• Home  /
  • Hydroxyl news   /
  • Understanding How New Indoor Air Treatment Systems Optimize Indoor Environmental Quality and Safety

Understanding How New Indoor Air Treatment Systems Optimize Indoor Environmental Quality and Safety


Sanitizing and deodorizing indoor environments are increasingly important as our homes, work, medical, recreational and travel spaces have become better insulated and less ventilated. These indoor spaces are increasingly concentrating chemicals, microorganisms, allergens and odors that contaminate both air and surfaces.1 The result can be unhealthy and unpleasant conditions often referred to as “sick building” syndrome. These health risks are well documented by the Environmental Protection Agency and scientific studies.2,3 We experience them as an increase in fatigue, headaches, irritated eyes and throat, coughing, dizziness, allergies, asthma, and bacterial and viral infections, among other symptoms.

The range of methods and devices used to sanitize and deodorize air and surfaces is broad and spans years of development. Methods include traditional approaches like the use of manual cleaning with detergents and sanitizing chemicals, ventilation and filtration/adsorption, as well as more advanced techniques involving the use of electrical and ultraviolet energy and/or catalytic surfaces to generate sanitizing agents (oxidants) that react with and decompose chemicals and kill microorganisms.

The challenge in selecting the right methods and devices is that there is so much information – and misinformation – in the marketplace and literature. The best way to separate fact from hyperbole – hype – is to rely on science based data and information; that is results obtained using reliable scientific methods from reputable sources.

The goal of this web site is to separate scientific fact from marketing fiction and be a resource for the facts about advanced air and surface sanitization technologies. The site will discuss the use of various airborne oxidants like ozone and atmospheric ions. Its primary focus will be on the most recent advances that use hydroxyl radicals (hydroxyls) – an oxidizing agent – generated in various ways to sanitize spaces as small as a room and up to millions of cubic feet. The emphasis will be on the use of these hydroxyl systems to sanitize large, occupied spaces. Methods such as ultraviolet irradiation and vapor phase or aerosolized chemical oxidant treatment that can only be safely used in unoccupied spaces will not be addressed.

Evolution of Methods to Clean Indoor Environments

The trend to sealing buildings better in order to increase energy efficiency can reduce indoor environmental quality and safety as volatile organic chemicals (VOCs) and pathogens accumulate to levels as much as five times higher than outdoors.1,2 The kinds and amounts of indoor VOCs have increasingly been measured and better documented since the 1970’s and have evolved over time.1 Initially the focus was the measurement of noxious pollutants that originated outdoors, like:

  • Sulfur dioxide
  • Nitrogen oxides
  • Ozone
  • Airborne particulates
  • Radon

Over time other chemicals that outgassed from increasingly man-made furnishings and greater use of chemical cleaners, deodorizers etc. became a concern and were tracked. For example:

  • Formaldehyde
  • Asbestos
  • Tobacco smoke
  • Volatile organic chemicals

Indoor environmental cleaning technology has had to evolve over time to be more effective at reducing the increased amounts of particulates, VOCs, inorganic gases, and microorganisms in air and on surfaces in large, occupied spaces.

Traditional methods for cleaning and sanitizing are fundamentally static approaches. They focus on sanitizing surfaces by treatment with chemical agents like soaps and detergents and oxidizing agents like bleach and peroxides to remove dirt and kill microorganisms on surfaces. They can be effective if the chemicals are in contact with the surfaces long enough; generally sanitization and sterilization require contact times of 15 to 30 minutes to be effective.4 Once cleaned, however, the surfaces are immediately prone to recontamination from airborne sources and physical contact from people and materials. Most importantly, these methods do not address airborne chemicals and microorganisms. Volatile organic chemicals outgassing from building materials, furnishings, cleaning, outdoor pollution, human respiration, etc. are responsible for most of the symptoms associated with “sick building” syndrome. Toxic particulates like mold spores and dust contaminated with microorganisms also represent health risks.1,2

As technology evolved, newer methods became more comprehensive and dynamic. The most commonly used methods focus on effectively cleaning air and surfaces of particles, VOCs, inorganic gases, bacteria, viruses and mold; they include:

