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Atmospheric hydroxyl radicals (•OH or OH•) are the most remarkable molecule you’ve never heard about. Known as “nature’s detergent,” OH• naturally occurs in the Earth’s atmosphere and is responsible for eradicating volatile organic compounds (VOC), viruses, bacteria, and mold.
Hydroxyls are a green, non-toxic solution safe for humans, animals, and plants, making them ideal from an environmental, health and safety perspective for a wide variety of industries and uses. Hydroxyls can:
During daylight hours, the Earth continuously receives the Sun’s ultraviolet (UV) energy. When this UV energy is imparted on water molecules in the atmosphere (relative humidity), a reaction occurs. One hydrogen atom is removed from a water molecule, leaving behind a negatively charged oxygen and hydrogen molecule. This negatively charged molecule is known as the hydroxyl radical.
Hydroxyls are non-toxic, non-corrosive, and they don’t bleach cloth or deteriorate elastomers. OH• molecules are able to break down VOCs by transferring sufficient energy to break single and double bonds. Hydroxyls also kill virus, bacteria, and mold by breaking down their outer membranes, resulting in cell lysing. This kills the microorganism and prevents replication. OH• also decomposes endotoxins and pyrogens that result from killing microorganisms.
Because of these amazing properties NASA began researching ways to produce hydroxyls in enclosed spaces for use onboard their space shuttles. While successful in developing an OH• generator, most early renditions only produced very low volumes of extremely weak hydroxyl radicals. These devices were photocatalytic, meaning they required an additional chemical.
Over the years, OH• generators have evolved and are now capable of producing super-charged OH• molecules that can be deployed in large enclosed spaces as stand-alone units or integrated into existing ventilation or HVAC systems.
Figures 1, 2 and 3 show hydroxyl radical effectiveness at eliminating different microorganisms – specifically E. coli, Coronavirus, and Aspirgillus Niger (black mold). These tests were conducted by an independent biological sciences laboratory using one experimental and one control group. The charts depict the results of the samples that were exposed to hydroxyl radicals.
Note the high elimination rate and short OH• exposure time for E. coli and Coronavirus. Even black mold, which is extremely difficult to eliminate, was subdued after only 72 hours. Keep in mind, OH• treatment is a continuous process; microorganisms won’t be able to get a foothold or propagate when OH• is abundant.
Hydroxyl radicals are naturally occurring within our atmosphere, and as such, humans, animals, and plants have all evolved in and thrive in this hydroxyl-rich environment. In fact, according to the National Institute of Environment Health Sciences (NIEHS), “[There is] no science or research indicating that hydroxyl radical generation is harmful to humans. This applies to both naturally occurring and man-made generation.”
In the practice of industrial hygiene, the mantra for dealing with hazardous environments is “Destroy, Dilute, PPE.” This means your priority should be destroying the toxic substance if possible. If destruction is not possible, then diluting it to an acceptable level is the next best solution. Finally, if neither option works, providing and requiring personnel to wear PPE is the remaining option.
While diluting and providing PPE are options, they aren’t good answers; both have associated costs. For example, dilution doesn’t change the volume of toxic substances that are released into the environment, it simply reduces the concentration within an immediate area while increasing heating and cooling costs. PPE, which must be provided and worn, must be properly cared for and disposed of. There is also an increased risk of the equipment not be used or worn appropriately, resulting in dangerous health and safety issues.
With the use of hydroxyls, which sanitize indoor environments and remove VOCs and odors, the destruction of hazards elements is possible.
Other benefits include:
There are three key reasons why you might not have heard of hydroxyl radicals.
Hydroxyl generators capable of producing the necessary volumes of OH• required to properly sanitize areas were only developed and commercialized within the past several years. Before that, photocatalytic hydroxyl generators were not effective enough to solve sanitation and VOCs issues on a large scale.
Early OH• generator companies lacked sufficient experience and knowledge of commercial and industrial applications. They had difficulty speaking the language of manufacturing, industrial, and facility customers, and they struggled to develop branding and a message that properly educated customers on the technology.
In the beginning, OH• generator companies engaged with early adopter facilities, and these facilities often required confidentiality agreements to maintain their competitive advantage. This slowed the adoption of this technology.
In January of 2023, I conducted an experiment to demonstrate the effectiveness of hydroxyl radicals using packages of blackberries. Two packages were selected from a local supermarket from the same display case, so they were likely picked, packed, and shipped at the same time and under the same conditions.
One blackberry package was placed in a room with a photolytic hydroxyl radical generator running. The other package was placed in a comparable room, but without a generator. Both rooms had temperature and humidity reading devices to show the two samples remained under similar conditions.
The experiment ran for nine days, and by the sixth day, the blackberries in the non-hydroxyl treated room were showing early signs of mold growth, and over the next three days, it continued to spread until most of the blackberries in the package were affected.
In the hydroxyl-treated room, the blackberries didn’t show signs of mold until the seventh day, however, the mold did not spread to any other blackberries in the package. The mold that appeared on these blackberries was also different in appearance than the non-hydroxyl treated blackberries. As shown in Figure 4, the mold was greyish and not fuzzy, instead of white and fuzzy. It appeared the hydroxyl-treated mold was dead.
At the experiment’s end, both samples were inspected (see Figure 5). Of the 29 blackberries in the non-hydroxyl sample, 20 showed signs of mold. Of the 33 blackberries in the hydroxyl sample, only three had evidence of mold.
But why, if hydroxyl radicals are so impressive, did the treated sample still develop mold? With botrytis mold, spores often enter flowering plants and become encapsulated within the fruit. When the berry’s skin ruptures, the mold spores begin to grow. The good news is the hydroxyls began killing the mold immediately and prevented the spread to adjacent blackberries.
You can think of hydroxyl radicals as an immune system for indoor environments. They:
Not only does OH• produce health and sanitation benefits, but it also encourages increased profitability and safety by lowering or avoiding the cost of dangerous cleaning chemicals that threaten the well-being of employees.
Man-made hydroxyl radicals are green, nontoxic, and noncorrosive molecules that can be continuously deployed, whether people are present or not, which reduces interruptions to maintenance and production activities. This technology can supplement or even replace other systems and procedures to minimize disruptions, increase efficiency, and create exciting new opportunities for facility advancement.
This article was originally produced for use at Reliable Plant Conference and materials from this article were featured in Plant Services.
Tom Moriarty will be a featured speaker at the 2023 Reliable Plant Conference and Exhibition.