What Are Hydroxyl Generators in Fire and Smoke Restoration?

What a hydroxyl generator is

A hydroxyl radical is a very reactive molecule made of one oxygen atom and one hydrogen atom, written •OH.

A manufacturer’s spec sheet describes the radicals as forming naturally when ultraviolet light reacts with water vapor and oxygen in the atmosphere, and says its machines copy that process indoors ¹.

Inside the machine, the maker says, UV light and a catalytic surface generate the radicals, which then react with airborne contaminants ².

The restoration trade uses these units after fires, and for other odors, as an air treatment.

A Restoration Industry Association blog post, credited to a trade magazine, says hydroxyl generators produce oxidizing gases that break apart odor molecules, and that they may address a wider range of odors and volatile organic compounds than ozone ³.

It also says they can run while normal cleaning goes on, which may let clients stay in a home or business.

That is the industry’s story. Government guidance is more cautious. EPA’s fact sheet for people returning to ash-contaminated homes says to avoid products that add chemicals to the air to remove or neutralize smoke odors, naming ozone and hydroxyl generators and other oxidizers ⁴.

The two positions sit side by side, and this page reports both rather than choosing.

The wording matters. “Hydroxyl generator” is a category label, and devices vary in power, design and extra modes. A unit sold with a hydroxyl label can include an ozone mode.

The maker of one such unit lists ozone as unsafe for occupied spaces, where the area must be vacated during treatment ². Always ask what a specific machine produces.

For the wider picture, see the smoke odor removal entry. The fire and smoke damage restoration page shows where air treatment sits in a job.

How makers say hydroxyl treatment works

Hydroxyl generator operation, as manufacturers and a trade body present it, follows these steps. They are not tested findings, and they are not instructions for running a machine.

1. UV light meets moisture and oxygen. The maker says its photocatalytic chamber uses a germicidal UVC bulb with a catalytic surface to produce hydroxyl radicals ¹².
2. Radicals leave the unit. A fan moves air through the chamber. One model’s sheet lists a fan output of 130 cubic feet per minute and carbon pre- and post-filters ¹.
3. Radicals react with odor molecules. The trade post says the gases break apart odor molecules ³.
4. Placement and spacing matter. The trade post advises choosing the right generators and spacing them properly so oxidant concentrations do not get high in small, unventilated occupied spaces ³.
5. Treatment runs with other work. The same post says contractors can run units during normal cleaning ³.
6. The unit is maintained. The maker lists UV bulb and filter kits and says any device that moves air needs routine cleaning and preventive maintenance ¹.

None of these sources gives a measured concentration, an exposure limit or an independent test result for hydroxyl units. The trade post offers no specific limits ³. That gap is why caution is the safer reading.

Kinds of units, and what each claims

Example: what to check about a hydroxyl line on a quote

The hydroxyl proposal example illustrates how to read a quote. The quote and the answers are made up. The questions follow what the sources say about placement, occupancy and device modes ³²⁴.

A written answer to each question shows whether the treatment is a plan or a sales line. The estimate review tool and the hiring guide help you compare quotes.

Where hydroxyl generators show up

On a fire or odor estimate. They appear as “hydroxyl generator,” “hydroxyl machine” or “air treatment,” as a line item with a count of units.

In the work authorization. The paper should say whether the machines run while you are present, and what the contractor is doing about cleaning and removal in the same period.

In other jobs. The trade post names them for odors in general and for spaces where operations continue ³. This page covers smoke only.

In the claim file. Adjusters may ask what the equipment was for and how long it ran. Policies vary on odor scopes; see the insurance claims guide.

Next to thermal fogging and ozone. They can be offered together or as alternatives. The thermal fogging entry and the ozone treatment entry compare them.

What the sources say hydroxyl units offer

• Makers say they can run in occupied spaces, which can let work and living continue ¹³. Treat this as a maker claim, and note EPA’s caution ⁴.
• The trade post says they may address a wider range of odors and VOCs than ozone ³.
• They are portable, and the maker lists coverage by square footage and a continuous run mode ¹.
• They can run alongside cleaning, so they need not hold up other work ³.
• The maker says the radicals react without leaving residues. This is a maker claim; no independent test was loaded.

Limits and unknowns

• No independent evidence was loaded. The sources for effectiveness and occupant safety are a maker and a trade association post. The trade post gives no concentration thresholds, exposure limits or test data ³.
• A regulator advises avoiding them. EPA’s ash fact sheet says to avoid products that add chemicals to the air for smoke odor, naming hydroxyl generators ⁴. That advice is aimed at people returning to ash-contaminated homes, but it is the only government statement found.
• They do not clean. UF/IFAS says soot must be removed before textiles are cleaned or deodorized, and FEMA’s steps rely on cleaning, filtering and absorbing ⁸⁷.
• Oxidant levels depend on use. The trade post says placement and ventilation matter in small, unventilated spaces ³.
• Device modes vary. Some units include ozone ², which carries well-documented lung risks ⁵.
• Odor control is not safety. FEMA says cleanup can reduce but not eliminate contaminants and recommends environmental testing after cleanup before long-term occupancy ⁷.

What drives the cost of hydroxyl treatment

No price for hydroxyl treatment is given here. Hydroxyl work can be priced as a rental-style charge inside a larger fire scope, and costs vary by region and contractor, so no figure here would meet our cost standard.

