**4.3 High concentration H2O2 vapor**

High concentration H2O2 devices are roughly the size of a medium file cabinet, wheeled around facilities on four castors and can be very heavy, weighing up to 500 pounds (227 kg). They are operated via touchscreen displays and the range of treatment area is between 8,800 to 20,000 ft<sup>3</sup> (249 to 566 m<sup>3</sup> ), depending on the device. One system can connect up to 10 devices via ethernet cables linking one device to another and enabling the treatment of larger spaces. Validation of these vaporous systems is determined using chemical and biological indicators, often *G. stearothermophilus* (1 <sup>10</sup><sup>6</sup> ) an international standard for determining success in sterilization procedures [44]. These systems may not offer handspray or port-in capabilities; however, they can integrate into various chambers or rooms.

High concentration vaporous H2O2 systems traditionally employ a 35–59% H2O2 liquid solution, heated to a vaporous state [29]. These chemicals must be handled with care, since human contact with the liquid or vapor can be harmful and has been known to result in second- and third-degree burns [29–31]. Once heated, these chemicals are delivered to the treatment space, where vapor concentrations can reach peak levels of up to 1,400 ppm H2O2 [36], often necessitating precise operating conditions and continuous monitoring of the treatment cycle by the operator(s). A myriad of sensors precisely measures peak concentrations and these aid in delivering a specific combination of conditions to result in efficacy. These systems can be highly complex, accompanied by user manuals nearing a hundred pages of instructions. The four-part fogging process—dehumidification, conditioning, decontamination, and aeration—may require a technician to be present during the entire cycle of several hours [34, 69]. One reason for this vigilant monitoring may be to respond quickly should the system over or under deliver the high concentrations of H2O2 required. Another reason for persistent oversight may be a valid fear of escaped H2O2 vapor, which could migrate out of the treated space at high concentrations and affect personnel [38, 40].
