A hydroxyl generator provides an alternative to ozone generators because hydroxyls can oxidize contaminants,
remove odors and disinfect bacteria and viruses from indoor air without using high concentrations of ozone. Hydroxyl
generators create reactive hydroxyl molecules that safely convert contaminants into harmless carbon dioxide and
water without harming occupants. Hydroxyls also perform a more gentle cleaning and don't require wait times to
re-enter a building that has been treated.
But how does a hydroxyl generator work and are they always safe to use around people and pets? Can a hydroxyl generator
work better than an ozone generator? Are all hydroxyl generators basically the same or do some use different technology?
What is the best hydroxyl generator to buy? These are the questions that consumers face when they consider buying
a hydroxyl generator for the first time. This article will explain the pros and cons of various types of hydroxyl
units to consider before buying a hydroxyl generator.
Hydroxyl Generator Topics
How BoiE Builds
the Best Hydroxyl Generators
is an EPA-registered, American company that builds high-power hydroxyl generators using the latest hydroxyl generation
technologies and high-quality, energy-efficient components that last. We design hydroxyl generators for both home
and commercial use. BoiE high output hydroxyl generators remove strong odors without damaging sensitive materials
or necessarily having to vacate a property. Our portable hydroxyl machines are powerful and durable units, generating
hydroxyls using multiple methods and lasting much longer than overseas imports.
When you have serious odor problems and challenging conditions you'll want to buy a BoiE hydroxyl generator to
do the job right. Simply put, BoiE manufactures serious commercial-grade hydroxyl generators for consumers who
want high-performing products directly from an American manufacturer at the best price point. Plus, BoiE stands
behind every machine we build with a money-back guarantee and lifetime customer support. BoiE hydroxyl generators
will never become obsolete and with minimal maintenance, can last a lifetime.
Highlights of BoiE Hydroxyl Generators
- Portability: Compact, rugged cases with carry handles, designed for transporting and storing. No permanently-attached
cords or permanent flanges. Easy to take apart, change filters and clean.
- Safe Mode: All units can operate in Safe Mode, allowing units to be left running continuously in occupied
areas without ventilation.
- More Cleaning Power: Unique 360-Surround permanent nano-anatase TiO2 reactor grid maximizes the hydroxyl
reactive surface area without restricting airflow, and is easy to replace.
- Quality Optics: Machines use safe and cool-operating true 368nm anatase titanium dioxide (TiO2)-coated
optics or 253nm UV-C (germicidal) optics (depending on application), that are energy-efficient, last thousands
of hours and are simple to change.
- Hydroxyl Accelerator: BoiE's ultrasonic water vapor optional accessory adds moisture in low humidity
environments to increase hydroxyl reactions and works will all models.
- GFCI Protection: Integrated into the wiring through an accessory outlet for electrical safety in wet
- Warranty: Unlimited 1-year warranty and lifetime customer support, plus unlimited warranty extensions
for an annual maintenance fee.
What is a Hydroxyl Radical?
The hydroxyl, more specifically the hydroxyl radical (ĽOH), is a short-lived molecule containing a single
oxygen and a single hydrogen atom. The word "radical" means that it is neutral with no net charge. A
hydroxyl radical has an unpaired electron and is always searching for another electron to become paired again.
The easiest way to become complete is for a hydroxyl radical to abstract (take away) an atom from something nearby,
which is how hydroxyls break down contamination.
In the atmosphere, Hydroxyls are formed by the reaction of ultraviolet (UV) light from the sun disassembling water
vapor (H2O) into hydroxyl radicals (ĽOH). Atmospheric hydroxyl radicals react with and neutralize concentrated
pollutants in outdoor air. The atmospheric cleaning effects of hydroxyls are massive, breaking down the smoke from
forest fires and many other pollutants.
However, with an average life of less than a second, naturally-formed hydroxyls are limited to the outdoors and
have little to no effect on indoor air. Most naturally-produced hydroxyls never pass through an exterior doorway.
Hydroxyls are not harmful to people, pets or plants, as we are directly exposed to hydroxyls every time we are
What is a Hydroxyl Generator?
