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Plutonium is an alpha-particle producer, which means it releases alpha particles as it undergoes radioactive decay. Alpha particles are made up of two protons and two neutrons, with a positive charge. Additionally, plutonium readily oxidizes to \(\mathrm{Pu}^{4+}\) in the environment.

The layer of dead skin on our bodies is sufficient to protect us from most alpha-particle radiation. This is because alpha particles have a low penetration depth and are easily stopped by a thin layer of material, like dead skin or even paper.

The chemistry of \(\mathrm{Pu}^{4+}\), plutonium in the oxidized state, is similar to that of \(\mathrm{Fe}^{3+}\), which is an iron ion. This similarity means that plutonium ions can potentially be taken up by biological systems, which have evolved to utilize iron ions for various functions.

Despite the layer of dead skin protecting us from alpha particles, plutonium is still highly toxic because of its ability to enter our bodies through ingestion or inhalation. Once inside the body, given its chemical similarity to iron, plutonium ions are taken up by cells, where they can cause severe damage. The alpha particles emitted by plutonium can ionize cellular molecules, damaging them and potentially leading to cancer or other health issues. In conclusion, plutonium is one of the most toxic substances known because it is an alpha-particle producer that can enter the human body and cause significant cellular damage. Its chemistry similarity to iron allows it to be taken up by cells, further increasing its potential harm.