How can retailers know if plant protein powders are safe from heavy metals?

A recent hullabaloo asserted that animal-sourced protein powders were cleaner than protein derived from plant sources. There's no controversy around the idea that plants uptake all manner of compounds from the soil, from nutrients to—yes—heavy metals. But just because heavy metals like lead and cadmium are detected, does that mean that they are anywhere close to being unsafe to consume? And more to the point, what can retailers do to make sure the protein powders they stock are as clean as can be? We spoke with Yolanda Fenton, product formulator for Natural Factors, for some answers.