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Hydrogen Peroxide as an Air Cleaner

Hydrogen peroxide has been around for a long time.  The brown bottle you may keep in your bathroom as an antiseptic for treating wounds has many, many more uses!  It was discovered in 1818 by scientist Louis Jacques Thénard as he reacted barium peroxide with nitric acid.  Today, it’s still used medically, as well as in many diverse applications such as launching rockets and satellites into space, or as a more environmentally-friendly alternative to chlorine-based bleaching products in the manufacture of paper.   (Peroxide Power)

Hydrogen peroxide is chemically written as H2O2, meaning it has 2 hydrogen atoms and 2 oxygen atoms.  It is an oxidizing agent, releasing an oxygen atom when it decomposes.  Decomposition happens quickly in the presence of organic matter like microbes or reactive compounds (hence the bubbling fizzing action on wounds or with baking soda), but it will also decompose slowly in storage, which is why it’s sold in those brown bottles to protect it from light and the ambient air.  

Hydrogen peroxide can be used as a disinfectant in appropriate dilutions on surfaces, in laundry, and in the air.   In the air, hydrogen peroxide is safe in concentrations up to 1ppm according to the Occupational Safety and Health Administration (OSHA). Because it’s chemically very similar to water, it can be produced from water and decomposes into water.  Yet as common and beneficial of a substance as it is, bulk hydrogen peroxide is surprisingly hard to produce and transport.  Currently, large quantities of hydrogen peroxide are made through what’s known as the “anthraquinone process.” This method is energy-intense, requires large-scale production, and produces large quantities of carbon dioxide (CO2) as a byproduct. While directly reacting hydrogen and oxygen to make hydrogen peroxide would be ideal, thermodynamics prefers to form the more stable water (H2O) over hydrogen peroxide.  (Producing hydrogen peroxide when, and where, it’s needed)  However, since only a minimal amount of hydrogen peroxide is needed and proven safe to kill microbes in the air, purifiers are now using different technologies to produce “dry” hydrogen peroxide and distribute it for air cleaning.  Here are some examples:

• Photohydroionization (PHI) is a technology developed by RGF Environmental Group that uses a broad-spectrum, high intensity UV light targeted on a hydrated quad-metallic catalyst. The UV light in conjunction with the catalyst promotes the conversion of naturally occurring water vapor into airborne molecules of hydrogen peroxide (H2O2). These airborne H2O2 molecules revert to oxygen and hydrogen once they have come in contact with a pollutant. (PHI) This company produces standalone and in-duct products.
• The TADIRAN AIROW technology fractures Oxygen (O2) into two separate “O” molecules by using a discharge current. These “free O” atoms combine with the H2O molecules in the airflow, transforming into hydrogen peroxide (H2O2). The H2O2 is then distributed through the indoor unit of the air conditioner into the conditioned living space. The amount of hydrogen peroxide that Tadiran’s new TADIRAN AIROW releases into the conditioned space is below the safety requirement as determined by OSHA of 1ppm. TADIRAN AIROW has been proven to release less than 7ppb of hydrogen peroxide. (HYDROGEN PEROXIDE TECHNOLOGY FOR INDOOR AIR PURIFICATION)
• AirROS purifiers utilize and create 7 species of ROS (Reactive Oxygen Species).  The first stage, which occurs inside the device, includes 5 of these ROS (atomic oxygen, singlet oxygen, hydroxyl radicals, superoxide and peroxynitrite), and 2 species (gas-phased H2O2- dry hydrogen peroxide and low concentration levels of O3-ozone) leave the reactor and move into the room for further disinfection.  According to AirROS, “...Dry Hydrogen Peroxide purifiers technology can only provide short-distance surface treatment within the air purifier because of the short life of hydrogen peroxide. If you have a surface not close to the purifier, it will be untreated and left vulnerable to contamination.  AirROS commercial air and surface purifiers offer long-distance surface treatment because of the Trioxidane that forms from O3 and H2O2 combined, which means you can treat any surface, no matter how far away it is from the purifier. As a result, it provides an added layer of protection against surface contamination and eliminates odor effectively.  Trioxidane decomposes very quickly in water but has a half-life of 16 minutes in normal ambient conditions, making it one of the longest lasting hydroxyl radicals. It’s theorized that the human body also produces trioxidane as a powerful oxidant against invading bacteria because the body also produces singlet oxygen and has lots of water, the two ingredients for making trioxidane.  (Trioxidane)
• AsepticSure Oxidation by Medizone International (UK company) is a system that uses hydrogen peroxide and ozone to clean unmanned rooms. According to EPA registration, personnel must be trained, the room must be sealed, and the ozone generated can have severe effects on certain materials, such as natural rubber and nylon.  The time to disinfect, personnel required to operate the system and limitations (not to be used with contraindicative materials or with life-saving equipment or with personnel in the room), all seem to be quite restrictive, yet the system has been sold to and installed at many medical facilities.
• A hydrogen peroxide generator composed of a TiO2 catalyst that is activated with UV light was studied in 2022.  The photocatalyst becomes activated by light given off by a nearby UV-A bulb which excites electrons across the bandgap of TiO2, converting water vapor in the air stream passing through the catalyst into H2O2.  The researchers were aware that it is theoretically possible that H2O2, OH radicals, and hydroperoxide radicals can enter an air stream that passes through an operating photocatalytic TiO2 structure. From an indoor air space standpoint, however, only H2O2 will survive long enough to be detected at distances greater than about 1 cm from the photocatalyst. Over time, the H2O2 that has entered the room will either react with organic species within the indoor space or decay naturally into the benign products, water and oxygen. Hydrogen peroxide can last up to 30 minutes, depending on temperature, humidity, and reactive contents in the room.

