OEHHA Moves to List Four New Carcinogens Under Prop 65

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On May 8, 2026, the California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment (OEHHA) announced its intent to list four substances as known to the state to cause cancer under Proposition 65:

  • Welding fumes
  • Hydrochlorothiazide
  • Voriconazole
  • Tacrolimus

All four listings are being proposed pursuant the labor code mechanism.  They follow International Agency for Research on Cancer (IARC) determinations that those substances are Group 1 carcinogens— meaning there is sufficient evidence of carcinogenicity in humans.

Comments on the proposed listing are due June 8, 2026.  OEHHA has opened two separate comment portals: one for welding fumes and one for the other three substances.

According to IARC, exposure to hydrochlorothiazide, voriconazole, and tacrolimus occurs primarily through their use as medications.  Hydrochlorothiazide is a diuretic prescribed for hypertension and edema; voriconazole is an antifungal used to treat or prevent invasive aspergillosis and other serious fungal infections; and tacrolimus is an immunosuppressant.

Should any of these listings be finalized, businesses that have significant exposures to the listed chemicals will have one year before Prop 65 warning requirements take effect.