The tsunami and nuclear disaster in Fukushima, Japan, now threatens the developing brains of children in Cambodia — but not for reasons that were ever expected.

Cambodia has long struggled with iodine deficiency. The element is crucial to early brain growth: When pregnant women and their infants have low levels, the children can permanently lose 10 to 15 I.Q. points. Iodine deficiency is considered the world’s leading preventable cause of mental impairment.

Cambodia was making great progress against iodine deficiency until 2011, according to a report published in 2015 by the Iodine Global Network, a public-private partnership combating the deficiency.