Currently, smallpox is known to be kept only at those two locations. Both house the virus in biosafety level four (BSL-4) facilities. This level of safety the highest possible requires researchers to wear a positive pressure suit that resembles those worn in space and includes a tethered air supply. All manipulation with the virus, which is stored in liquid nitrogen tanks, is done in bio-safety cabinets. When exiting the lab, researchers spend seven minutes in a chemical shower to disinfect themselves, followed by a water rinse. No air escapes from within the BSL-4s rooms, and each year the CDC lab is shut down for a month to undergo preventative maintenance.