Except that chemophobia isn’t irrational just because people use the wrong words. It’s irrational because there is no fundamental, meaningful definition of “synthetic” in this context. What’s synthetic? Is refined sugar, even if it’s chemically identical to unrefined sugar (minus impurities)? Is chemically identical lab-grown insulin?

Furthermore, even if we could draw a precise demarcation that would make sense, it would still be irrelevant: things can be beneficial and harmful regardless of whether they occur naturally or are synthetic. This is known as the naturalistic fallacy. In a similar vein, people tend to overstate the importance of coevolution for biological tolerance. Yes, it has its role in assessing safety but it’s not the ultimate argument that people make it out to be.

> So, the OP was quite reasonably saying that chemicals not naturally present at certain concentrations should be tested for safety when high concentrations are proposed to be used.

Sure but nobody is disputing this in the first place, so making such a statement is at best irrelevant and at worst a bad argument designed to derail a discussion.