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Economics is politics by other means

Both lead and bismuth are also an excellent radiation shield, blocking gamma radiation while simultaneously being virtually transparent to neutrons. In contrast, sodium will form the potent gamma emitter sodium-24 (halflife 15 hours) following intense neutron radiation, requiring a large radiation shield for the primary cooling loop.

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Lead and LBE coolant are more corrosive to steel than sodium, and this puts an upper limit on the velocity of coolant flow through the reactor due to safety considerations. Furthermore, the higher melting points of lead and LBE (327 °C and 123.5 °C respectively) may mean that solidification of the coolant may be a greater problem when the reactor is operated at lower temperatures. Finally, upon neutron radiation the bismuth in LBE coolant will undergo neutron capture and subsequent beta decay, forming polonium-210, a potent alpha emitter. The presence of radioactive polonium in the coolant would require special precautions during refueling of the reactor.

Although lead poisoning is one of the oldest known work and environmental hazards, the modern understanding of the small amount of lead necessary to cause harm did not come about until the latter half of the 20th century. No safe threshold for lead exposure has been discovered--that is, there is no known amount of lead that is too small to cause the body harm.

In medical usage, heavy metals are loosely defined and include all toxic metals irrespective of their atomic weight: "heavy metal poisoning" can possibly include excessive amounts of iron, manganese, aluminium, mercury, cadmium, or beryllium (the fourth lightest element) or such a semimetal as arsenic. This definition excludes bismuth, the heaviest of approximately stable elements, because of its low toxicity.

Bismuth poisoning exists and mostly affects the kidney and liver. Skin and respiratory irritation can also follow exposure to respective organs. As with lead, overexposure to bismuth can result in the formation of a black deposit on the gingiva, known as a bismuth line.

In the industry, it is considered as one of the least toxic heavy metals.

Scientific literature concurs with the idea that bismuth and its compounds are less toxic than lead or its other periodic table neighbours (antimony, polonium) and that it is not bioaccumulative. Its biological half-life for whole-body retention is 5 days but it can remain in the kidney for years in patients treated with bismuth compounds.

Bismuth's environmental impacts are not very well known. It is considered that its environmental impact is small, due in part to the low solubility of its compounds. Limited information however means that a close eye should be kept on its impact.