Duplicating the process in the atmosphere, a hydroxyl generator creates Hydroxyl Radicals. The generator circulates
indoor air through a chamber where hydroxyl radicals are continuously formed. Hydroxyl generators attack indoor
contaminants the same way as naturally-occurring hydroxyls would. As a hydroxyl generator runs, the air in the
room moves through its chamber where pollutants are gradually broken down into carbon dioxide and water. By continuously
running the generator, the overall level of room air contamination decreases.
In addition to creating hydroxyl radicals, a hydroxyl generator produces peroxides and superoxides. All of these
are called Reactive Oxygen Species (ROS) because they contain oxygen, are very reactive and break down airborne
contamination instantly. Because of ROS, hydroxyl generators can safely eliminate airborne fungal spores, viruses
(including corona-type viruses¹), bacteria and dangerous biological agents in milliseconds. In fact, following
the Amerithrax attacks in 2001, the federal government invested billions on biodefenses.² One countermeasure
developed was a system that generated ROS to safely neutralize weaponized anthrax.³
Hydroxyl generators are widely used by water and fire damage restoration companies along with mold remediation
professionals because the hydroxyl radicals generated are non-toxic to humans. Also, hydroxyl generators work exceptionally
well in hot, humid environments without causing damage to sensitive materials or requiring wait time to reoccupy
a treated area.
Where are Hydroxyl Generators Used?
Hydroxyl generators are used in the restoration industry in environments where concentrated ozone is undesirable
either because it could damage materials or because workers or occupants are present. Hydroxyl generators are often
used after floods or fires have been extinguished and the building is full of smoke odor because hydroxyl generators
easily break down smoke orders.
Hydroxyl generators can be left running at the job site in the days before work begins on a restoration project
to help purify the air and prevent mold from taking hold. Hydroxyl radicals react with over 5,000 compounds and
will break apart odors from fire and smoke, floods, chemical fumes, fuel oil, VOCs and trauma without damaging
What are the Methods Used
to Generate Hydroxyls?
There are two basic methods to generate hydroxyls for indoor air purification: Photocatalytic Oxidation (PCO)
or an Advanced Oxidation Process (AOP). Each method has advantages and disadvantages. PCO is the most common and
uses a light-activated catalyst to generate hydroxyls that oxidize contaminants in the airflow that pass over the
activated catalyst. The catalyst creates hydroxyls by abstracting hydrogen atoms from passing water molecules in
the air. The chief advantage of PCO is that the process is the safest method to produce hydroxyls without also
generating potentially unsafe byproducts. Photocatalytic Oxidation is FDA-approved for use in Class II medical
devices. Examples of Class II medical devices include air purifiers, powered wheelchairs and infusion pumps.
PCO technology, originally developed in 1994 by NASA to purify air onboard a spacecraft,4 is the only
hydroxyl generation method that is always safe to use around people. Safe-to-view, UV-A black light or shielded
UV-C germicidal light shines on a nano-catalyst where massive numbers of hydroxyls form whenever there is water
vapor in the air. Contaminants that come in contact with the light-activated catalyst are instantly oxidized. The
UV-C wavelength has the added effect of sterilizing airborne germs, viruses and bacteria that pass close to the
light source. PCO is the only method of hydroxyl generation that can be left running unmonitored in occupied spaces
without requiring fresh air ventilation.
An AOP refers to any process that generates hydroxyls from chemical reactions without requiring contact with a
light-activated catalyst. Also called direct hydroxyl production (DHP), an AOP may combine ozone, hydrogen peroxide
or UV-C radiation to directly generate hydroxyls that oxidize contaminants in air or water. An AOP generates more
hydroxyls because the reaction area is not limited to a catalyst surface. The disadvantage is that byproducts including
ozone and hydrogen peroxide off-gassing may reach unsafe levels for workers, so monitoring is needed for occupied
What Methods Do BoiE Hydroxyl
BoiE manufactures both PCO and AOP hydroxyl generators. The BoiE Hydra INDUCT
and Hydra QUAD are pure PCO hydroxyl generators, along with the Hydra
FUSION (includes a 32K Shock Ozone Generator for optional unoccupied use). These PCO models use safe-to-view
blacklight and are safe to use in all occupied environments.