  • Filtration and adsorption – solids and gases
  • Ozone air purifiers
  • Ionization and “non-thermal” ionic plasma air purifiers
  • Photocatalytic ultraviolet (UV) air purifiers – photocatalytic oxidation (PCO)
  • Hydroxyl UV air and surface purifiers – multiple wavelength UV

The first four categories are described in a review by Kadribegovic et. al. with regard to function requirements and energy efficiency.5 The last category, about hydroxyl UV air and surface purifiers, is the newest. There is much information in the literature about the generation and chemistry of atmospheric hydroxyls outdoors by the action of the sun’s UV energy on ozone and subsequently water vapor.6,7 Within the last few years purifying systems based on the generation of atmospheric hydroxyls indoors have been described in the literature8 and some are now available commercially.9

Older systems use filters, adsorption, ionization and ozone to remove particulates, deodorize and sanitize, tailoring applications to individual problems. Each approach has benefits and limitations. Newer systems use photocatalytic adsorption to effectively sanitize small volumes of space. The newest systems use a range of powerful UV energy to generate nature’s most effective and safe cleansing agent – the hydroxyl radical oxidant – to sanitize even large residential and commercial occupied spaces. They restore nature’s outdoor balance indoors by decomposing volatile organic compounds and destroying bacteria, viruses and mold in air and on surfaces.

Advanced analytical methods10 like integrated gas chromatography-mass spectrometer (GC-MS) systems and proton transfer reaction-mass spectrometry (PTR-MS) have enabled an objective assessment of the contamination levels in indoor environments as they are able to identify and measure even low part per billion (ppb) levels of volatile chemicals. Reliable, repeatable measurements of VOCs and inorganic gases provide a quantitative means to evaluate the effectiveness and safety of purification systems, a subject that will be addressed below for each category of device, as applicable.

There has been a lot of hype about the capabilities of these different systems. We will provide an overview of each of the above approaches to explain their attributes and limitations; what they are good at and what they are not good at, based on limiting factors inherent to their technology and use. Then, we will focus on the use of hydroxyl based sanitizing systems since they have the broadest application for sanitizing air and surfaces in large occupied commercial spaces.

Filtration, Adsorption and Ionization

Filtration and adsorption remove particulates and low part per billion (ppb) levels of VOCs and inorganic gases, but cannot mitigate higher levels of VOCs unless very large surface area materials are used and replaced frequently. Mesh filters remove particles larger than 10 microns, but ninety percent of all airborne particles are smaller and require HEPA filtration, which removes 99.97% of particles above 0.3 microns. Some microorganisms can be removed by HEPA filters with this rating. Most virus particles are too small to be removed by mechanical filtration. However, virus particles rarely exist individually in air and are more commonly present in larger aerosolized, heterogeneous particles that include mucus, which enables them to be trapped in HEPA filters.11

Filtration can be improved by the concurrent use of ionization. Low energy ionizers generate positive and negatively charged particulates which then clump together to improve filtration, or can also be trapped on charged collection plates. Filters, adsorbents and collection plates must be changed frequently to maintain efficiency and flow rates, and to prevent bacterial and viral contamination. A consideration in the use of filtration, adsorption and ionization systems is the limitation on rates of air flow for optimal use. While applicable to small spaces such as a residential room or office, maintenance, material replacement costs and throughput limitations often make these systems too costly to be used in large commercial spaces.

Ozone Air Purifiers

Ozone, a natural oxidant, decomposes some types of VOCs and kills bacteria, viruses and mold. The US Environmental Protection Agency provides a summary of ozone generators that are sold as air cleaners on the web site http:www.epa.gov/iaq/pubs/residair.html. Ozone reacts rather slowly, so high concentrations are required for it to be effective. The electric arc method is used most commonly for ozone generation and results in ozone formation rates of 6-12 g/hour which typically result in concentrations of ~ 500 to 1000 ppb. That is much higher than the safety limits set by OSHA and other international safety and health agencies of 50 to 100 ppb for an eight-hour exposure under various working conditions.12 Therefore, these systems, while effective at eliminating odor and killing microorganisms, have the limitation that that they can only be used in unoccupied spaces. There are further practical impediments to their use, since ozone bleaches dyes in textiles and artwork and causes damage to fabric, paper, latex, leather, and electronics.

