At a hospital, modern surgery is often done in an operating theater using surgical instruments, an operating table for the patient, and other equipment. The environment and procedures used in surgery are governed by the principles of aseptic technique: the strict separation of "sterile" (free of microorganisms) things from "unsterile" or "contaminated" things. All surgical instruments must be sterilized, and an instrument must be replaced or re-sterilized if it becomes contaminated (i.e. handled in an unsterile manner, or allowed to touch an unsterile surface). Operating room staff must wear sterile attire (scrubs, a scrub cap, a sterile surgical gown, sterile latex or non-latex polymer gloves and a surgical mask), and they must scrub hands and arms with an approved disinfectant agent before each procedure. There is moderate-quality evidence that usage of two layers of gloves compared to single gloving during surgery reduces perforations and blood stains on the skin, indicating a decrease in percutaneous exposure incidents.