The BoiE Hydra Pro XL uses a combination of PCO and AOP technology. The unit can
operate solely in PCO mode for occupied spaces, or in PCO and AOP mode simultaneously. The AOP technology uses
UV-C germicidal light to break down low-concentration hydrogen peroxide vapor (HPV), low-concentration UV ozone
or a combination of both, to generate hydroxyls directly into the air. Contaminants are oxidized both in the air
stream and on the catalyst surface, which increases the overall coverage area.
Note that AOP mode requires an unoccupied area or close monitoring and worker protection to ensure that HPV and/or
ozone do not build up to unsafe levels* during the treatment cycle before they naturally revert to harmless water
and oxygen, respectively. Both PCO and AOP hydroxyl generation technologies are safer than concentrated ozone and
can be more effective in certain situations. AOP hydroxyl generators are highly-effective after a flood or a fire.
Can a Hydroxyl Generator Kill Mold?
A hydroxyl generator can kill airborne mold spores and removes mold odors, but has a limited effect on surface
mold. This is because hydroxyl generators do not penetrate materials or kill mold that has already become established
on surfaces due to hydroxyls not lasting long enough to reach them. It is important to understand that a hydroxyl
generator is not a magic bullet. A hydroxyl generator that is safe to use in occupied areas acts primarily on the
air inside the machine and the air nearby. The further away the contamination, the less chance that hydroxyls can
While there are anecdotal stories of hydroxyl generators decontaminating distant surfaces through cascading reactions
of contaminants, there is no third-party study proving that hydroxyl generators safe to use in an occupied area,
can kill surface mold or disinfect porous surfaces across a room while maintaining EPA-safe air quality for occupants.
For a typical building environment it is fair to say that most reactions that involve hydroxyls disinfecting mold
will occur either within or very close to the hydroxyl generator.
There are certain hydroxyl generators that can kill mold on surfaces by using a fogging or misting system that
carries reactants that form hydroxyls all the way to surfaces, however, these devices exceed the EPA's permissible
limits, requiring appropriate personal protective equipment (PPE) to be worn 100 percent of the time, and therefore
cannot be used within occupied areas.
When Do You Need a PCO Hydroxyl
A Photocatalytic Oxidation (PCO) hydroxyl generator is most beneficial when the hydroxyl machine needs to be
run for long periods of time with occupants or workers present. A PCO hydroxyl generator is recommended for 24x7
home use or commercial use. A PCO hydroxyl generator can be left behind after a project is complete to eliminate
contaminants that later become airborne. PCO hydroxyl generation will not harm the young, the old or those with
allergies, asthma or compromised immune systems.
PCO was originally developed by NASA with a specific intent not to harm occupants in a closed environment onboard
a space station. PCO can use either safe-to-view UV-A black light or shielded UV-C germicidal light and several
types of permanent catalysts. Both UV-A and UV-C will activate the catalyst in the same manner. However, the UV-C
wavelength has the added effect of sterilizing airborne germs, viruses and bacteria that pass close to the light
source, but requires the optics to be shielded from view.
PCO technology requires water vapor to fuel reactions, so humidity of at least 50 percent is needed for good results.
When conditions are too dry, a PCO hydroxyl generator can be used with a water misting system to increase the moisture
level of the air that is drawn into the machine. BoiE manufactures an Ultrasonic
Hydroxyl Accelerator for this purpose. All BoiE hydroxyl generators have a PCO mode option for occupied areas
where a water misting system is useful.
When Do You Need an AOP Hydroxyl
An Advanced Oxidation Process (AOP) hydroxyl generator is most beneficial when the hydroxyl machine can be run
for long periods of time without occupants or unprotected workers present. An AOP hydroxyl generator is recommended
for commercial work including fire and water damage restoration projects or mold remediation work areas because
AOP is a more powerful method. An AOP hydroxyl generator uses ozone-producing UV-C light, hydrogen peroxide vapor
(HPV) and other combinations to generate hydroxyls. Contaminants are oxidized in the air around the reactants,
which increases the coverage area.
