It would be interesting to see the results from NSF and ROHS-approved home machines from manufacturers like Nuova Simonelli, La Marzocco, and Astra.
I also wonder how effective water scale deposits on the boiler are at mitigating lead - doing tests on new-ish machines without much scale buildup could be a factor in these results.
Stainless is becoming more of a popular alternative to brass and copper, but isn't as effective of a thermal conductor and is not as malleable. For steam boilers and heat exchanger systems, being able to expand and deform under high pressure is a desirable feature.