Ionization – Non-Thermal Plasma Generators (NTP)

NTP generators use an electric discharge to generate a mixture of high energy electrons (3-6 eV) and other particles called a non-thermal plasma.13 By means of various methods such as corona or dielectric barrier discharge, spark discharge is prevented and sustainable plasmas can be maintained. The plasma particles within the device react with oxygen and water in air to form ozone and other oxidants that oxidize and decompose VOCs and kill airborne microorganisms. It is commonly stated that NTP systems result in the formation of organic ions from VOCs, and that, once formed, these organic ions continue on a path to decomposition, thereby cleaning the air. However, there appears to be no scientific basis for this claim. The other postulated reactive oxygen species such as the hydroperoxyl radical, the superoxide radical anion and the hydroxyl radical are transient and there are no quantitative data to support how much, if any, are formed.13
The general configuration for these devices is to have electrodes separated by a dielectric material inside a cylinder or other similarly shaped reaction chamber. Air is treated as it is circulated through the device with a fan. A common type of non-thermal plasma generator uses a reactor that consists of two electrodes; one in the form of a metal pipe, and the other in the form of a metal wire running down the middle of the pipe. These electrodes are separated by a void space and are lined with a dielectric material and filled with glass beads. This type of reactor is called a dielectric-barrier discharge system. The contaminated air flows through of the pipe, and the oxidation reactions to remove the unwanted gases and aerosolized contaminants and microorganisms occurs within.14, 15 When the voltage across the electrodes exceeds the insulating effect of the beads; millions of micro-discharges occur. The duration of these discharges is measured in nano-seconds and the overall effect produces a silent glow.15 It is claimed that in this environment electrons become energized enough to be removed from molecules, forming organic and inorganic free radical species, although there are few if any data to support this claim.

Given that NTP devices use an electric discharge to generate the required plasma from ambient air, ozone is a major by-product of their operation. Ozone concentrations resulting from the use of these systems vary with device design and have been measured in the mid to high ppb and even ppm range. Because most systems generate levels of ozone much higher than the 100 ppb permissible for use in occupied spaces, NTP systems have not been commonly used to sanitize indoor environments. Rather, most applications have been in industrial facilities, for example, to remove mercury from industrial emissions or sulfur from diesel emissions. NTP’s will convert elemental mercury to mercury oxide, which is a solid and can be removed using a fabric filter or an electrostatic precipitator. NTP’s will also convert elemental sulfur to sulfur dioxide, sulfurous and sulfuric acids which are removed by filters and scrubbers.15

Because NTP systems use an electric arc as the energy source, the generation of oxidants is not selective and results in a complex mixture of oxidized organic and inorganic by-products which could represent a health hazard. There is little information available about the safety of these systems and, to our knowledge, no toxicology studies have been conducted to evaluate the safety of long-term use in occupied spaces.

Photocatalytic UV Air Cleaners

Photocatalytic UV cleaners – also called photocatalytic oxidation or PCO systems – purify air using a photosensitive semiconducting catalyst like titanium dioxide that adsorbs VOCs and microorganisms. The catalyst is irradiated and activated by 254-315 nm UV light enabling the formation of oxidants, presumed to be superoxide (O2¯ ) and hydroxyl radicals (OH) at the surface, where they immediately react with VOCs, other unwanted gases, and microorganisms also adsorbed onto the surface.16 A major benefit of this method is that no ozone is formed at those UV wavelengths. There is no scientific evidence that PCO systems generate oxidants that circulate throughout the treatment space to decompose VOCs or kill microorganisms. The prevailing opinion is that VOCs and microorganisms need to be adsorbed onto the catalyst surface so they are in proximity with the reactive oxygen species that are formed when the catalyst absorbs UV energy.16 Multiple catalysts can be used although the benefits of doing so are not quantified. “Photo hydro ionization” is a trade name for a PCO system that uses a 4-part catalyst.

PCO systems were developed under NASA support to treat small, occupied spaces where there were low ppb levels of VOCs, in particular for use on the space shuttle. This is an ideal and successful application because there is only a small amount of contaminated air to be pumped through the device where it is cleaned; and in the shuttle the formation of ozone must be assiduously avoided.

