That brass mouthpiece on your favorite trumpet, saxophone, or French horn could contain lead, a toxic metal that can cause neurological damage and developmental issues.

An Oakland-based environmental health nonprofit last year ordered brass mouthpieces and had them tested for lead. Thirteen had concentrations of the toxic metal at levels above what California deems safe, the Center for Environmental Health said Jan. 23.

The nonprofit, which has filed hundreds of lawsuits over toxic substances...