Limitations of dry hydrogen peroxide include:

• Position of the unit: position is very important, because dry hydrogen peroxide has relatively high reactivity, which can diminish its effective lifetime. For instance, H2O2 is known to react with metal surfaces such as those provided by the metal ductwork in the bypass duct. As the pathlength between the device and the room becomes longer, the H2O2  concentration could possibly become diminished (due to reactions with the metal ducting) to a point where MS2 inactivation is minimal or no longer even occurs (2022 study Evaluation of a Gaseous Hydrogen Peroxide Generating Device). 
• Sensitivity: The other product, trioxidane, is a product of ozone and hydrogen peroxide.  Although devices are restricted in ozone output in the US, those who have asthma or other respiratory issues may want to use them with caution. 

OH, the detergent of the atmosphere, and OH-, the ion that cleanses our homes’ air

Did you know that earth’s atmosphere is self-cleaning, to an extent?  We would be A LOT worse off if it wasn’t.

OH, the hydroxyl radical, is the most important oxidizing species in the atmosphere.  In this article, we’re going to discuss how it’s formed in nature, what it does, and how it’s different from the hydroxide ion OH- that is formed in bipolar ionizers.  

You’ve probably heard that there is ozone in the earth’s atmosphere.  The majority of ozone is found in the stratosphere (about 10-25 miles above the earth), shielding us from the sun’s UV light and cosmic radiation.  This is where ozone can be destroyed by molecules that contain chlorine and bromine, such as chlorofluorocarbons (CFCs). (EPA.gov) About 10% of the ozone is found a bit lower, however, in the troposphere (where clouds are formed and planes fly).  In the troposphere, ozone performs a very important function by being a primary ingredient for the production of OH.  Here, UV energy from the sun (mostly in the UV-B range of 290-310nm)(Treatise on GeoChemistry, ch.5.5.9.1 Chemistry of the Hydroxyl Radical (OH) in the Troposphere), breaks down ozone (O3) into O + O2.  Then, in the presence of water vapor (there’s very little water vapor in the stratosphere, so this has to happen in the troposphere), the lone O molecule reacts with H2O to form 2 molecules of OH (hydroxyl radical).  

In chemistry, a radical, also called free radical, is a molecule that contains at least one unpaired electron.  OH is a radical which is highly reactive because of the configuration of electrons in its outermost shell.  Normally, atoms and molecules prefer to have 8 electrons in their outermost shell, making them most stable (called the Octet Rule), but they will compromise and share electrons if necessary.  OH has 7 electrons in its outer shell: 6 electrons are from the O atom and 1 electron from the H atom.  Each electron has a negative charge, but it is balanced by the same number of protons in the nuclei of the atoms, so that the total “charge” of the molecule remains neutral.  Electrons also like to be “paired”, and although each has a negative charge, they have opposing spin directions which causes them to seek to be “paired” with another electron.  The OH molecule constantly seeks one more electron to “pair” with the 7th electron in its outer shell.  OH only survives for nanoseconds after it is formed–because it can immediately steal that missing electron from most of the chemicals found in the troposphere.  This reaction of the OH with other molecules is called oxidation. 

(Oxidation: Despite the name, the presence of oxygen is not a requirement in an oxidation reaction.  The reaction is part of a transfer of electrons between two substances.  Oxidation occurs simultaneously with reduction in a type of chemical reaction called a reduction-oxidation or “redox” reaction.  The oxidized atom loses electrons, while the reduced atom gains electrons.  On earth, oxidation is usually an undesirable reaction.  Oxidation is another name for rust, corrosion, and breakdown of materials around us and in us.  Our bodies produce “anti-oxidants” to prevent breakdown of our cells. ) 

There are limitless reactions that can happen in the atmosphere, but OH reacts primarily with carbon monoxide (40%) to form carbon dioxide. Around 30% of the OH produced is removed from the atmosphere in reactions with organic compounds and 15% reacts with methane (CH4). The remaining 15% reacts with ozone (O3), hydroperoxy radicals (HO2) and hydrogen gas (H2). (Oxidation and OH Radicals)  With its supreme oxidation potential, hydroxyl radicals can react with molecules and chemicals that are otherwise extremely stubborn and resist oxidation. (Hydrogenlink.com)

Since OH is primarily formed with energy from the sun, OH production mainly happens during daylight hours.  The following map is a snapshot of a model showing how OH is generated as sunlight illuminates a rotating earth.  (The Atmosphere: Earth’s Security Blanket)  Because OH is so short-lived, it’s really hard to detect, so the formation or degradation of other chemicals is used to determine how much OH is in the atmosphere at any one time.  For example, this model is generated from the Tropospheric Emissions Spectrometer (TES) equipment on a NASA satellite.  TES measurements of a number of other chemical elements influenced by OH, such as ozone, carbon monoxide and nitrogen dioxide, have enabled scientists to better represent OH in these models.

Did you know that humans also generate OH indoors? (Science Daily)  Indoor air can have higher (but not dangerous) levels of ozone, which reacts with certain oils on our skin. The reaction releases a host of gas phase chemicals containing double bonds that react further in the air with ozone to generate substantial levels of OH radicals.  It’s a very new discovery (2022), which was aided with extensive computer modeling.  This is important to know, because although they are great to have in the upper level of the troposphere,  we don’t want high levels of hydroxyl radicals indoors.  They can damage tissue and frequently initiate chain reactions with other radicals and VOCs, able to produce harmful chemicals like formaldehyde.