PCO systems can be scaled to treat larger spaces by increasing the surface area of the catalyst used, but are limited in the volume of space and the amount of VOCs they can effectively treat by the capacity and condition of the catalyst and other factors such as the requirement for relatively low fan speeds.16,17 Furthermore, there are other limitations:

  • Humidity inhibits the reaction rate as water competes for catalyst active sites
  • Inorganic contamination deposited during use deactivates the catalyst
  • Incomplete oxidation can result in levels of formaldehyde and acetaldehyde that are two to seven times higher than background amounts.17

New Hydroxyl Ultraviolet (UV) air and surface purifiers

Hydroxyl cleaners use UV energy to generate a sanitizing molecule called the hydroxyl radical (or just “hydroxyl”) and other resulting oxidants, which are also sanitizing agents. These chemicals are in the vapor phase and are similar in concentration to the hydroxyls and oxidants which are produced outdoors. Hydroxyls can be formed at different wavelengths beginning with photolysis of water vapor or ozone, but the dominant pathway in hydroxyl cleaners is via the generation of UV light with sufficient energy to break the H-O bond in ambient water molecules:


In nature, the upper atmosphere filters out these more energetic wavelengths of light and the generation of hydroxyls proceeds through steps that first involve the photolysis of ozone to generate a reactive oxygen atom species, O(1D), which then reacts with water to generate hydroxyls.

Most hydroxyl systems use quartz lamps (sometimes called optics) as the UV power source.  Manufacturers generally do not provide analytical data regarding how many hydroxyls are generated by each system as hydroxyls are extremely difficult to measure quantitatively, due to their low amount and short lifetime, which ranges between ~10 to 60 milliseconds in a typically contaminated environment.  As an example, in one study done by HGI Industries, hydroxyl formation rates were measured for an Odorox® Boss device in a clean room environment under well-controlled conditions via the quantitative rate of loss of a hydrocarbon.  A hydroxyl formation rate was measured that resulted in concentrations of about 3 million hydroxyls per cubic centimeter if averaged over the 120 cubic meter test space.18

Hydroxyls are also formed in nature in our lower atmosphere, predominantly by the photolytic decomposition of ozone by UV radiation at about 315 nm, to yield concentrations of about one to ten million molecules per cubic centimeter outdoors.6,10 They are typically referred to as atmospheric hydroxyls.  In nature, the UV radiation below 315 nm is filtered out by the upper atmosphere, so the alternate pathway of generating hydroxyls by scission of water vapor does not occur.  Hydroxyls power the atmospheric cleansing and sanitizing cycle.6,10 In typical outdoor environments where the concentrations of VOCs can range from ppb to low ppm levels, hydroxyls present in concentrations of the amount of 3 million molecules per cubic centimeter will react in less than 100 milliseconds with VOCs.  These initiate a series of fast, free radical chain reactions7 that decompose VOCs and their by-products, keeping air safe to breathe. Hydroxyls and the secondary organic oxidants that are formed by them can also kill bacteria, viruses and mold by causing the thin, delicate cell walls to rupture through reaction with the lipids and proteins in them.  Humans, animals and plants have evolved symbiotically to tolerate these natural levels of hydroxyls and their reaction by-products.

(Note that airborne hydroxyls are not to be confused with biological hydroxyls that are formed within the human body as part of the immune system response.19 When pathogens are detected within the body by the immune system, the first line of defense is the rapid formation of specialized cells called lymphocytes that attack foreign cells, microorganisms and other foreign material.  The principal lymphocytes formed are T cells, B cells and natural killer (NK) cells.  When the action of this initial immune system response is inadequate, certain cells in the blood, including mitochondria, will generate in-vivo hydroxyls at the site of infection/invasion to rapidly kill all cells near where they are formed.  Atmospheric hydroxyls cannot enter the blood stream or tissues within the body.  Skin and mucosal membranes have evolved to provide a barrier to entry and protect the body and are regenerated constantly to maintain their integrity and function.)

