Many lamps have a shortwave UV blocking coating on the envelope and are sold as "ozone free" lamps. These "ozone free" lamps are used commonly in indoor applications, where proper ventilation is not easily accessible. The company WACOM has also a long history of xenon lamp production. Some lamps have envelopes made out of ultra-pure synthetic fused silica (such as "Suprasil"), which roughly doubles the cost, but which allows them to emit useful light into the vacuum UV region. These lamps are normally operated in a pure nitrogen atmosphere.