On the internet, nobody mentions about lead's oxidation state when talking about lead poisoning. I assume it's Pb(II) and Pb(IV), as chelation therapy is suggested as a treatment. Also, its accumulation in bones and teeth suggest its ionic state. But what about elemental lead? How is it transformed in human body? Is it as toxic as Pb(II) and Pb(IV) compounds?

• In order for an element to be toxic, it has to be soluble in aqueous solution. If it is not soluble, it gets out of the body like sand or clay. Metallic lead is not soluble into water until it is oxidized into $\ce{Pb^{2+}}$. The main compounds of Pb(IV) are $\ce{PbO2}$ and $\ce{Pb3O4}$ which are insoluble and $\ce{PbCl4}$ which is decomposed by water. So $\ce{Pb(IV)}$ is probably not directly toxic. As a consequence, only $\ce{Pb(II)}$ is toxic – Maurice Sep 10 '20 at 11:07
• Here is a review of lead toxicity. I highly recommend to give it a thorough read. – Nilay Ghosh Sep 10 '20 at 11:11

Then there is a wholly different story of the covalent compounds of Pb. They are more toxic than ionic Pb(II), and some are capable of penetrating through the skin. In particular, the notorious $$\ce{Pb(C2H5)4}$$ has caused our civilization a great deal of harm.