California has added welding fumes, hydrochlorothiazide, voriconazole, and tacrolimus to the Proposition 65 list as chemicals known to cause cancer. While the pharmaceutical listings are important for life sciences and healthcare supply chains, the inclusion of welding fumes has broader implications for manufacturers, fabricators, construction firms, and other businesses with welding-related activities or downstream exposure scenarios.
California’s Latest Chemical Listing Changes
California’s Proposition 65 list continues to evolve, and the latest update is a reminder that regulatory obligations can be triggered not only by the chemicals intentionally placed on the market, but also by exposures generated through common industrial processes.
Effective July 17, 2026, the Office of Environmental Health Hazard Assessment (OEHHA) added the following substances to the Proposition 65 list as chemicals known to the State of California to cause cancer through the “Labor Code” listing mechanism:
- Welding fumes
- Hydrochlorothiazide
- Voriconazole
- Tacrolimus
These listings are based on evaluations by the International Agency for Research on Cancer (IARC), which classified welding fumes, hydrochlorothiazide, voriconazole, and tacrolimus as Group 1 carcinogens, meaning they are carcinogenic to humans. IARC also concluded that there is sufficient evidence of carcinogenicity in experimental animals for hydrochlorothiazide and tacrolimus. At this time, no Proposition 65 No Significant Risk Levels (NSRLs) have been listed or proposed for these substances.
Further information is available from OEHHA:
Welding Fumes Added to the Proposition 65 List for Cancer – OEHHA
New Cancer Listings and Their Business Impact
For many businesses, welding fumes are the most immediately relevant addition. Welding is widely used across construction, manufacturing, shipbuilding, automotive, aerospace, maintenance, and metal fabrication operations. The fumes generated during welding are complex mixtures of metal compounds, particulates, and gases, and their composition can vary depending on the welding process, base metals, filler materials, electrode coatings, ventilation, and surrounding conditions.
That complexity is important from a compliance perspective. Proposition 65 obligations are not limited to finished products with intentionally added ingredients. Businesses should also consider whether exposures may arise during manufacturing, installation, repair, maintenance, workplace activities, or downstream use in California.
The three pharmaceutical substances—hydrochlorothiazide, voriconazole, and tacrolimus—are also significant for companies operating in pharmaceutical, healthcare, packaging, distribution, and related supply chains. Hydrochlorothiazide is commonly used to treat hypertension and fluid retention; voriconazole is a broad-spectrum antifungal used for serious fungal infections; and tacrolimus is an immunosuppressant used in transplant medicine and certain dermatological applications. While these substances may appear sector-specific, businesses should evaluate whether they are present in products, packaging, waste streams, distribution channels, or other exposure scenarios relevant to California.
Assessing Your Chemical Compliance Obligations
Businesses that manufacture, import, distribute, sell, install, maintain, or use products or processes in California should treat this update as a prompt to reassess exposure pathways. The practical question is not simply whether one of these substances appears on a bill of materials, but whether individuals in California may be exposed above a level that requires a clear and reasonable warning.
Key actions to consider include reviewing product formulations, supplier declarations, safety data sheets, manufacturing and maintenance procedures, workplace exposure controls, and customer-facing warning strategies. For welding-related operations, companies may also need to consider whether exposures occur during fabrication, field installation, repair activities, or foreseeable downstream use.
The absence of established NSRLs increases the importance of documentation. Where a business concludes that a warning is not required, that decision should be supported by a defensible exposure assessment, clear assumptions, and appropriate technical rationale. Where warnings are needed, companies should ensure that they are timely, accurate, and aligned with Proposition 65 requirements.
How H2 Compliance Supports Proposition 65 Compliance
H2 Compliance can help businesses understand the implications of this Proposition 65 update and build a practical, risk-based response. Our services include:
- Proposition 65 applicability assessments for products, processes, and exposure scenarios
- Review of formulations, bills of materials, supplier data, and safety data sheets
- Exposure pathway analysis and support for warning determinations
- Development or review of Proposition 65 warning language and compliance documentation
- Regulatory monitoring and impact assessments for new or emerging chemical listings
For companies operating across multiple jurisdictions, this update also reinforces the value of proactive substance tracking and cross-functional communication between regulatory, EHS, legal, procurement, and operations teams.
Organizations that assess the impact early will be better positioned to manage compliance obligations, support customer inquiries, and reduce the risk of last-minute warning or documentation gaps.
Published August 5th, 2026
Article image generated with the assistance of Artificial Intelligence.