Thanks @Scott Buckingham
With regard to "...the dose makes the poison..." there's also the mode of exposure combined with whether there's an acute toxicity above a certain single dose or if accumulation is occurring, as the organism's excretion pathways may mitigate trace level of an exogenous chemical or its metabolite(s).
Additionally, as there's the lag phase between a chemical being introduced to the environment that's then found to pose an unintended issue, the amount of any persistent chemical is inevitably diffused to all of its possible endpoints-the bioaccumulation and biomagnification pathways playing a significant part.