Prior Exposure of Human Cells to Near Uv‐radiation Gives a Decrease in the Amount of the Unscheduled Dna‐synthesis Induced by Far Uv‐radiation

Pretreatment of human cells with near UV radiation (UVA) in fluences exceeding 5 × 104 Jm−2 caused a decrease in the amount of the unscheduled DNA synthesis induced by far UV radiation (UVC). The DNA repair synthesis, as measured by the incorporation of [3H] ‐thymidine, is reduced by nearly a factor of 2 for a UVA radiation exposure of 1.5 × 105 Jm−2. Since solar UVA fluence rate is rather independent of latitude, this figure corresponds to a UVA exposure time of 50‐60 min from noon sunlight in the summer time.