“How fast does it kill?” is the first question most QA managers ask about hypochlorous acid, and the most misunderstood. A kill time is not a property of a chemical on its own. It is the result of a specific test: one organism, one strain, one concentration, one surface, one temperature, one level of soil, measured against a defined reduction. Change any of those and the time changes.
This page separates the two things that are easy to blur together: the regulated contact times that appear on an EPA-registered label, which are the only numbers you can rely on as claims, and the science of kill kinetics, which explains why those times look the way they do and what moves them in practice.
About claims on this page
EcoloxTech generators produce a hypochlorous acid solution on site. They are not EPA-registered disinfectants and we make no efficacy claims for them or for the solution they produce. The only EcoloxTech product with disinfection claims is HOCl-AS200 (EPA Reg. No. 101112-1), used according to its label. Research cited here describes published studies of hypochlorous acid and electrolysed water; it is not a claim for any product.
What “kill time” and “contact time” mean
Contact time (also called dwell time) is how long a surface must stay visibly wet with the product for the claimed result to be achieved. It is the number on a disinfectant label, and it is what inspectors and auditors check against your SOP. If the surface dries early, the contact time was not met, whatever the product.
Kill time is the laboratory term. It comes from a standardised test in which a known population of a named organism is exposed to the product and the survivors are counted. Results are expressed as a log reduction: a 3-log reduction removes 99.9% of the organisms, 5-log removes 99.999%, 6-log removes 99.9999%. Disinfectant registration requires the product to meet the reduction the test method demands for each organism, within the stated contact time.
Two practical consequences follow. First, a laboratory kill time measured in seconds does not entitle anyone to shorten a labelled contact time on the floor. Second, a published study showing rapid inactivation under one set of conditions says nothing about another set of conditions until someone tests it.
The contact times you can rely on: HOCl-AS200
HOCl-AS200 is EcoloxTech’s ready-to-use hypochlorous acid disinfectant, registered with the EPA under Reg. No. 101112-1 (EPA Est. No. 101112-FL1). Its active ingredient is hypochlorous acid at 0.0174%. The accepted label lists the following organisms, all at a 5-minute contact time on hard, non-porous surfaces.
| Organism (as listed on the label) | Strain | Contact time |
|---|---|---|
| Pseudomonas aeruginosa | ATCC 15442 | 5 minutes |
| Staphylococcus aureus | ATCC 6538 | 5 minutes |
| Methicillin-resistant Staphylococcus aureus (MRSA) | ATCC 33591 | 5 minutes |
| Salmonella enterica | ATCC 10708 | 5 minutes |
| Enterococcus faecium | ATCC BAA-2320 | 5 minutes |
| Escherichia coli | ATCC 43888 | 5 minutes |
| Listeria monocytogenes | ATCC 19114 | 5 minutes |
| Streptococcus pyogenes | ATCC 12344 | 5 minutes |
| SARS-CoV-2 | USA-WA1/2020 | 5 minutes |
| Norovirus (as feline calicivirus) | F9 | 5 minutes |
| Murine norovirus | MNV-G | 5 minutes |
| Adenovirus type 5 | ATCC VR-5 | 5 minutes |
| Herpes simplex virus type 2 | ATCC VR-540 (strain MS) | 5 minutes |
| Avian influenza A H3N2 | A/Turkey/Kahle/04 | 5 minutes |
| Bovine viral diarrhea virus | ATCC VR-534 | 5 minutes |
| Influenza A H1N1 | ATCC VR-1520 (A/WS/33) | 5 minutes |
| Respiratory syncytial virus | ATCC VR-26 (strain Long) | 5 minutes |
| Rhinovirus type 14 | ATCC VR-284 (strain 1059) | 5 minutes |
| Herpes simplex virus type 1 | ATCC VR-260 (strain H) | 5 minutes |
| Duck hepatitis B virus (surrogate for human hepatitis B) | — | 5 minutes |
| Human immunodeficiency virus type 1 (HIV-1) | strain MN | 5 minutes |
The directions that go with those times matter as much as the times themselves:
- Pre-clean. All surfaces must be cleaned of visible soil before disinfection.
- Keep it wet. Spray from 6–8 inches, wet the surface thoroughly and leave it visibly wet for 5 minutes, then allow to air dry.
- Check strength if stored. If the product is not used immediately after production, test it with a chlorine test kit and do not use it below 149 ppm available chlorine.
- Hard, non-porous surfaces only. Stainless steel, glass, glazed tile and porcelain, sealed plastics, enamel and similar. It is not a terminal sterilant or high-level disinfectant for critical or semi-critical medical devices.
The label also qualifies HOCl-AS200 for EPA’s emerging viral pathogen claims across enveloped, large non-enveloped and small non-enveloped viruses, used according to the label directions for the supporting viruses. Full directions, use sites and surfaces are on the product page and the regulations page.
What decides how fast hypochlorous acid works
Every hypochlorous acid solution, generated on site or bottled, obeys the same chemistry. Five variables do most of the work.
1. Free available chlorine (concentration)
Concentration is measured as free available chlorine (FAC) in parts per million. Within a sensible range, higher FAC means faster inactivation, and the relationship is roughly reciprocal: halve the concentration and you need roughly twice the time for the same effect. Water treatment engineers express this as the CT value — concentration multiplied by time — for a target reduction of a given organism. The same logic explains why surface sanitising, food-contact work and biofilm remediation are usually specified at different concentrations.