The drivers are the number of units, the floor area, the days they run and whether the contractor bills for monitoring and moves. A maker’s sheet gives coverage by square footage for each model, and the number of machines on a quote should match the rooms treated ¹.

Lifespan and warranty belong to the equipment: one maker lists a UV chamber life of more than 25,000 hours and a two-year warranty on its unit ¹. That is a maker’s product term, not a promise about your odor.

The cost question for an owner is what else is on the estimate. Machine days without cleaning and removal are a weak plan. See the cost guide for what drives totals.

Who does what, and what to inspect

After treatment: ventilation, filters and follow-up

Ventilate when the building and the weather allow. EPA’s ash fact sheet advises opening windows and doors to air out the building during indoor cleanup, if conditions permit, and using an air cleaner to capture fine particles ⁴.

Keep filtering. FEMA suggests a filter rated MERV 11 or higher in constant fan mode and activated charcoal for light odor, once the bulk of soot is gone ⁷.

Smell the space again after several days. If odor returns, something that holds it was not removed. The soot removal entry and the protein smoke residue entry explain two usual causes.

Return rented equipment and keep the manual and run log with your claim papers.

Warning signs, and when to stop

• A sharp or electrical smell near the machine, or throat, eye or chest irritation while it runs. Leave, ventilate and tell the contractor.
• A claim that the machine is “totally safe” with no manual, label or limit to read.
• A plan that uses an ozone mode with people, pets or plants inside ⁵.
• Hydroxyl units offered in place of cleaning, removal and duct work.
• Units packed into a small room with no ventilation ³.
• A household member with asthma, heart or lung disease, who should ask a health professional first.

Standards, guidance and rules

Industry standard. ANSI/IICRC S700 covers fire odor management in general terms. The first edition is dated 2025 and is under revision ¹¹. The IICRC page does not detail hydroxyl equipment, the standard is paid, and this page does not describe its text.

Government guidance. EPA’s ash fact sheet (EPA-452/F-26-011) advises avoiding hydroxyl generators, ozone and other oxidizers for smoke odor ⁴. It is guidance, not a ban.

Equipment labeling. The maker’s sheets carry an EPA establishment number, which identifies a manufacturing site and is not an approval of use ¹. No source here shows a government approval of hydroxyl generators for occupied spaces.

Ozone-capable units. California regulates the sale of ozone-emitting air cleaners through CARB ⁶. See the ozone entry.

Worker safety and contracts. Employee protection falls under OSHA rules, and your contract sets what the contractor promises.

Hydroxyl compared with other odor treatments

Common myths about hydroxyl machines

Myth Hydroxyl is the same as ozone.

Reality They are different gases. Hydroxyl units make hydroxyl radicals, ozone generators make ozone. But some combination units contain both, so ask ².

Myth Because it is “natural,” it needs no precautions.

Reality The maker says the process mimics atmospheric chemistry ¹. EPA still advises avoiding these products for smoke odor ⁴, and the trade post warns about oxidant levels in small rooms ³.

Myth A hydroxyl machine cleans soot.

Reality No source describes it removing residue. Soot must be removed first ⁸.

Myth It is approved for occupied homes.

Reality The occupancy statements here come from a maker and a trade post. EPA’s ozone page says no federal agency has approved ozone devices for occupied spaces ⁵, and no source found approves hydroxyl units.

Myth One machine fixes the whole house.

Reality Odor is held in materials, cavities and ducts, and FEMA’s ladder starts with cleaning and filtering ⁷.

Where your location changes the picture

When this matters to you, and what to do next

Hydroxyl generators matter when a contractor proposes hydroxyl machines. Ask what is being cleaned and removed first, ask for the manual and ask whether you should stay out. If anyone in your home has lung disease, ask a health professional, and consider a plan that relies on cleaning, filtering and ventilation.

The scam avoidance guide flags guaranteed results. The how restoration works guide shows where odor work belongs in a fire job.

If the odor is only light, FEMA’s four steps are a cautious start before any equipment. If the odor is deep, plan for professional cleaning of ducts, contents and cavities.

Related terms

Air Scrubber

An air scrubber is a portable filtration machine that draws room air through a series of filters, such as a pre-filter and a HEPA filter, and returns it to the room to remove airborne particles while restoration work is under way.

HEPA Filtration

HEPA filtration is air filtering with a high-efficiency particulate air filter, a pleated mechanical filter rated to remove at least 99.97% of airborne particles 0.3 microns across when tested under set conditions.

Negative Air Machine

A negative air machine is a portable fan unit with a HEPA filter that pulls air out of a sealed work area and sends it outside, so the area sits at lower air pressure than the rooms around it and dust and mold spores do not leak out.

Restoration Scope of Work

A restoration scope of work is the written description of the work a restoration company agrees to do, covering what, where, how much and when, so that you, the company and your insurer can compare the same plan.

Salvageable vs. Non-Salvageable Materials

Salvageable materials are building materials and belongings that can be cleaned and dried back to a safe, sound state after water damage, while non-salvageable materials are those that cannot and must be removed and replaced.

IICRC Standards in Restoration

IICRC standards in restoration are voluntary, consensus-based industry standards published by the Institute of Inspection, Cleaning and Restoration Certification, a nonprofit, that describe how professionals should carry out work such as water damage restoration, mold remediation and fire and smoke damage restoration.