Isothiazolinone (CMIT/MIT) in Water Treatment: Uses, Dosage, Specifications and Safety

Isothiazolinone (CMIT/MIT, also called isothiazolone or isothiazolin; CAS 55965-84-9) is a 3:1 mixture of a chlorinated and a non-chlorinated isothiazolinone, dosed as a non-oxidizing biocide and supplied for water treatment mainly as a 1.5 % active-ingredient (a.i.) liquid. In that grade, 1.11 % is CMIT and 0.39 % is MIT, so every dose has to name its basis: as product or as active.

Cooling towers receive CMIT/MIT as a basin slug, 100 ppm as product held for 4 hours in one supplier's program, and the mixture is approved under the EU Biocidal Products Regulation until 2027-06-30 (Implementing Regulation (EU) 2016/131). CMIT/MIT is one of the biocides in this reference of water treatment chemicals, documented with the same identity, dose and compliance fields. This page covers industrial and utility water systems only.

The identity card below summarizes the attributes that define CMIT/MIT.

Every value on this page is checked against primary sources (EPA pesticide labels, EUR-Lex, ECHA, NSF listings, HSE); see our methodology and fact-checking process.

What Is Isothiazolinone (CMIT/MIT)?#

CMIT/MIT is the 3:1 reaction mass of 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT) and 2-methyl-4-isothiazolin-3-one (MIT), two heterocyclic rings built on a nitrogen-sulfur (N-S) bond. The mixture carries CAS 55965-84-9 but no EINECS number; ECHA lists it under list numbers 911-418-6 and 611-341-5, while EC 247-500-7 belongs to CMIT alone and EC 220-239-6 to MIT alone.

Trade names include Kathon, Preventol D and Preventol IT, and PubChem records Kathon 886 as a synonym. The registered water-treatment grade KATHON 886 MW 1.5 % (alternate brand KATHON LX 1.5 %, EPA Reg. No. 707-134) contains 1.5 % a.i. in water, the strength every dose on this page refers to unless stated otherwise.

What is the difference between CMIT and MIT?#

CMIT is the chlorinated component (C4H4ClNOS, 149.60 g/mol) and MIT the non-chlorinated one (C4H5NOS, 115.16 g/mol); in a 1.5 % product, CMIT supplies 1.11 % and MIT 0.39 % of the active ingredient. Methylchloroisothiazolinone (searched as methyl chloro isothiazolinone) is CMIT; methylisothiazolinone is MIT.

The two also differ in law and hazard. MIT has its own EU approvals, separate from the 3:1 mixture, and its harmonised classification carries H311 (toxic in contact with skin) where the mixture carries H310 (fatal in contact with skin).

How Does Isothiazolinone Work as a Non-Oxidizing Biocide?#

CMIT/MIT kills by reacting the electron-deficient sulfur of its N-S bond with cell thiols such as cysteine and glutathione, which inhibits growth within minutes and kills cells within hours (Silva et al., 2020, Molecules). The thiol reaction works without the oxidation step that chlorine or bromine relies on, which is why CMIT/MIT counts as a non-oxidizing biocide.

Thiol-reactive kill covers 3 organism groups in water systems: bacteria, algae and fungi. The same review lists cooling-tower slug dosing, RO membrane storage and UF biofouling control among the applications.

How do pH and temperature affect CMIT/MIT stability?#

CMIT has a half-life below 2 hours at pH 9.6 and 60 °C (140 °F), against 46 days at room temperature and pH 8.5 (Silva et al., 2020). The degradation rate of CMIT rises about 2,000-fold between pH 4.5 and pH 11, so alkaline cooling water consumes the chlorinated component far faster than neutral water.

Stability decides how long a slug stays active. A cooling tower running at high pH and warm return temperatures loses CMIT within hours, which is why contact time is counted from the moment of the shot and the residual is checked before the next one. High pH therefore appears with sulfides and amines in the incompatibility list further down this page.

Where Is Isothiazolinone Used? Cooling Towers, Closed Loops and Membranes#

CMIT/MIT is used in 3 water systems: open recirculating cooling towers, closed loops and RO or UF membrane plants. The table below compares the dose, basis, feed point and control in each system.