How Hydroxyl Cleansing Systems Work

Atmospheric hydroxyls do not exist naturally indoors at levels high enough to sanitize.20 Hydroxyls formed outdoors react so fast following their creation (within a fraction of a second) that they do not have an opportunity to be transported indoors. Hydroxyl cleansing systems work by irradiating the water vapor and other gases in air as it is circulated through chambers equipped with quartz lamps (also called optics), generating hydroxyls and other oxidants. Ideally they do so by producing concentrations similar to those found in nature. Hydroxyl formation rates vary widely with the type and number of lamps used and relative humidity.

processing chamber

The hydroxyls, and the resulting organic oxygenated species they form, react with and decompose mid to high ppb levels of VOCs and kill a broad range of bacteria, viruses and mold in air and on surfaces. Systems are typically equipped with from one to three lamps for use in room sized spaces. Larger systems are also available with as many as 48 optics and can be used individually or in groups to treat millions of cubic feet of space and decompose even high ppm levels of VOCs. Systems can be portable or designed to be integrated into heating, ventilation and air conditioning systems.

Airborne hydroxyls oxidize and decompose VOCs by a series of free radical chain reactions which are very fast and efficient. For example, compared to ozone, which undergoes molecular addition reactions, hydroxyls react via abstraction at a rate some million times faster. When hydroxyls react with VOCs they remove a hydrogen atom to regenerate the configuration H-O-H, which is water. This reaction creates an organic free radical, depicted as R∙, which in turn rapidly reacts with oxygen to create a mixture of organic oxidants called oxy and peroxy compounds R-O-O∙ and R-O∙. These free radical oxidants participate in a complex series of radical transfer and abstraction reactions; this is sometimes referred to as a cascade of oxidants since they continue to react quickly to form other R’-O-O∙ species with increasingly shorter carbon chains as CO2 is lost. Oxidation of VOCs need not completely transform all of the carbon atoms to CO2 in order to be effective. For example effective elimination of noxious odors can be rapidly accomplished when only the area with the functional group responsible for the odor is oxidized. Typically functional groups like O, S, Cl, N that are the source of odor polarize the carbon atoms next to them, making it easier for hydroxyls to abstract the hydrogen atoms attached to them, thereby altering the chemical structure sufficiently to eliminate the odor.7,22 The organic oxidants are themselves very powerful sanitizing and deodorizing chemicals. They are more stable and react less quickly than hydroxyls, so they can circulate throughout the treatment area to complete the purification process. The contaminated air should run continuously to recirculate by-products through the photolysis chamber to continue the oxidizing process. Continued oxidation cleaves off carbon atoms ultimately forming carbon dioxide and water. Smaller VOCs react fast, so oxidation by-products like formaldehyde or acetaldehyde don’t accumulate.


As a category, the FDA and other regulatory agencies do not regulate or require pre-market approval for UV air and surface cleaning devices used in residential and commercial applications, because they irradiate ambient air and sanitize in a manner similar to that found in nature. They do require that the devices prove they do what they claim and comply with Occupational Safety and Health Agency (OSHA) industrial safety standards. To do so, manufacturers of sanitizing systems should provide scientific data for efficacy and safety, although this is not often done since non-regulated devices are rarely challenged by government regulatory agencies.

Data concerning efficacy of a sanitizing/cleansing system can be generated in-house, and ideally corroborated by independent laboratory test data. In either event, descriptions of the experimental arrangement and conditions, and the data themselves, should be publicly available. When appropriate, such experiments could be published in the peer-reviewed literature.

The types of information that would be extremely useful to potential users of a sanitizing system are:

  • Measurement of the reduction of VOC levels during extended operation in specified volumes of space, including:
    • Concentrations of total VOCs – combination of different compounds grouped together
    • Concentrations of individual compounds removed in controlled experiments
    • Information on the formation of oxidized products, such as aldehydes, ketones etc.
    • Measurement of the potential influence of inorganic chemicals like ozone, CO, NO etc. during extended operation
  • Surface microorganism kill rates of representative bacteria, virus and mold species
  • Microorganisms kill rates for aerosolized representative bacteria, virus and mold species


Manufacturers of all sanitizing/cleansing systems must meet applicable standards for safety and performance defined by government agencies in published standards. These include standards for electrical, mechanical and radiation safety, among others. Typically independent testing laboratories like Underwriter Laboratories (UL) and Intertek (ETL) conduct such studies and issue the right to label devices with their mark to indicate that they have been successfully tested. They provide detailed reports of their testing results to the manufacturer who can then provide this information to consumers.

