Many water suppliers require this, and plumbing code may legally require permanently installed backflow preventers. To reduce the risk of contamination, a backflow preventer can be fitted. The backflow of urine is prevented by valves known as ureterovesical valves. Also sewage lifting stations provide comprehensive protection against sewer backflow. The proximal region contains valves which prevent the backflow of sperm when muscles dilate. There are valves in most veins to prevent backflow. Valves to prevent cement backflow did not close. The semilunar valves close to prevent backflow into the heart. Backflow prevention devices such as "anti-siphon valves" are required in such designs. All plumbing codes require backflow prevention in several ways. Many states and/or local municipalities require annual testing of backflow prevention assemblies. The Lean combustor reduces NOx gases with required pressure loss and backflow margin. The simplest, most reliable way to provide backflow prevention is to provide an air gap. Lastly, an air gap or barometric loop are methods to prevent backflow from taking place. Serum urea and creatinine determinations may be necessary to rule out backflow kidney damage.