Even 200 ppm of sodium hypochlorite produced only a little more than a 1-log reduction in norovirus genome copies in one groundwater experiment, according to a groundwater study of human norovirus genotype GII.4. That single result reframes the disinfection efficiency of chlorine treatment. A big number on a bottle doesn't tell you how much infectious virus remains, because chlorine only works while a measured residual stays in contact with the water, at a given pH and temperature, against a specific virus. You'll get further by reading chlorine claims as exposure, not as dose.
Sun exposure for your skin is a useful comparison. Brightness matters, but minutes matter too, and shade, cloud, and sunscreen change what reaches you. Chlorine works the same way, and the sections below build that idea one step at a time.
CT Values and Log Reductions for Chlorine
Free-chlorine CT targets for virus control
| Condition | CT (mg·min/L) | Reduction target | Practical reading |
|---|---|---|---|
| EPA, pH 6 to 9, 0.5°C | 12 | 4-log virus (99.99%) | Longest contact of the listed rows |
| EPA, pH 6 to 9, 5°C | 8 | 4-log virus (99.99%) | Matches the 4-log free-chlorine figure in the guidance |
| EPA, pH 6 to 9, 10°C | 6 | 4-log virus (99.99%) | Mid-range cold-to-warm value |
| EPA, pH 6 to 9, 15°C | 4 | 4-log virus (99.99%) | Shorter contact than at 5°C |
| EPA, pH 6 to 9, 20°C | 3 | 4-log virus (99.99%) | 0.5 mg/L for 6 minutes meets this CT |
| EPA, pH 6 to 9, 25°C | 2 | 4-log virus (99.99%) | Lowest listed value |
| EPA free chlorine, about 5°C | 4 | About 2-log (99%) | Lower log target at the same temperature |
| EPA free chlorine, about 5°C | 6 | About 3-log (99.9%) | Step between 2-log and 4-log |
| WHO drinking-water benchmark, pH below 8 | 15 | Many bacteria and viruses | 0.5 mg/L for 30 minutes; not a 4-log virus target |
What CT actually measures
CT means free-chlorine concentration in milligrams per litre multiplied by contact time in minutes. The unit is written mg·min/L, and you can read it as the total chlorine exposure the water received. A residual of 2 mg/L held for 5 minutes and a residual of 1 mg/L held for 10 minutes both give a CT of 10 mg·min/L, even though the doses look different on paper.
That's why a bottle dose isn't efficiency. The amount you pour in is only a starting point, since some chlorine gets used up reacting with material in the water before it can act on viruses. What counts is the residual, the free chlorine still present after that demand, and how long that residual stays in contact.
Practical rule: never judge a chlorine claim by concentration alone. Multiply the measured free-chlorine residual by the minutes it actually lasted.
Reading the temperature rows
Start with the warm row. The U.S. Environmental Protection Agency disinfection profiling guidance lists CT values for 4-log virus inactivation at pH 6 to 9 of 12 mg·min/L at 0.5°C, 8 at 5°C, 6 at 10°C, 4 at 15°C, 3 at 20°C, and 2 at 25°C. At 20°C, a CT of 3 mg·min/L could be reached with 0.5 mg/L free chlorine maintained for 6 minutes, because 0.5 times 6 equals 3.
Now run the cold row with the same residual. At 0.5°C, the listed CT is 12 mg·min/L, so 0.5 mg/L would need to hold for 24 minutes to deliver the same target. That 12 mg·min/L value is six times the 2 mg·min/L listed at 25°C, which is why treatment plants can't apply one fixed dose all year and expect the same result in winter and summer.
What a log reduction means in plain fractions
Log reductions sound technical, but they're just a counting shortcut. Each extra log knocks off another nine out of ten of what's left.
- 2-log reduction: 99% fewer viable or infectious organisms, so 1 in 100 remains.
- 3-log reduction: 99.9% fewer, so 1 in 1,000 remains.
