Hmmm.... drag. This would have to do with where the crops are grown and/or where they're harvested. I know that years ago Chinese herbs had been compromised too because many of the herbs were picked near roadsides where high pollution from car exhaust was a concern. I know this was true for the herb gota kola (or maybe it was ginko) that came from India and China. Herbs are in high demand and anyone who needed money can pick these herbs and sell them to a company.
This goes back to knowing a little about the company and harvesting practices -- are crops grown in the country using organic practices, etc.?
I came across something recently and there is another problem with herbal medicines.... fungus. The worldwide prevalence of fungus growth is a concern too on herbal crops.