Methyl Isobutyl Ketone

Methyl isobutyl ketone (MIBK) is a medium-evaporation ketone solvent used in lacquers, printing inks, and surface coatings as a co-solvent for cellulose nitrate and acrylic resin systems. Its solvency and evaporation rate make it a useful modifier in automotive touch-up paints alongside faster-evaporating solvents like MEK. ACGIH lowered its TLV from 50 ppm to 20 ppm in consideration of limited reproductive toxicity data in animal studies.

MIBK is California Prop 65 listed under its own CAS number (108-10-1), both as a carcinogen and as a developmental toxin. This file previously stated it was "not classified as carcinogen or Prop 65 listed" and that "the Prop 65 status has not been triggered as of 2026." Both statements were incorrect: MIBK's CAS number matches the OEHHA list directly, on two separate listing entries. This was found and corrected during a catalog-wide Prop 65 citation audit, while investigating the related but chemically distinct 4-methylpentan-2-ol (MIBC, CAS 108-11-2), whose own file had wrongly borrowed MIBK's Prop 65 status. The two substances need to be tracked separately, and each is now flagged with the status that actually belongs to it.

Regulatory status

• California Prop 65 listed for cancer: "Methyl isobutyl ketone," listed 2011-11-04 (mechanism: Labor Code). Also listed for developmental toxicity: "Methyl isobutyl ketone (MIBK)," listed 2014-03-28 (mechanism: Authoritative Body). Both confirmed directly on the OEHHA list by CAS 108-10-1.

Sources

• OEHHA Proposition 65 list cross-check: data/prop65/oehha-list.json (986 entries, published 2025-12-05), direct CAS match on 108-10-1, two entries (cancer 2011-11-04; developmental 2014-03-28).
• Blast-radius check: all 4 products currently referencing this ingredient file already resolve prop65_warning: true, sourced independently from their own SDS §15 disclosures of Methyl Isobutyl Ketone / CAS 108-10-1: content/products/mouse-milk/mouse-milk-penetrating-oil/index.md, content/products/bondo/bondo-scratch-rock-chip-repair-kit/index.md, content/products/gasoila/gasoila-free-all-rust-eater/index.md, and content/products/dupli-color/dupli-color-scratch-fix-all-in-1-universal-black/index.md. None of their published values change from this correction; each already overrode this file's previously-wrong false flag with real SDS evidence. Correcting the file removes a dormant gap where the automated Prop 65 chemistry arm (computeProp65Raise()) would silently fail to flag MIBK-containing products that do not happen to have an agent manually citing SDS §15 text.