What is 2-Mercaptobenzothiazole?
2-Mercaptobenzothiazole (abbreviation: MBT; trade name: Captax) is a sulfur-containing heterocyclic compound used as a rubber vulcanization accelerator and as an industrial biocide. In rubber manufacturing, it facilitates cross-linking of rubber polymers during the vulcanization process. In industrial applications, it acts as a corrosion inhibitor and biocide in metalworking fluids, cooling systems, and some adhesive formulations.
MBT belongs to the benzothiazole chemical class, which is distinct from the thiuram and carbamate classes. It is a standard component of shoe-material contact dermatitis patch test panels and is included in the ACDS and European Standard Series for rubber chemical allergy screening.
The compound gained particular clinical significance through its association with shoe contact dermatitis — a condition where rubber chemicals in shoe materials (outsoles, insoles, adhesives used in shoe construction) leach through to contact foot skin, particularly under sweaty, occluded conditions.
Why does 2-Mercaptobenzothiazole cause reactions?
MBT sensitizes through Type IV delayed hypersensitivity. Its thiol (-SH) functional group is chemically reactive and can form disulfide bonds with cysteine residues in skin proteins, creating hapten-protein conjugates that the immune system recognizes and responds to.
Key clinical aspects:
- Shoe contact pattern: MBT causes a characteristic bilateral symmetrical foot dermatitis that follows shoe coverage — dorsum of feet, toe spaces, and sometimes the plantar surface
- Summer exacerbation: Hot weather increases foot sweating, which accelerates MBT leaching from shoe rubber, leading to seasonal worsening
- Occupational exposure: Metal workers and automotive industry workers may be exposed to MBT through industrial cooling fluids — a distinct exposure route from shoe contact
- Cross-reactivity: MBT may cross-react with other benzothiazole derivatives and with dibenzothiazyl disulfide (MBTS), another rubber accelerator formed by oxidation of MBT
Where is 2-Mercaptobenzothiazole found?
In consumer products:
- Rubber-soled shoes: The most important source; present in both outsoles and insoles
- Athletic footwear: Running shoes, sports shoes with significant rubber components
- Some adhesives: Rubber-based adhesives used in shoe construction
In industrial/occupational contexts:
- Metalworking coolant fluids: Corrosion inhibitor in water-based cutting fluids
- Cooling tower water treatment: Biocide component
- Rubber industry manufacturing: Occupational exposure for production workers
How to identify MBT exposure
Since MBT is not labeled on consumer products:
- Note the bilateral, symmetrical pattern of foot dermatitis corresponding to shoe coverage
- Observe improvement when wearing open sandals or going barefoot
- Patch test at a contact dermatitis clinic using the MBT test concentration and, if possible, pieces of the actual shoe material
In Indian products 🇮🇳
Rubber-soled footwear is nearly universal in India across all demographics — from inexpensive rubber chappals to athletic shoes to synthetic leather shoes with rubber outsoles. MBT-containing rubber is used in Indian-manufactured footwear as well as imported athletic footwear. The hot and humid Indian climate, combined with the common practice of wearing enclosed rubber-soled shoes for long periods, creates conditions particularly conducive to MBT leaching and skin contact.
Indian dermatologists managing persistent bilateral foot dermatitis — especially in patients who do not respond to antifungal treatment (ruling out tinea pedis) — should consider shoe contact dermatitis and MBT as potential causes. Patch testing with MBT is available at specialized contact dermatitis clinics in major Indian cities.
Safer alternatives
- Open leather sandals: Reduce shoe-to-skin rubber contact dramatically
- Cotton socks: Create a barrier between shoe rubber and skin
- MBT-free footwear: Shoes with non-rubber soles (natural leather or polyurethane) or those specifically certified rubber-accelerator-free
- Sandals over closed shoes: For sensitized individuals, switching to open sandals during warm months addresses seasonal exacerbation