In addition, it is important for the manufacturers of chemical and mechanical sanitizing systems to specify the conditions for safe use and to provide data to support their recommendations. For example:

  • All chemical cleaners should be used with ventilation, safety glasses and gloves (and other protective gear) for safe use. Specific instructions for proper dilution need to be followed and directions for proper mixing of chemicals must be adhered to. For example, mixing household cleaners like bleach and ammonia generates a toxic chlorine gas that is extremely hazardous.
  • Filter and catalyst systems must be cleaned and the filters, adsorbents and catalysts replaced on schedule to ensure that they function properly and do not harbor and circulate microorganisms and mold spores. Intervals for filter/catalyst replacement should be provided as filters and catalysts can degrade and produce particulates, some of which – like titanium dioxide – are harmful.
  • Systems that clean using ozone must specify that they can only be used in unoccupied spaces and that treated areas need to be well ventilated before permitting occupancy. They also need to warn that materials like electronics, leather, rubber etc., that are decomposed by ozone, be removed from treatment areas.
  • Systems that generate useful gas phase concentrations of OH using UV radiation also necessarily generate ozone and oxidized VOCs. When used as directed, most systems generate levels that remain within OSHA safe ranges.21 It is important that manufacturers provide test data to support safe use:
    • Report ozone and VOC formation rates for given volumes of treatment space
    • Recommend the proper system/model to use for each range of space being treated
    • Specify the levels of ventilation needed for safe use

The FDA does require 510(k) premarket approval of UV sanitizing systems for use in medical facilities. Having FDA approval of the technology used inside a sanitizing system is a valuable confirmation of its intrinsic safety. Note that the FDA requires that each and every model be individually reviewed and approved, even if the design changes are modest. Each vendor must address the safety of each model and should provide surface and aerosol microorganism test data, as required, to prove efficacy.

We illustrate some of these safety requirements, and the testing needed to show compliance, with reference to HGI’s systems. For example, the Odorox® MDU/Rx™ system was reviewed and approved by the FDA for use in occupied spaces in medical facilities in 510(k) (#133800, 2014). In order to receive approval, the FDA reviewed the following data, among others, from independent testing laboratories.

  • Intertek testing results across a broad range of specifications for radiation, mechanical and electrical safety
  • Concentrations of total and speciated VOCs formed during extended use, including formaldehyde, acetaldehyde and other VOC oxidation products
  • Confirmation of 3-4 log kill rates of representative aerosolized bacteria and selected virus
  • 13-week toxicology study involving a statistically significant population of animals under FDA “Good Laboratory Practices”

HGI also provided data showing high kill rates across a broad range of bacteria, virus and mold species on hard and porous surfaces. It is important that each manufacturer furnish these types of data for each model they sell.

The FDA does not regulate UV sanitizing systems that are integrated with heating and ventilation systems in commercial or medical facilities, such as the Odorox® Induct™, MVP14™ and MVP48™ systems.

Regulatory agencies such as OSHA do monitor the chemicals that can be generated from UV sanitizing systems. Acetaldehyde and formaldehyde can build up as larger VOCs are decomposed by hydroxyls. They are among the last products produced before complete oxidation to carbon dioxide and water. A device that produces sufficient concentrations of hydroxyl radicals will keep the steady state levels of these terminal oxidation products near background levels as these small VOCs react with hydroxyl radicals more rapidly than larger VOCs. This is what happens in nature. For example, studies commissioned at LRRI (Albuquerque, NM) and Columbia Analytical Laboratories (Sunnyvale, CA) confirmed that HGI systems produce sufficiently high concentrations of hydroxyls to efficiently decompose ambient VOCs and their by-products. They also showed that formaldehyde and acetaldehyde rapidly reached low steady state levels that remained near ambient baseline levels of 10-15 ppb for extended periods for the models tested.18

