Hydroxyl Radicals and their safety to humans and animals

Airora generates hydroxyl radicals through a cascade reaction between ozone and plant-derived essential oils called terpenes.

This White Paper discusses the safety aspects of the hydroxyl radicals.

What are Hydroxyl Radicals?

Hydroxyl radicals (OH) are highly reactive small molecules. The hydroxyl radical, •OH, is the neutral form of the hydroxide or hydroxyl ion (see Figure 1.).

The unpaired electron makes the hydroxyl radical very reactive as it tries to create hydroxy groups in other molecules. 

Figure 1. A hydroxyl radical has a free electron

Hydroxyl radicals are highly oxidizing, non-selective molecules, and therefore many susceptible organic molecules can easily be removed or degraded using hydroxyl radicals (e.g., acids, alcohols, aldehyde, aromatics, amines, ethers, ketone, etc.) [1; 2]. Hydroxyl radicals play a significant role in the regulation of volatile gasses such as formaldehyde and methane in the atmosphere.

Hydroxyl radicals and their role in atmospheric chemistry

hydroxyl radicals (OH) play a key role within tropospheric chemistry, and are abundant molecules in the outside air. Depending on the global latitude the exposure in the daytime in outside air can be anywhere between 0.5 and 40 Million radicals per cubic centimeter [3]. This makes it an extraordinarily ubiquitous molecule that plays a significant role in the atmospheric chemistry in reducing volatile organic compounds such as methane and formaldehyde.

 

Figure 2. Annual mean OH concentrations near the earth’s surface.

The units are 10 million radicals per cm³ [5].

 

The major daytime sources of OH include the photodissociation of ozone (O3) and nitrous acid (HONO), the photo-oxidation of formaldehyde (CH2O) and acetone (CH3C(O)CH3), together with the ozonolysis of alkenes. Nighttime processes leading to OH production also include the ozonolysis of alkenes and, more importantly, nitrate radical (NO3)-facilitated decomposition of peroxyacyl nitrates (RC(O)O2NO2) and NO3-initiated oxidation of alkenes.

The major sink processes for OH are to initiate the oxidation of carbon monoxide (CO), methane (CH4), and a wide range of reactive volatile organic compounds (VOCs), and to form nitric acid (HONO2) via reaction with NO2 [4].

Use of hydroxyl radicals as antimicrobial and antiviral agents

Hydroxyl radicals are already widely in use as antimicrobial and antiviral agents in several varieties.

First, hydroxyl radicals are generated through reactions of hydrogen peroxide, a common antibacterial cleaning product [7; 8]. In addition, popular air purifiers that emit positive and negative ions also work on the principle of hydroxyl radical action, as they bind to the surfaces of bacteria and viruses and generate hydroxyl radicals. These hydroxyl radicals quickly break down proteins and cell membranes to destroy the pathogens [9]. Thirdly, a number of technologies such as Airora produce hydroxyl radicals directly, which exhibit their destructive action on pathogens as described above [10, 11].

Hydroxyl Radicals and their effects on humans and animals

Humans, animals, and non-pathogenic bacteria have always existed in environments with high levels of hydroxyl radicals, so they have evolved mechanisms to protect themselves against the strong destructive activity that these molecules have [6]. This defence is based on a number of factors.

1. The outer layer of our skin consists of a number of layers, of which the outer most, the Stratum Corneum is a layer of 20-30 cells thick that is made up of keratin and horny scales of dead keratinocytes. This layer is impenetrable to hydroxyl radicals and provides protection against these molecules [12].
2. Hydroxyl radicals play a role in the intercellular metabolism. As such, cells have a built-in protection against reactive oxygen species such as hydroxyl radicals. Inter- and extracellular protection of living cells against reactive oxygen species such as hydroxyl radicals occurs through antioxidants and enzymes such as superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase [14; 15; 16; 17; 18]. As hydroxyl radicals come into contact with soft tissue in the face such as eyes, nasal or oral cavity, they are rendered inactive by the action of these enzymes and naturally present vitamins such as vitamin C, E [18].

A recent 13-week toxicology study on rats indicated that the output of hydroxyl radicals, oxidants, ozone and reaction by-products was well tolerated by the rats and did not induce any detectable toxicity under the conditions used in the study [13].

The Airora technology generates at maximum output 2M hydroxyl radicals per cubic centimetre. As Figure 2. indicates this is about 50% of the average level exhibited in the outdoor air in Northern Europe on a continuous basis.