Cooling towers: slug dosing with a 4-hour contact#

Cooling towers receive CMIT/MIT as a slug into the basin, 100 ppm as product held for 4 hours of contact in Dober's published program. HSE HSG274 Part 1 (2024) sets the same order of contact for non-oxidizers, typically at least 4 hours. Oxidizer and non-oxidizer schedules are compared under cooling tower biocides.

Contact depends on what the water contains. Sulfides, amines and thiols consume the active, and CMIT breaks down in hours at high pH and temperature, so a tower with sulfide or amine contamination or alkaline chemistry holds less active for the full 4 hours than the nominal dose suggests.

Closed loops: chilled and hot-water systems#

Isothiazolin is one of 5 non-oxidizing biocides used in closed loops, beside glutaraldehyde, tris nitro, potassium dimethyldithiocarbamate and polyquats (NATCOLL closed-system options). One study (Critchley, 2009) reports that isothiazolinone, like bromine, may need higher doses in loops treated with molybdate or nitrite programs, a single-study result that a plant confirms by counts before changing a dose.

RO and UF membranes: label doses and off-line soaks#

PermaClean PC-56 (EPA Reg. No. 1706-183) is labeled for UF and non-potable RO at 2.75 to 16.5 fl oz of product per 1,000 gal of feed, about 21 to 129 mL/m³ (calculated), fed at the system inlet. For an off-line soak, the label gives 100 to 400 ppm as product (13.75 to 55 fl oz per 1,000 gal); permeate and reject go to drain during use, and the system is rinsed until permeate conductivity is at or below pre-treatment values.

MIT alone also appears in RO cleaning and storage, according to the Silva et al. (2020) review. RO and NF slug doses and contact times follow the bulletin of the membrane manufacturer for the installed element, not a generic figure. Element-specific biocide choices are covered under membrane biofouling control.

How Is the Isothiazolinone Dose Calculated? Product vs Active Ingredient#

The dose as active equals the dose as product multiplied by the a.i. fraction: 100 mg/L of a 1.5 % product is 1.5 mg/L as active (calculated). The KATHON label (EPA Reg. No. 707-134) states its doses both ways, 500 to 1,167 ppm as product equal to 7.5 to 17.5 ppm a.i., for fouled systems in a non-cooling use.

Supplier cooling figures such as 100 ppm do not state a product strength. The 1.5 mg/L a.i. value holds only for a 1.5 % grade, so a dose transfers between products only on an active basis.

Worked example: a cooling tower slug dose#

A worked example: 100 mg/L as product in a 100 m³ (26,417 gal) system needs 10 kg (22.0 lb) of product, which is 0.15 kg (0.33 lb) as active for a 1.5 % grade (calculated). The slug equals system volume times target dose; kg = mg/L x m³ / 1,000 (EOCP) and lb = mg/L x MG x 8.34 / purity (Florida DEP), with a purity of 1 for a dose as product.

Product volume needs the density of the delivered grade from its SDS. For other volumes, the dosage and feed rate calculator converts a product dose to kg and lb.

How Is a CMIT/MIT Biocide Program Controlled and Tested?#

A CMIT/MIT program is controlled with 4 checks: a colorimetric field test for the active, aerobic plate counts, Legionella sampling and, on membranes, a conductivity rinse. HSE HSG274 Part 1 (2024) sets the count levels for cooling systems. Sampling rules and action levels are set out under Legionella control in cooling towers.

The 4 checks and their values are listed below.

• CMIT/MIT colorimetric field kit (HSG274): the sample is filtered at 0.45 µm or read against a field blank.
• Aerobic counts: at or below 1 x 10^4 cfu/mL (incubated at 30 °C (86 °F) for at least 48 h) is under control; above 1 x 10^5 cfu/mL calls for action.
• Legionella sampling: taken immediately before a non-oxidizer shot; not detected or at most 100 cfu/L is the target, above 1,000 cfu/L calls for immediate action.
• Membrane conductivity rinse: the PC-56 label requires rinsing until permeate conductivity is at or below pre-treatment values.

Why do programs alternate CMIT/MIT with a second biocide?#

Programs alternate because HSE HSG274 (2024) advises 2 non-oxidizing biocides with different kill mechanisms, each typically held for at least 4 hours. CMIT/MIT attacks thiols, so its partner works another way: DBNPA binds to cell walls, and glutaraldehyde cross-links cell-wall amines. Alternating two mechanisms keeps one surviving population from carrying through every shot, and the Legionella sample is timed before the shot so the result reflects the program, not the last dose.

