It's not clear where the heavy metals are coming from. One possibility mentioned in the article is the soil itself. I wonder if even potting soil is tested for heavy metals? If heavy metals get through whatever food testing is done by the FDA, McCormick, etc., I don't have great confidence that soil testing is any better. That said, I would think growing your own is safer.
> If heavy metals get through whatever food testing is done by the FDA
About that...
> In addition, the limited testing the FDA has done on spices has been focused on harmful bacteria, such as salmonella, not heavy metals, Ronholm says.
The reason for these is probabaly the extreme depths from which water is pumped. The normal shallow ground waters arent this bad with arsenic I would assume.
I wonder if hydroponically grown herbs would lack them. Could be a business opportunity, if people care enough (or the government steps in to regulate).
To be really certain you'd have to test your soil.
I don't think you could test seeds without destroying them. But if your soil was clean then you could grow a plant from potentially contaminated seeds and that plants seeds would be far less contaminated.
Probably the only reasonable way of guaranteeing clean plant food sources is to use hydroponics, and to manufacture fertilizer yourself. Distilling water is easy, and buying the chemicals for fertilizer seems doable. But you'll want to make your own hdpe containers, and silicone tubing, so you don't get unlisted phthalates or plasticizer. GRAS and "food safe" classifications are fine, for things that are actually looked for, but mistakes are made, and most failures seem to favor producers, not consumers.
The question boils down to the actual harms you're mitigating and what constitutes the appropriate response.