AOP technology requires either water vapor (H2O) or hydrogen peroxide vapor (H2O2) to fuel reactions and less humidity
is needed compared to PCO. However, the more humidity the better the results. An AOP hydroxyl generator can also
be used with a water misting system such as BoiE's Ultrasonic Hydroxyl Accelerator
to increase the moisture level of the air that is drawn into the machine, resulting in greater hydroxyl generation.
AOP mode requires an unoccupied area or close monitoring and worker protection to ensure that HPV and/or ozone
off-gassing do not build up to unsafe levels* during the treatment cycle before they naturally revert to harmless
water and oxygen, respectively. Despite the off-gassing hazards, an AOP hydroxyl generator provides a much more
gentle cleaning process than concentrated ozone and will not harm plants or sensitive materials.
The Verdict: Hydroxyl
Generator versus Ozone Generator
Hydroxyl generators react much faster than ozone, remove thousands more contaminants, do not damage natural
rubber and certain adhesives that concentrated ozone can, and require no wait time to reoccupy a treated area.
However, since contaminated air has to move through a chamber, hydroxyl cleaning takes a longer period of time
and has a limited effect on surface contamination.
Ozone can spread throughout a room, saturate porous materials and attack surface contamination over a shorter period
of time. However, concentrated ozone can damage sensitive materials such as natural rubber, nylon and adhesives.
An ozone treatment also requires an unoccupied area with all living things removed and a wait time for ozone to
revert back into normal oxygen before re-entering an area after a treatment.
- Instant reaction
- More cleaning of the air
- Cleans air as it passes through chamber
- Safe for humans, animals and plants
- Slower reaction as compared to hydroxyls
- Floods room, more cleaning of the surfaces
- Contacts all areas, Ozone is heavier than air
- Harmful to humans, plants and animals
- Harmful to natural rubber, some adhesives
If the question is which odor removal technology is better -- ozone or hydroxyls, the answer is neither because
both methods remove odors from the air very well. If the question is which technology is safer, then a hydroxyl
generator clearly wins. And if the question is which technology saturates and disinfects surfaces the best, then
the answer is an ozone generator. The truth is that neither technology can claim superiority over the other and
results will vary based on the type of use and the environmental conditions.
BoiE manufactures both ozone generators and hydroxyl generators and we know from our customer experiences that
the product needed depends on how the customer intends to use it, and that can quickly change with each new project.
A professional service company should consider investing in both ozone and hydroxyl technologies for that reason.
A service business should consider buying the Hydra Pro XL because you get the advantage
of having both PCO and AOP and can decide which one to use.
An individual or small business should consider buying the Hydra Fusion to get a
safe hydroxyl generator and an optional shock ozone generator in one unit, the Hydra QUAD
to run continuously in an occupied problem area or the Hydra INDUCT to deal with
HVAC problems. Hydroxyl generators, despite being available for over a decade, are still a relatively new technology
to many, and a potential buyer should invest time in learning about the science behind the technology and its pros
and cons before purchasing.
1. Foster, Howard & Ditta, Iram & Varghese, Sajnu & Steele, Alex. (2011). Photocatalytic disinfection
using titanium dioxide: Spectrum and mechanism of antimicrobial activity. Table 7. Viruses shown to be killed by
photocatalytic disinfection: SARS coronavirus. Applied Microbiology and Biotechnology. 90. 1847-68. 10.1007/s00253-011-3213-7.
2. Hylton, Wil S. "How Ready Are We for Bioterrorism?" New York Times, Oct. 30, 2011: A4. Print.
3. Titan Systems Corporation, Defense Advanced Research Projects Agency (DARPA). Project Name: Immune Building
(ROS technology for the neutralization of anthrax spores), DARPA Contract No. MDA97203C0092 Special Projects Office
4. Graf, John C. "Using Spacecraft Trace Contaminant Control Systems To Cure Sick Building Syndrome"
NASA, Washington, Technology 2003: The Fourth National Technology Transfer Conference and Exposition, Volume 1;
p. p 258-265 (1994)
* The U.S. Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL), averaged over
an eight-hour work shift, for ozone is 0.10 ppm and 1 ppm for HPV. When used in AOP modes in confined spaces, the
HYDRA Pro XL may exceed PEL and require appropriate personal protective equipment (PPE). Monitoring is required
if workers are present.