- 4-log reduction: 99.99% fewer, so 1 in 10,000 remains.
Say you start with 10,000 infectious virus particles in a litre of water. A 2-log result leaves about 100, a 3-log result leaves about 10, and a 4-log result leaves about 1. Those survivors are the reason a 4-log result is not sterilization, since sterilization would mean nothing viable is left at all.
The steps between logs also aren't evenly spaced in CT terms. A review of U.S. EPA free-chlorine guidance identifies CT values of 4, 6, and 8 mg·min/L for approximately 2-log, 3-log, and 4-log virus inactivation respectively, with 8 mg·min/L giving 4-log, or 99.99%, inactivation of viruses at 5°C and pH 6 to 9. Going from 99% to 99.99% at that temperature takes roughly double the exposure, not a small extra splash.
Where the WHO benchmark fits
Many readers meet chlorine through household or emergency water advice, and that's where a familiar benchmark helps. The World Health Organization drinking-water guidance recommends at least 30 minutes of contact time with a free-chlorine residual of 0.5 mg/L at pH below 8, which produces a CT of 15 mg·min/L. Under those conditions chlorine is generally effective against many bacteria and viruses.
Don't mistake that benchmark for a universal virus guarantee. It's a general drinking-water operating point, and it sits in the table as a comparison row rather than a 4-log virus target. Some organisms also sit outside its reach, since chlorine-resistant parasites need very different treatment, so a residual that looks adequate for routine bacterial and viral control doesn't protect equally against every pathogen.
Why concentration and time trade against each other
Because CT is a product, you can reach the same number two ways. A lower residual held longer can match a higher residual held briefly, as long as the residual persists for the full period and the water conditions stay the same. This is the part product labels often hide, since they advertise the starting concentration but stay quiet about whether it survives to the end of the contact period.
There's a catch readers often miss. If chlorine decays during contact, the actual exposure is less than the opening concentration multiplied by the total minutes. Operators handle this by measuring the residual through the process, not just at the dosing point, and by designing tanks so the slowest-moving water still gets its full share of time.
A CT copied from a warm-water row won't protect cold water. Match the temperature row first, then check pH and water quality.
So the habit to build is simple. Before you copy any CT from a table, a label, or a study, confirm the temperature, the pH range, and whether the number describes the residual that remained or merely the dose that went in. Those three checks decide whether the figure applies to your water at all.
pH Temperature and Organic Load Effects
A measured dose doesn't guarantee virus removal. What matters is the effective exposure, the free chlorine that's still active after the water has taken its share, held for the time the water spends in contact. pH and temperature change how much work each milligram of that residual can do.
CT for 99 percent virus inactivation by pH and temperature
| Temperature | pH band | CT (mg·min/L) | Change from favorable pH |
|---|---|---|---|
| 10°C | 7.0 to 7.5 | 8 | Reference point at this temperature |
| 10°C | 8.5 to 9.0 | 22 | Nearly three times the CT |
| 0 to 5°C | 7.0 to 7.5 | 12 | Cold water at favorable pH |
| 0 to 5°C | 8.5 to 9.0 | 35 | Highest listed combination |
Reading the pH shift
WHO-linked data summarized in an Engineers Ireland review of chlorine demand in Irish water supplies show the CT required for 99% virus inactivation at 10°C rising from 8 mg·min/L at pH 7.0 to 7.5 to 22 mg·min/L at pH 8.5 to 9.0. At 0 to 5°C, the requirement rises from 12 to 35 mg·min/L across the same pH range. Nothing about the virus changed between those rows, only the water around it.
Picture a balance scale. Below about pH 7.5, more of the free chlorine sits as hypochlorous acid, the more active form, so the scale tips toward fast inactivation. As pH rises, the balance tips toward hypochlorite, a weaker disinfectant, so the same measured residual buys you less virus kill per minute and you need a higher CT to reach the same endpoint.