What Is CMIT/MIT Incompatible With? Sulfides, Amines and High pH#

CMIT/MIT is deactivated by nucleophiles that attack its N-S bond and by high pH, which speeds CMIT degradation about 2,000-fold from pH 4.5 to 11. The 4 nucleophile groups are listed below.

• Sulfides, including hydrogen sulfide (H2S).
• Amines, which open the N-S bond like the other nucleophiles.
• Thiols, the same chemical group the biocide attacks in cells.
• Metals, which the Silva et al. (2020) review lists with the nucleophiles.

Molybdate and nitrite programs may also raise the dose needed, a single-study observation (Critchley, 2009). Incompatible pairs across all programs are listed under water treatment chemical compatibility.

Which Standards Cover Isothiazolinone? NSF/ANSI/CAN 60#

No AWWA or EN product standard covers CMIT/MIT; NSF/ANSI/CAN 60 applies only where a formulated membrane biocide is certified for a drinking-water treatment train. Those products are listed by trade name under the product function "Reverse Osmosis Membrane Biocide", not under a chemical name.

Is CMIT/MIT certified to NSF/ANSI/CAN 60?#

No CMIT/MIT-based listing was verified: formulated products appear under "Reverse Osmosis Membrane Biocide", with maximum use levels from 1 to 100 mg/L as product across all listed biocides (NSF listings, 2026-10-20). The function covers 42 companies, 186 products and 242 product-function rows; 179 rows give no maximum use level, and 30 mg/L as product is the most common stated value (NSF listings, 2026-10-21).

A maximum use level is the total over all feed points and appears on the product label and listing. Certification ends when a product is repackaged, diluted or blended at an uncertified site. MULs and listing rules are covered under NSF/ANSI/CAN 60 certification.

What Is the Regulatory Status of CMIT/MIT? FIFRA, EU BPR and 21 CFR 173.310#

CMIT/MIT is a registered active under US FIFRA (CMIT PC code 107103, MIT 107104), approved under the EU BPR from 2017-07-01 to 2027-06-30 and not listed in 21 CFR 173.310 (status 2026-10-21). Under 40 CFR 152.15, a product making slimicide or algicide claims is a pesticide and may be sold only with an EPA Reg. No.

The table below compares the instruments that apply to CMIT/MIT by value and type.

Is CMIT/MIT approved for cooling systems under the EU BPR?#

Yes, with conditions: C(M)IT/MIT (3:1) is approved for PT11 (preservatives for liquid-cooling and processing systems) until 2027-06-30, but not for large open recirculating cooling systems unless risks are reduced, and small open recirculating systems need mitigation of air deposition (Implementing Regulation (EU) 2016/131). The same approval covers PT12 slimicides and PT2, PT4, PT6 and PT13.

MIT alone holds separate approvals: PT11 under Regulation (EU) 2017/1278 from 2019-01-01 to 2028-12-31, PT12 under Regulation (EU) 2017/2004 to 2029-03-31, and PT13, whose expiry Decision (EU) 2026/2127 of 2026-09-24 postponed from 2026-09-30 to 2029-03-31.

How Should CMIT/MIT Be Handled and Stored Safely?#

The 3:1 reaction mass carries the harmonised classification Danger with H330, H310, H301, H314, H318, H317, H400 and H410 (CLP Annex VI, index 613-167-00-5), under pictograms GHS06, GHS05 and GHS09. MIT alone (index 613-326-00-9) differs only by H311 in place of H310, while CMIT alone has no Annex VI entry shown by PubChem; its notified classification lists H314 (98.3 % of notifications), H317 (98.4 %) and H400 (94.5 %).

A diluted product is classified by its concentration, so labelling, protective equipment, storage and spill response follow the SDS of the delivered grade. No OSHA PEL, NIOSH REL or IDLH exists, and CMIT/MIT is not listed under EPA RMP or OSHA PSM.

The table below summarizes the hazard and handling data for CMIT/MIT.

What does an isothiazolinone safety data sheet (SDS) cover?#

An isothiazolinone SDS (the older MSDS) is the document that gives the classification, handling, exposure and transport data for one product at one a.i. strength. The 4 checks below apply to the delivered grade.

