1. (radioactive fall-out) Radioactive particles deposited from the atmosphere either from a nuclear explosion or from a nuclear accident. Local fall-out, within 250 km of an explosion, falls within a few hours of the explosion. Tropospheric fall-out consists of fine particles deposited all round the earth in the approximate latitude of the explosion within about one week. Stratospheric fall-out may fall anywhere on earth over a period of years. The most dangerous radioactive isotopes in fall-out are the fission fragments iodine–131 and strontium–90. Both can be taken up by grazing animals and passed on to human populations in milk, milk products, and meat. Iodine–131 accumulates in the thyroid gland and strontium–90 accumulates in bones.

2. (chemical fall-out) Hazardous chemicals discharged into and subsequently released from the atmosphere, especially by factory chimneys.