structural:scholar_edition, structural:reformatting, stylistic:style_transfer
Biological role and precautions

The early lanthanides have been found to be essential to some methanotrophic bacteria living in volcanic mudpots, such as Methylacidiphilum fumariolicum: lanthanum, cerium, praseodymium, and neodymium are about equally effective. Cerium is otherwise not known to have biological role in any other organisms, but is not very toxic either; it does not accumulate in the food chain to any appreciable extent. Because it often occurs together with calcium in phosphate minerals, and bones are primarily calcium phosphate, cerium can accumulate in bones in small amounts that are not considered dangerous.

Cerium nitrate is an effective topical antimicrobial treatment for third-degree burns, although large doses can lead to cerium poisoning and methemoglobinemia. The early lanthanides act as essential cofactors for the methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV, for which lanthanum, cerium, praseodymium, and neodymium alone are about equally effective.

Like all rare-earth metals, cerium is of low to moderate toxicity. A strong reducing agent, it ignites spontaneously in air at 65 to 80 °C. Fumes from cerium fires are toxic. Cerium reacts with water to produce hydrogen gas, and thus cerium fires can only be effectively extinguished using class D dry powder extinguishing media. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Cerium is not toxic when eaten, but animals injected with large doses of cerium have died due to cardiovascular collapse. Cerium is more dangerous to aquatic organisms because it damages cell membranes; it is not very soluble in water and can cause environmental contamination.

Cerium oxide, the most prevalent cerium compound in industrial applications, is not regulated in the United States by the Occupational Safety and Health Administration (OSHA) as a hazardous substance. In Russia, its occupational exposure limit is 5 mg/m. Elemental cerium has no established occupational or permissible exposure limits by the by the OSHA or American Conference of Governmental Industrial Hygienists, though it is classified as a flammable solid and regulated as such under the Globally Harmonized System of Classification and Labelling of Chemicals. Toxicological reports on cerium compounds have noted their cytotoxicity and contributions to pulmonary interstitial fibrosis in workers.