Opinion: The germicidal effect of ambient air (open-air factor) revisited

Abstract

Abstract. The term open-air factor (OAF) was coined following microbiological research in the 1960s and 1970s which established that rural air had powerful germicidal properties and attributed this to Criegee intermediates formed in the reaction of ozone with alkenes. We have re-evaluated those early experiments applying the current state of knowledge of ozone–alkene reactions. Contrary to previous speculation, neither Criegee intermediates nor the HO radicals formed in their decomposition are directly responsible for the germicidal activity attributed to the OAF. We identify other potential candidates, which are formed in ozone–alkene reactions and have known (and likely) germicidal properties, but the compounds responsible for the OAF remain a mystery. There has been very little research into the OAF since the 1970s, and this effect seems to have been largely forgotten. In this opinion piece we remind the community of the germicidal open-air factor. Given the current global pandemic spread by an airborne pathogen, understanding the natural germicidal effects of ambient air, solving the mystery of the open-air factor and determining how this effect can be used to improve human welfare should be a high priority for the atmospheric science community.

An Old Defence Against New Infections: The Open-Air Factor and COVID-19

“…Rather, they believed it to be due to a mixture of unidentified reactive chemicals. A recent review of the evidence reached the same conclusion [17]. While the causative agents have yet to be appropriately identified, the most likely candidates that contribute to the OAF may be a range of distinct germicides which are formed when alkenes in the air react with ozone.…”

“…By the 1970s, scientists had concluded that the OAF was not a single molecule [15] but a mixture of highly reactive chemical species which varied in composition [16]. The authors of a 2021 review paper on the subject supported this position, arguing that no single molecule or class of molecules can achieve the level of bactericidal activity reported in the Porton Down experiments [17]. One potential component of the OAF, the hydroxyl radical (HO), has been generated artificially and used to kill airborne pathogens [18][19][20][21].…”

“…Instead, a range of products -each acting as distinct germicides -appear to be combining and contributing to the OAF. But they conceded that 'the compounds responsible for the OAF remain a mystery' [17]. This may explain why the OAF continues to be neglected in public health and infection control.…”

Chemical Fate of Oils on Indoor Surfaces: Ozonolysis and Peroxidation

“…It is well-known that both ozonolysis ,, and autoxidation reactions of lipid aerosol impact air quality and climate. , In particular, organic peroxide formation in aerosol has drawn substantial research attention recently. Not only may it affect the oxidizing ability of the atmosphere through decomposition, − but also ROS exposure through inhalation of particulate matter is often linked to aerosol toxicity. − As well, the presence of lipid-base peroxides in the ambient air could also contribute to the germicidal power of the atmosphere …”

Opinion: Atmospheric Multiphase Chemistry: Past, Present, and Future

“…Lastly, the pandemic prompted a re-examination of an overlooked aspect of our atmosphere that it has an as-yetunidentified germicidal component referred to as the Open Air Factor (Cox et al, 2021). In particular, it was shown many decades ago that fresh air led to better outcomes for tuberculosis patients and injured World War I soldiers than indoor air.…”