I can think of a couple of examples where something toxic at a high level is not necessarily toxic at a low level.  Vitamin C and A are both toxic at high levels, but who wants to argue that there is no safe level of exposure or no benefit.  Certain metals, such as selenium,  are toxic at high levels, but may be beneficial at low levels.  Isn't the concept of a low dose challenge to the immune system behind the theory of vaccines.  The linear no-threshold approach does not fit every situation.  That is not to say that every chemical or bacteria is beneficial, but neither is it true that every chemical or microbe is dangerous an any level, no matter how low.  I think this article misrepresents and overgeneralizes the work of Dr. Calabrese.   

Tom Slavin