OSHA requires that indoor ozone levels are below 100 ppb for safe, long-term exposure. Under certain work conditions and in different states ozone safety levels are set as low as 50 ppb.12,21 Typical natural ozone levels range from 20 to 60 ppb – or more – in highly populated urban areas. HGI technology maintains these same natural levels through the use of customized optics, system design optimization, recommended ventilation practices and machine selection for given volumes of treated air. For its larger industrial systems, like the MVP14™ and MVP48™ systems, HGI has integrated real-time interactive process controls so that oxidant levels can be sampled remotely and measured continuously, enabling machine settings to be adjusted automatically to maintain required oxidant levels. Vendors should provide operational use guidelines indicating which models are designed for various sized spaces. System designs vary among vendors resulting in different formation rates for hydroxyls, secondary oxidants and oxidation by-products. Treatment spaces should be normally ventilated (1-5 exchange rates/hour) to maintain safe oxidant levels. Some vendors have conducted third party laboratory testing and can provide such data to corroborate claims; it is important to request this data as claims abound and data are scarce. Measuring hydroxyls requires specialized equipment like ultra-clean rooms and gas chromatograph-mass spectrometers. It is not possible to measure hydroxyls in a typical chemical laboratory; so it is important to understand the experimental design and how the data were collected, not just the conclusions.


Weschler and Shields have speculated in the environmental chemical literature on the potential health hazards of the myriad of organic oxidation products – aldehydes, ketones, alcohols etc. – resulting from the formation of hydroxyl radicals indoors from natural chemical reactions involving the reaction of ozone with certain types of hydrocarbons with double bonds called alkenes.6 HGI data indicate that, when their systems are used according to operation guidelines, all of these organic intermediates (measured as Total and speciated VOCs) are present in low ppb levels that are similar to the distribution and concentration found in nature. It has been presumed that they are therefore safe to breathe. A comprehensive review of government safety and health databases, including PubMed and the National Library of Medicine resulted in a statement from the National Institutes of Health (NIH) that they “cannot find any hard science or research indicating that hydroxyl radical generation is harmful to human health. That applies to both atmospheric and man-made generation” (Colleen Chandler, NIEHS Office of Communications and Public Liaison, August 8, 2010).

Although no adverse effects from the use of UV hydroxyl generators have ever been reported, there have been no published toxicology studies to verify this until recently. HGI commissioned a comprehensive 13-week toxicology study using the Odorox® Boss™ unit that involved the exposure of forty (40) test rats and twenty (20) control rats to two to three times the concentration of hydroxyls normally used. The study was conducted in compliance with the US Food and Drug Administration’s Good Laboratory Practices (GLP) regulations (21 CFR Part 58) by an industry leading clinical contract research company, Comparative Biosciences, Inc. (Sunnyvale, CA). Extensive data were collected including behavioral, physiological, neurological, ophthalmological and hematology data, as well as clinical chemical analysis, and gross histopathology studies. The study results indicated that the test animals tolerated the exposure well with no abnormal clinical observations. There were no histopathology or cytopathology (cellular level) differences between the control rats and the exposed rats. During analysis, specific attention was paid to the skin, eyes, nasal turbinates, larynx/pharynx, and respiratory system. There were no changes in these organs and they appeared to be within normal limits in both the control and treated animals. These results are applicable only to HGI Odorox® systems, which use UV energy of particular wavelengths and custom optics in certain configurations. Other systems that use different optics, catalysts, or incorporate other methods such as adding different oxidizing agents or organic chemicals would need to be separately evaluated as the resulting mixture of by-products would be “unnatural” and could pose health problems.


