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Brass contains lead as part of the alloy. Stainless steel is about the only food-safe nozzle that's manufactured for 3D printers.
Lead has a lower melting point than the other components of the alloy, so it tends to end up on the outermost layers of the boundaries of the grains of the material and the machining process further exacerbates this by smearing the lead as it's machined. This leads to more lead on the outside of the machined surfaces, increasing it's ability to leech out.
And seeing as how most every nozzle manufactured comes out of China, the amount of lead in their alloys is probably above the normal 2% or so.