which could lead to personal injury or property damage. Instead, water or a vacuum is often used to test for leaks and ensure system integrity during the installation and inspection process. The use of water as a testing medium is preferred because it is non-compressive, which ensures that any failures do not result in the violent shattering that could occur with air or other compressive testing means.