**4.4 Hydrogen peroxide silver ion devices**

Chemical solutions, even within the range of H2O2 technologies, differ not only in concentration, but also in their formulation. Some available H2O2 solutions contain additional active ingredients, such as the heavy metal silver nitrate [70]. Although silver has a long history of use in wound care, it is also known to cause a permanent retention of silver once in the body [71]. Silver ions are one of the most toxic known forms of heavy metal [70]. Accidental ingestion of these invisible silver residues can cause problems for the microbiome of the human digestive system, since these metals lack the ability to differentiate beneficial bacteria from pathogenic bacteria [72]. Silver persists not only in the body, but also in the environment, where it remains toxic and can be lethal to organisms [70]. As a result of a growing understanding of these unintended negative consequences, the use of silver for disinfection is regulated by the European Union (BPR, Regulation (EU) 528/2012) which states that "It may unnecessarily expose humans, animals and the environment to biocidal active substance, generate health and/or environmental risks and impacts, and may also contribute to the development of resistance to biocides leading to other health and/or environmental issues" [73]. Likewise the EPA acknowledges the potential health hazards related to exposure to silver, and has issued cautionary documents to this effect [74]. Due to the high level of potential exposure during residue cleanup, and the resulting inhalation or dermal absorption of this heavy metal, proper protocols and control should be always employed [74]. Devices for aerosolizing H2O2 with silver vary in size from toolbox-sized fixed systems in mobile transportation to large, stand-alone portable systems. Some of these systems spray in a mist, while others use a more wet delivery method which may impede the generation of floating aerosols [75].
