**2. Dry hydrogen peroxide and vaporized hydrogen peroxide**

While DHP and vaporized hydrogen peroxide both utilize hydrogen peroxide to reduce infectious pathogen burdens in a treated indoor space, there are several notable differences between the two technologies. The most apparent difference between DHP and vaporized hydrogen peroxide is that DHP is a true gas composed of individual molecules exhibiting near ideal gas behavior [18], whereas VHP is an aerosol of highly concentrated aqueous droplets. As a result, vaporized hydrogen peroxide effectively sterilizes a room, but it also may lead to aerosol H2O2 concentrations which exceed the safety limits for human exposure. Vaporized hydrogen peroxide may only be used in vacated areas. Other precautionary measures, such as sealing doors, windows, and HVAC systems, must be taken before use as well, in order to prevent unintended dissemination of H2O2 to adjacent spaces [5, 6, 9–11]. Further, in aqueous form, hydrogen peroxide forms a weak acid which is corrosive to some materials, equipment, and furnishings. Dry Hydrogen Peroxide, on the other hand, is much less concentrated, and does not cause such material compatibility issues. Dry Hydrogen Peroxide can be applied for an unlimited time of exposure and can be used in spaces occupied by humans. Dry Hydrogen Peroxide therefore represents a highly effective adjunct to the intermittent usage of harsher disinfectants.

#### **2.1 Dry hydrogen peroxide and bipolar ionization**

Bipolar ionization creates a plasma consisting of positive ions, negative ions, and free radicals, with the intention of releasing them into a space. This plasma can be generated in multiple ways, but the two primary types of bipolar ionization are

corona discharge and needlepoint. Both types of bipolar ionization utilize sets of oppositely charged electrodes to ionize ambient humidity and oxygen as the indoor air passes through the device. Corona discharge bipolar ionization is rarely utilized currently, due to the potential for generation of ozone; accordingly, most manufacturers have switched to needlepoint ionization [19]. Manufacturers of needlepoint bipolar ionization (NPBI) claim that the electrodes used in the devices produce an electric field with a voltage below 12 eV to eliminate the potential for ozone generation [20]. Dry Hydrogen Peroxide and bipolar ionization each utilize ambient humidity and oxygen in their generation processes and continuously disperse their products throughout treated spaces; however, DHP is produced as stable H2O2 molecules, while bipolar ions are an unstable plasma. Additionally, neutrally charged H2O2 generated from DHP can travel long distances, whereas the oppositely charged ions created by bipolar ionization may rapidly recombine, diminishing the effective concentration as distance from the device increases [21, 22].