By contrast, the continuous outdoor exposure in Northern America, India and Central Africa averages at about 20M hydroxyl radicals per cubic centimeter. There is no record of any adverse effects of continuous exposure to OH in these regions. Therefore, exposure to hydroxyl radicals generated by Airora is comparatively limited and considered completely harmless.

 

References

[1]  Tchobanoglous, G., Burton, F., & Stensel, H. D. (2003). Metcalf & Eddy, wastewater engineering, treatment, disposal & reuse (4th ed.). New York: McGraw-Hill.

[2] Munter, R., 2001. ADVANCED OXIDATION PROCESSES – CURRENT STATUS AND PROSPECTS. Proc. Estonian Acad. Sci. Chem., 50(2), pp.59-80.

[3] Riedel, K. and Lassey, K., 2008. Detergent of the atmosphere. Water & Atmosphere, 16(1), pp.22-23.

[4] Clemitshaw, K., 2015. Encyclopedia Of Atmospheric Sciences (Second Edition). 2nd ed. Academic Press, pp.232-238.

[5] J. Lelieveld, F. J. Dentener, W. Peters, M. C. Krol. On the role of hydroxyl radicals in the self cleansing capacity of the troposphere. Atmospheric Chemistry and Physics, European Geosciences Union, 2004, 4 (9/10), pp.2337-2344.

[6] Imlay, J., 2008. Cellular Defenses against Superoxide and Hydrogen Peroxide. Annual Review of Biochemistry, 77(1), pp.755-776.

[7] Hernandez, P., Sager, B., Liang, T., Lozano, C. and Khazzam, M., 2018. Bactericidal Efficacy of Hydrogen Peroxide on Propionibacterium acnes. Journal of Shoulder and Elbow Surgery, 27(4), p.e131.

[8] Ibi, H., Hayashi, M., Yoshino, F., Tamura, M., Yoshida, A., Kobayashi, Y., Shimizu, K., Lee, M., Imai, K. and Ogiso, B., 2017. Bactericidal effect of hydroxyl radicals generated by the sonolysis and photolysis of hydrogen peroxide for endodontic applications. Microbial Pathogenesis, 103, pp.65-70.

[9] CORPORATION, S., 2020. What Is Plasmacluster?. [online] Sharp Plasmacluster | Sharp Global. Available at: <https://global.sharp/pci/en/about_pci/> [Accessed 8 October 2020].

[10] Airora. 2020. The Chemistry Behind Airora. [online] Available at: <https://www.airora.com/chemistry.html> [Accessed 8 October 2020].

[11] Pyure Dynamic Protection. 2020. Scientific Evidence - Pyure Dynamic Protection. [online] Available at: <https://pyureco.com/pyure-technology/scientific-evidence/> [Accessed 8 October 2020].

[12] Yousef, H., Alhajj, M. and Sharma, S., 2020. Anatomy, Skin (Integument), Epidermis. [online] Ncbi.nlm.nih.gov. Available at: <https://www.ncbi.nlm.nih.gov/books/NBK470464/> [Accessed 8 October 2020].

[13] 2020. 13-Week GLP Toxicology Study Of The Odorox Boss Hydroxyl Processor Air Cleansing Machine In Rats. [ebook] Sunnyvale, CA: Comparative Biosciences Inc. Available at: <https://pyureco.com/wp-content/uploads/2020/08/Toxicology_Study_Report.pdf> [Accessed 8 October 2020].

[14] Lipinski, B., 2011. Hydroxyl Radical and Its Scavengers in Health and Disease. Oxidative Medicine and Cellular Longevity, 2011, pp.1-9.

[15] Yim, M., Chock, P. and Stadtman, E., 1990. Copper, zinc superoxide dismutase catalyzes hydroxyl radical production from hydrogen peroxide. Proceedings of the National Academy of Sciences, 87(13), pp.5006-5010.

[16] Willcox JK, Ash SL, Catignani GL. Antioxidants and prevention of chronic disease. Review. Crit. Rev. Food. Sci. Nutr. 2004;44:275–295.

[17] Genestra M. Oxyl radicals, redox-sensitive signalling cascades and antioxidants. Review. Cell Signal. 2007;19:1807–1819.

[18] Young, I., 2001. Antioxidants in health and disease. Journal of Clinical Pathology, 54(3), pp.176-186.