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

R Anthony Cox,Timothy J Wallington,Erik H Hoffmann,Michael E Jenkin,John N Crowley,Christopher J Penkett,Markus Ammann,Abdelwahid Mellouki,Paul T Griffiths,Andreas Tilgner,V Faye Mcneill,Hartmut Herrmann

doi:10.5194/acp-21-13011-2021

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.

Highlights

Flowing ambient air over microorganisms supported on spider-web filaments resulted in a loss of viability, which was reduced to zero after ∼ 3 h of exposure unless the air was first passed through a brass tube, which removed the germicidal agent
Laboratory experiments exposing Escherichia coli to ozone, sulfur dioxide (SO2), nitrogen oxides (NOX), formaldehyde (HCHO) and ionised air at ambient levels showed that none of these were the responsible agent, and fluctuations of temperature and RH were ruled out
Druett and May (1968) postulated that the open-air factor (OAF) was identical, or closely related, to the “ozone–olefin complex” identified earlier in phytotoxic air pollution. This postulate was supported by additional laboratory work at Microbiological Research Establishment at Porton Down (MREPD) by Dark and Nash (1970), who showed that the ozonolysis of a series of alkenes resulted in significant, but variable, inactivation of E. coli and Micrococcus albus

Summary

The history of the open-air factor

The public health benefits of fresh air have been recognised since ancient times (Brimblecombe, 1995; Hobday, 2019). Druett and May (1968) postulated that the OAF was identical, or closely related, to the “ozone–olefin complex” identified earlier in phytotoxic air pollution. This postulate was supported by additional laboratory work at MREPD by Dark and Nash (1970), who showed that the ozonolysis of a series of alkenes resulted in significant, but variable, inactivation of E. coli and Micrococcus albus. De Mik and De Groot (1978) at the Medical Biological Laboratory (TNO, Rijkswike, the Netherlands) found substantial damage to the DNA in aerosolised E. coli after exposure to air containing ozonised cyclohexene, indicating that OAF(s) could enter the outer lipid or fatty-acid protective layer(s) of the microorganisms and that they or free radicals deriving from them are accessing and interacting chemically with DNA within the cell. While the formation of the key species from alkene–O3 reactions has been demonstrated experimentally, it is likely that the same species (and/or similar species) are formed from additional pathways in ambient air (e.g. HO- or NO3initiated oxidation of volatile organic compounds) with a number of sources contributing to OAF

Revisiting the open-air factor

Findings

Implications