Chlorine demand eats the dose first
Chlorine demand is the portion of your dose consumed by things that aren't viruses. Dissolved organic matter, ammonia, iron, sediment, and deposits on pipe walls all react with chlorine, and dirty water can consume most of a dose before disinfection properly starts. That's why practitioners pre-filter cloudy water when they can, since removing solids also removes hiding places where microbes sit shielded from the residual.
This is also why free chlorine and total chlorine aren't interchangeable on a test strip. Total chlorine includes chlorine that's already combined with other substances and largely spent, while free chlorine is the fraction still available to inactivate viruses. A strong total reading with a weak free reading means the water won the first round.
Short-circuiting steals contact time
Tanks create a quieter problem. In a poorly baffled contact tank, some water finds a fast path from inlet to outlet and leaves early, a flaw called short-circuiting, while other water sits in stagnant corners far longer than intended. The average residence time can look fine on paper even though the fastest fraction received only part of the planned CT.
Good design pushes flow toward plug flow, where every parcel of water spends roughly the same time moving through. Baffles, rounded corners, and perforated baffle walls all exist to narrow that spread of times, because disinfection is only as strong as the exposure received by the water that leaves first.
Four checks that make a residual meaningful
- Measure free chlorine: test the active residual, not total chlorine or the amount added.
- Record pH and temperature: both set which CT row applies to your water.
- Measure at the point of use: a residual at the dosing pump says little about the far tap or tank outlet.
- Confirm the slowest water: the earliest-leaving fraction must still receive the full contact time.
One more nuance keeps expectations honest. CT tables are conservative design tools, and individual laboratory studies sometimes show faster early inactivation under their own controlled conditions. That's a reason to respect the safety margin in the tables, not a shortcut for cutting contact time at home.
Rotavirus Adenovirus and Norovirus Results
No single efficiency percentage fits every virus. Viruses differ in structure, and chlorine attacks those structures unevenly, so a result measured on one virus can mislead you badly about another.
Chlorine results by virus or surrogate
| Virus or surrogate | Condition | Reported result | What it does not prove |
|---|---|---|---|
| Human rotavirus | pH 10, 5°C | CT 0.14 mg·min/L for 99.99% | Not a dose for every non-enveloped virus |
| Adenovirus | Free-chlorine exposure in comparative study | Protein needed for replication may be blocked | Genome may persist while a protein needed for replication is blocked |
| Norovirus GII.4 | Groundwater, up to 200 ppm sodium hypochlorite | 1.15 log10 genome copies per µL | Genome loss is not proof that infectivity is gone |
| Norovirus suspension | 3.75 mg/L free chlorine, 30 minutes | Still infectious in an earlier review | A modest residual can fail |
| Tulane virus (norovirus surrogate) | Agricultural water, 3-log target | 9.6 to 23 ppm free chlorine | Surrogate result; compare with 0.6 to 0.9 ppm for E. coli |
Rotavirus looks easy, and that is the trap
Human rotavirus is a large non-enveloped virus, a category often treated as a harder disinfection target than enveloped viruses. Yet a comparative study of enteric adenovirus and feline calicivirus reports a human rotavirus CT value of 0.14 mg·min/L for 99.99% inactivation at pH 10 and 5°C. That works out to roughly 0.1 mg/L free chlorine maintained for 1.4 minutes, a tiny exposure beside the regulatory virus CTs built to cover harder targets in variable real water.
Read that number narrowly. It describes one virus, in one experiment, at an unusually alkaline pH you'd rarely use for drinking water. Borrowing it as a general non-enveloped virus dose would be like timing one runner downhill and setting everyone's qualifying standard from that run.
Adenovirus fails differently
Adenovirus shows why the endpoint you measure changes the answer. Free chlorine may inhibit expression of a protein required for the viral life cycle without necessarily destroying the genome or eliminating the virus's ability to bind to a cell receptor. A test that only looks for genetic material can therefore report the virus as still present even when it can't complete an infection, or the reverse concern can arise with other methods.