1. Confirm the a.i. %, the CMIT and MIT split (1.11 % and 0.39 % in a 1.5 % grade) and the EPA Reg. No. against the label.
2. Compare the hazard section with the harmonised classification of the 3:1 reaction mass.
3. Read section 14 for the transport entry of that formulation.
4. Check that sulfides, amines, thiols, metals and high pH appear as incompatible.

What Are the Alternatives to Isothiazolinone?#

The 3 main alternatives to CMIT/MIT are DBNPA, glutaraldehyde and bronopol, non-oxidizing biocides that differ most in kill speed and pH stability. The table below compares them by identity, cooling dose, stability and status.

Isothiazolinone vs DBNPA#

DBNPA acts fastest of the non-oxidizers, 60 minutes of contact against 4 hours for CMIT/MIT in Dober's programs, but it hydrolyzes within hours at neutral to alkaline pH (half-life 63 h at pH 7, 73 min at pH 9). That speed suits remedial shots, while CMIT/MIT needs the longer contact that HSE HSG274 (2024) sets at typically 4 hours or more. In the EU, DBNPA holds a PT11 approval only for short-term preservation of up to 14 days from 2027-07-01. Hydrolysis and RO use of the fast-acting biocide are covered under DBNPA.

Isothiazolinone vs glutaraldehyde#

Glutaraldehyde reacts with cell-wall amines rather than thiols and keeps a half-life of 63.8 days at pH 9 and 25 °C (77 °F), while CMIT/MIT lasts under 2 hours at pH 9.6 and 60 °C (140 °F). That stability gives glutaraldehyde the longer hold in alkaline water.

The AQUCAR GA 50 label sets a slug of 40 to 200 ppm as product, 20 to 100 mg/L as active for a 50 % product, more than ten times the active of a 100 ppm CMIT/MIT slug at 1.5 % (calculated). Label slugs and the Glutatect test are covered under glutaraldehyde.

Isothiazolinone Grades, Suppliers and Sourcing#

Isothiazolinone is sold as a liquid CMIT/MIT product, typically at 1.5 % a.i., under EPA registrations such as 707-134 (KATHON 886 MW 1.5 %, LANXESS). Under 40 CFR 152.15, the registration number on the drum is what makes a slimicide claim lawful in the US. Distributors and producers by region are listed under water treatment chemical suppliers.

The table below lists makers and catalog sellers of CMIT/MIT named in registrations and catalogs, alphabetically.

Listed alphabetically; no company is a partner of this site and none pays for inclusion.

Buyers should match the EPA Reg. No. or the EU BPR product authorisation to the intended label use, because the 3:1 mixture's PT11 approval excludes large open recirculating cooling systems unless risks are reduced.

How is isothiazolinone priced and quoted?#

Isothiazolinone offers compare per kg of active: the price per kg of product divided by the a.i. fraction, 0.015 for a 1.5 % grade (calculated). Two products at different strengths only compare on that basis.

The quote factors are a.i. % and CMIT:MIT ratio, the registration required (EPA Reg. No. and state, or EU BPR product authorisation), annual quantity, package (drum, IBC or bulk), delivery point and documents (SDS, label, certificate of analysis per lot). Send one specification and request quotes for water treatment chemicals on the same a.i. basis.

Where Does Isothiazolinone Sit Among Biocides for Water Treatment?#

CMIT/MIT is one of the non-oxidizing biocides, beside glutaraldehyde, DBNPA, THPS and quaternary ammonium compounds, that complement oxidizers such as chlorine and bromine in cooling and process water. Oxidizing and non-oxidizing classes are compared under biocides for water treatment. Kill mechanisms of each class are compared under non-oxidizing biocides.

Isothiazolinone FAQs#

Is isothiazolinone an oxidizing biocide?#

No: isothiazolinone is a non-oxidizing biocide that kills by reacting with cell thiols such as cysteine, not by oxidation. Growth stops within minutes and cells die within hours (Silva et al., 2020).

Is Kathon the same as isothiazolinone?#

Kathon is a LANXESS trade name family for CMIT/MIT products, such as KATHON 886 MW 1.5 % (EPA Reg. No. 707-134). The active ingredient is isothiazolinone; Kathon names the formulated product.

Is methylisothiazolinone (MIT) used alone in water treatment?#

Yes: MIT holds its own EU approvals, including PT11 to 2028-12-31 and PT12 to 2029-03-31, and review literature reports its use in RO cleaning and storage.