Pure MDI (4,4'-Methylene Bisphenyl Diisocyanate)CAS 101-68-8
The 'A-side' monomer at the heart of spray foam and most polyurethane chemistry.
Toxic Chemical Databases
Substance-specific listings — these flags are for Pure MDI alone, not for the broader family.
Chemical Type
Aromatic Diisocyanate — Monomeric
Chemical Description
Pure 4,4'-MDI is the standard reference monomer used to characterize MDI toxicology. It is less volatile than TDI but a confirmed respiratory sensitizer. IARC places it in Group 3 (not classifiable as to human carcinogenicity) — a meaningfully weaker carcinogen signal than TDI. Once cured, the polyurethane polymer is stable; the hazard window is the aerosolized spray during installation and any residual unreacted monomer left by off-ratio or under-cured batches.
Biological Activity
Severity scores specific to this substance, NOT the parent family average. Differences between siblings are real and meaningful.
Top Health Consequences
- Respiratory sensitization with occupational asthma the leading endpoint
- Hypersensitivity pneumonitis with chronic low-level exposure
- Allergic contact dermatitis on skin contact
- IARC Group 3 — weaker cancer signal than TDI
Pathways of Exposure
- Inhalation of aerosolized A-side during spray foam application
- Off-gassing of residual monomer from off-ratio or under-cured foam
- Re-aerosolization during demolition, sanding, or thermal degradation
Building Materials with Pure MDI
Listed alphabetically — product categories where this specific substance appears.
Engineered Wood Adhesives (Pure-MDI Bonded)
Spray Polyurethane Foam Insulation
Two-Part Industrial Coatings