Genome copies and infectivity are different endpoints. A falling RNA signal doesn't by itself prove the virus can no longer infect anyone.
Norovirus demands caution
Norovirus sits at the stubborn end of this comparison. In groundwater, sodium hypochlorite at 10, 30, 50, 100, 150, and 200 ppm against genotype GII.4 produced reductions of only 0.21, 0.31, 0.48, 0.74, 0.95, and 1.15 log10 genome copies per microliter. Separately, norovirus suspensions have remained infectious after 30 minutes of exposure to 3.75 mg/L free chlorine, so a modest residual that handles bacteria comfortably can still leave infectious norovirus behind.
Surrogate work points the same way. A 2025 Journal of Food Protection study found that a 3-log10 reduction of Tulane virus, a human norovirus surrogate, in agricultural water required 9.6 to 23 ppm free chlorine, compared with 0.6 to 0.9 ppm for E. coli. Feline calicivirus plays a similar stand-in role as a small non-enveloped surrogate, useful for comparison but never identical to norovirus itself, and our detailed review of norovirus transmission and disinfection efficacy works through that distinction.
Enveloped viruses and the missing number
Readers often ask about influenza A (H1N1) and SARS-CoV-2 here. Enveloped viruses are generally considered easier to inactivate than non-enveloped ones, since their outer lipid layer is vulnerable to oxidants and detergents. But there's no comparable chlorine CT for influenza or SARS-CoV-2 in the evidence behind this guide, so you shouldn't borrow a rotavirus or norovirus number by analogy. Look for a named study that reports a measured CT, the water conditions, and whether it tested infectious virus or only genetic material before you trust any percentage.
Plant Targets versus Laboratory CT Values
Set two numbers side by side and the gap looks absurd. A laboratory rotavirus result sits at a fraction of 1 mg·min/L, while one public water reuse standard demands 450 mg·min/L. Both numbers are real, and neither is wrong, because they're answering different questions.
Three chlorine decisions compared
| Decision | Target | Reported figure | What it does not mean |
|---|---|---|---|
| Laboratory virus CT | One virus in one experiment | Rotavirus 0.14 mg·min/L for 99.99% at pH 10, 5°C | Not a design target for variable water |
| Surface-water drinking-water credit | At least 4-log virus, 3-log Giardia | Filtration plus disinfection can share the credit | Not chlorine alone doing the whole job |
| California reuse criteria | Disinfected tertiary recycled water | CT at least 450 mg·min/L, about 90-minute modal contact | Not a home dose; compliance depends on residual measured through the basin |
Drinking-water credit is shared work
Surface-water plants are expected to reach at least 4-log virus removal or inactivation, with a matching 3-log target for Giardia. That credit doesn't have to come from chlorine alone, since filtration physically removes particles and organisms before disinfection ever starts. A plant might earn part of its virus credit at the filters and the rest from the chlorine contact basin, which is why quoting a plant's chlorine dose tells you little about its total virus control.
The reuse standard and its safety margin
For recycled water, the bar rises sharply. The U.S. EPA summary of California water reuse criteria requires a chlorine CT of at least 450 mg·min/L, generally paired with a minimum modal contact time of about 90 minutes, for specified disinfected tertiary recycled water, based on the Pomona Virus Study. One theoretical way to reach 450 is 5 mg/L chlorine maintained for 90 minutes, though actual compliance depends on how the residual is measured and maintained throughout the process.
Treat that arithmetic as an illustration, not a recipe. Modal contact time describes when the bulk of the water passes through, and operators verify it with tracer testing because short-circuiting can waste much of a basin's nominal volume. Tanks sometimes get redesigned with better baffling for exactly this reason, since recovering wasted contact time can deliver more disinfection than adding chlorine.
A laboratory CT describes one virus under controlled conditions. A plant target buys confidence across changing water, imperfect mixing, and every pathogen that might be present.