1. Charles J. Weschler, “Changes in indoor pollutants since the 1950’s”. Atmospheric Environment, 43 (2009),153-169.
2. “Sick Building Syndrome”, United States Environmental Protection Agency, 2009-02-19. United States Environmental Protection Agency. (1994). “Indoor Air Pollution: An introduction for health Professionals”, EPA 402—R-94-007.
3. Sundell, J; Lindval, T; Berndt, S (1994). “Association between type of ventilation and airflow rates in office buildings and the risk of SBS-symptoms among occupants.” Eviron. Int. 20 (2): 239–251.
4. CDC, “Guidelines for Disinfection and Sterilization in Healthcare Facilities”, William A. Rutala, Ph.D., M.P.H.1,2, David J. Weber, M.D., M.P.H.1,2, and the Healthcare Infection Control Practices Advisory Committee (HICPAC), 2008.
5. R. Kadribegoric, L. Ekberg and P. Fahlen, “Air Cleaning technologies – Function requirements and energy efficiency”, Indoor Air 2008, 17-22.
6. D. R. Crosley, The Measurement of OH and HO2 in the Atmosphere, J. Atm. Sci. 52, 3299 (1995).
7. B. J. Finlayson-Pitts and J.N. Pitts, Jr., “The Chemistry of the Upper and Lower Atmosphere”, Academic Press San Diego, 1999.
8. M. S. Johnson, E. Nilsson, E. Svensson, S. Langer, “Gas-Phase Advanced Oxidation for Effective, Efficient in Situ Control of Pollution”, Environmental Science and Technology, 2014, 48, 8768-8776.
9. www. hgiind.com, www. rgf.com
10. D. E. Heard, Ed. “Analytical Techniques for Atmospheric Measurement”, Blackwell Publishing, Ltd.,2006.
11. Air Filtration and the Use of HEPA Filters in Biological Safety Cabinets. www.nuaire.com/pdf/ use of heap filters, 2014.
12. OSHA ozone safety guidelines. http://www.osha.gov/dts/chemicalsampling/data/CH_259300.html
13. A. Chirokov, A. Gutsol, A. Fridman; “Atmospheric pressure plasma of dielectric barrier discharges”, Pure Appl. lcehm. 77 (2005), pp 487-495.
14. H. Heberer, E. Nies, M. Dietschi, A. Moller, “Reflections on the efficiency and toxicological implicatioins of NTP air cleaning devices“, BGIA Institut fur Arceitsschutz der Deutschen Gesetzlichen Unflaaversicherung, LUFT 65 (2005) no. 10, p. 419-424.
15. US Environmental Protection Agency report 456/R-05-001, L. Cox, D. Russell, Clean Air Technology Center, Feb. 2005
16. J. MO, Y. Zhang, Q. Xu, J. Joaquin Lamson, R. Zhao, “Photocatalytic purification of volatile organic compounds in indoor air: a literature review”, Atmospheric Environment 43 (2009) 2229-2246.
17. A. T. Hodgson, H. Destaillats, D.P. Sullivan, W. J. Fisk, “Performance of ultraviolet photocatalytic oxidation for indoor air cleaning applications”, Indoor Air 2007; 17: 305-316.
18. Columbia Laboratory reports and LRRI report available upon request.
19. Julio F. Turrens, “Mitochondrial formation of reactive oxygen species, Journal of Physiology, Volume 552, Issue 2, pages 335–344, October 2003.
20. C. Weschler and H. Shields, Environmental Science and Technology, “Production of the Hydroxyl Radical in Indoor Air”, Vol. 30, No. 11, 3250-3258, 1996.
21. OSHA Ozone Air Contaminants Standard, 29 CFR 1910.1000
22. NASA compilation of reaction rate coefficients: http://jpldataeval.jpl.nasa.gov/pdf/JPL_15_AllInOne.pdf

Additional References:

1. R. Atkinson, “Kinetics and Mechanisms of the Gas-Phase Reactions of the Hydroxyl Radicals with Organic Compounds”, Journal of Physical and Chemical Reference Data, Monograph No.1, 1989.
2. D. Berry, G. Mainelis, D. Fennell, “Effect of an Ionic Air Cleaner on Indoor/Outdoor Particle Ratios in a residential Environment”, Aerosol Science and Technology, 41:316-328, 2007.


The terms radical and free radical are used interchangeably when referring to chemical moieties with multiple atoms and an unpaired electron. These include moieties such as OH, R-O-O, R-O etc. The unpaired electron is not shown, but would be depicted as: OH∙, R-O-O∙, RO∙ etc. where the “dot” denotes the unpaired electron. Elements with an unpaired electron are referred to at atoms, for example when referring to the hydrogen aton, For H, the unpaired electron is not depicted. The convention used in this web site is to omit the depiction of the unpaired electron when referring to raidical/free radical moieties or H.

More information:


Written by Hydroxyl