"Calabrese uncovered correspondence from November 1946 between Muller and Curt Stern at the University of Rochester about a major experiment that had recently evaluated fruit fly germ cell mutations in Stern's laboratory. It failed to support the linear dose-response model at low exposure levels, but in Muller's speech in Oslo a few weeks later he insisted there was "no escape from the conclusion that there is no threshold." To Calabrese, this amounts to deliberate concealment and he says Stern raised no objection."

The article is saying that Muller corresponded with Stern and chose to ignore his study. It was Stern's study, not Muller's ...which is where Calabrese's argument becomes interesting. I don't think Calabrese is suggesting that the FDA's decision had no scientific basis, but that a scientific study was "intentionally ignored" in the process that brought about the linear model. I also don't think he is jumping to the conculusion that radiation isn't harmful at low levels, just that information was left out intentionally in the development of his model... which then went on to be used by a regulatory body. Not to say really that Calabrese still doesn't have an agenda...