Keep the three decisions separate in your mind. A laboratory virus CT tells researchers how susceptible one virus is, a drinking-water disinfectant credit tells regulators how much of a shared target chlorine must carry, and a reuse standard tells operators how much exposure guarantees public confidence. Blurring them is how a tiny lab number ends up misused as a design target, or how a huge regulatory number ends up feared as a household dose.
Disinfection By-Products and Study Limits
A chlorine treatment has two chemical jobs: inactivate viruses and react with other substances in water. Dissolved organic matter can consume chlorine and form disinfection by-products, or DBPs, so the same treatment can produce different by-product levels in different source waters.
Source water sets the chemical bill
A 2025 study across four source waters reported trihalomethane formation after chlorination of 49 to 111 µg per mg dissolved organic carbon at 48 hours in surface water, compared with 21 to 27 µg per mg dissolved organic carbon in groundwater. Surface water can contain more humic material from soil and decaying vegetation, which helps explain why the same nominal chlorine treatment may produce a different by-product burden. If trihalomethanes in tap water are a concern, water reports can help guide whether to choose filters for THM removal.
Trihalomethanes are only part of the picture. Researchers have identified many other by-products of chlorination, so checking one category does not describe every compound that may be present. Advanced UV/chlorine systems can also form some chlorate, another reason to assess the treatment process as a whole.
Two readers, two different decisions
For household water treatment, increasing chlorine to match a laboratory virus result does not by itself prove greater protection. A higher dose is not the same as a measured loss of infectivity, and it may increase chemical exposure or affect taste. Use the residual and contact time specified for the treatment method.
Plant operators must weigh virus inactivation against by-product formation as source water changes. They may also consider another disinfectant for part of the process. The overview of chlorine dioxide disinfection explains this alternative. When reading a study, check which virus was tested, the CT conditions, and what organic material was present. A reduction in genome copies alone does not establish that infectious virus was inactivated.
Check the Conditions Before Trusting a Dose
You can test any chlorine claim, whether it comes from a product label, a journal paper, or a utility report, with six checks in order. Work through them before you repeat the headline percentage to anyone else.
- Confirm free chlorine: use the active residual, not total chlorine and not the amount originally added.
- Do the multiplication: residual in mg/L times the contact time the water or surface received gives the CT in mg·min/L, and our explainer on what contact time means for disinfectants shows how to judge whether that time was real.
- Match pH and temperature: find the table row for your conditions instead of borrowing a warm, neutral-pH figure.
- Name the virus: note whether it's enveloped or non-enveloped, and whether a surrogate stood in for it.
- Prefer infectivity data: a study that cultured infectious virus tells you more than one reporting genome copies alone.
- Note the organic load: dirty water consumes chlorine before disinfection starts and supplies the precursors for by-products.
If you can't fill in one of those steps from the source in front of you, say so plainly. The honest verdict is that efficiency isn't yet known for your conditions, not that chlorine succeeded or failed. That restraint is what separates a careful reader from someone quoting a percentage stripped of its context.
From here, pick the next virus page that matches your concern, whether that's norovirus, rotavirus, or a respiratory virus, and run the same six steps against its disinfection section. Our Science articles cover mutation mechanisms, virus-host interactions, disinfection strategies, environmental factors, and real-world applications of virus control, while our Education articles highlight practical tips for staying safe, cleaning effectively, and recognizing common risks. Between the two, you'll find the named evidence each virus deserves instead of one universal kill percentage stretched across all of them.
Put the checklist to work today. Take the next disinfectant claim you meet, find its free-chlorine residual, contact time, and named virus, and calculate the CT yourself before you trust it. For everyday surface cleaning between deep disinfection jobs, choose disinfecting wipes that name their target viruses and required wet contact time on the label, keep the surface visibly wet for that full period, and subscribe to VirusFAQ.com updates so each new virus guide reaches you with the same six checks already applied.
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