2. pH (which form of chlorine is present)
Free chlorine in water exists as two species: hypochlorous acid (HOCl) and the hypochlorite ion (OCl−). They interconvert with pH around a pKa of about 7.5. Below that, HOCl dominates; above it, hypochlorite does. The distinction matters because HOCl is an uncharged molecule that crosses microbial cell walls far more readily, and it is widely reported to be a much stronger biocide than hypochlorite at the same FAC. This is why a bleach solution and a near-neutral HOCl solution with the same ppm reading behave so differently. Our research hub indexes a study that tested sodium hypochlorite adjusted to different pH values: antimicrobial efficacy of 4.2% sodium hypochlorite adjusted to pH 12, 7.5 and 6.5.
| pH | Approx. share as HOCl | Approx. share as OCl− |
|---|---|---|
| 5.0 | ~100% | ~0% |
| 6.0 | ~97% | ~3% |
| 7.0 | ~75% | ~25% |
| 7.5 | ~50% | ~50% |
| 8.0 | ~25% | ~75% |
| 9.0 | ~3% | ~97% |
3. Organic load
Hypochlorous acid is an oxidiser and it does not discriminate. Protein, fat, sugar, blood and soil consume available chlorine exactly as a cell wall does. Chlorine spent on soil is not available for microorganisms, which is why every serious protocol, and the HOCl-AS200 label, starts with cleaning. On a dirty surface, no contact time printed anywhere is meaningful.
4. Temperature
Like most chemical reactions, inactivation is faster when warmer and slower when colder. Cold rooms, chillers and refrigerated processing areas are the places where contact times deserve the most discipline.
5. The organism, and whether it is protected
Microorganisms differ widely in resistance. As a broad rule used in infection control, vegetative bacteria and enveloped viruses are the most susceptible; non-enveloped viruses such as noroviruses are tougher; fungi and mycobacteria tougher still; and bacterial spores are the most resistant. On top of that, organisms living in a biofilm — the slimy matrix that forms in drains, on conveyors and in water lines — are shielded by it and need longer exposure, higher concentration and mechanical action. Research such as studies of Listeria monocytogenes biofilms treated with electrolysed oxidising water looks specifically at this problem.
What the published research examines
Our research hub indexes more than 300 published studies on hypochlorous acid and electrolysed water. The table below points to representative work by organism group. Study conditions (concentration, pH, surface, soil, temperature, method) vary widely, so read each study for its own conditions rather than lifting a single number out of context.
| Organism group | What the studies look at | Example in the research hub |
|---|---|---|
| E. coli O157:H7, Salmonella, Listeria | Inactivation on stainless steel and in suspension; stability of the solution over time | Neutral electrolysed water on stainless steel; EO water against E. coli, Salmonella and Listeria |
| Salmonella, Listeria (solution stability) | How storage conditions affect available chlorine and activity | Stability of EO water and its efficacy |
| Campylobacter | Carcass treatment under plant operating conditions | Electrolysed water and Campylobacter on poultry carcasses |
| MRSA | Electrochemically activated saline versus dilute hypochlorite | Hypochlorous acid solutions and MRSA |
| Norovirus | Surface inactivation by liquid and fog application; a laboratory evaluation of an EcoloxTech solution | Liquid and fog application against human norovirus; Laboratory evaluation against murine norovirus |
| SARS-CoV-2 surrogate | Inactivation of oral pathogens and a coronavirus surrogate | HOCl, oral pathogens and a SARS-CoV-2 surrogate |
| Avian viruses | Neutral hypochlorous acid against avian influenza and other avian viruses | Neutral HOCl and avian influenza |
| Fungi | Candida albicans and the biochemical mechanism of action | EO water and Candida albicans |
| Bacterial spores | Bacillus cereus cells and spores, alone and combined with citric acid | Electrolysed water and citric acid against B. cereus spores |
Browse by organism on the microbe pages or by sector on the industry pages.
Turning contact times into a working SOP
A contact time only protects you if it is met every time, on every surface, by every shift. The programmes that work tend to share the same habits:
- Write the contact time into the SOP for each surface and task, taken from the label of the registered product you are relying on for disinfection.
- Clean first, visibly. Remove soil and residues before any sanitising or disinfecting step.
- Apply enough to stay wet. Coverage that dries in two minutes has not achieved a five-minute contact time. Adjust volume, method or re-application.
- Verify strength. Check free available chlorine and pH at the start of each shift with test strips or a photometer, and log the result.
- Verify outcome. ATP swabs and periodic microbiological swabbing tell you whether the programme is working, which no chemical specification can.
- Review after change. New surfaces, new soils, colder rooms or a new product all justify re-checking the procedure.
If you are replacing an existing chemical programme, the free sanitation audit maps each task to the right solution and contact time and flags where a registered disinfectant is required.
Frequently asked questions
Can I shorten the contact time if a study shows faster results?
No. For disinfection claims, the contact time on the registered product’s label is the legal and practical minimum. Laboratory studies describe their own conditions and do not change label directions.
What contact time does HOCl-AS200 need?
Five minutes on pre-cleaned, hard, non-porous surfaces, kept visibly wet throughout, for every organism on its label. See the full table above.
Why does a bleach solution with the same ppm behave differently?
Because of pH. Household bleach is alkaline, so most of its free chlorine is present as hypochlorite ion, the weaker form. Near-neutral and slightly acidic solutions keep most of their free chlorine as hypochlorous acid.
Does cold slow it down?
Yes. Chemical inactivation is slower at low temperature, so cold rooms and chilled areas are where contact-time discipline matters most.
How do I know the solution is still at strength?
Measure it. Free available chlorine and pH take seconds to check with test strips or a photometer, and HOCl-AS200’s label requires a check if the product is not used immediately after production (minimum 149 ppm available chlorine).