Caesium-137 has a relatively low boiling point of 671 °C (1,240 °F) and easily becomes volatile when released suddenly at high temperature, as in the case of the Chernobyl nuclear accident and with atomic explosions, and can travel very long distances in the air.

After being deposited onto the soil as radioactive fallout, it moves and spreads easily in the environment because of the high water solubility of caesium's most common chemical compounds, which are salts.

[12] Caesium-137 is not widely used for industrial radiography because it is hard to obtain a very high specific activity material with a well defined (and small) shape as caesium from used nuclear fuel contains stable caesium-133 and also long-lived caesium-135.

The latter has been used in demonstration of chemically stable water-insoluble forms of nuclear waste for disposal in deep geological repositories.

As an almost purely synthetic isotope, caesium-137 has been used to date wine and detect counterfeits[13] and as a relative-dating material for assessing the age of sedimentation occurring after 1945.

[14] Caesium-137 is also used as a radioactive tracer in geologic research to measure soil erosion and deposition; its affinity for fine sediments is useful in this application.

[21][22][23] In 2003, in autopsies performed on 6 children who died in the polluted area near Chernobyl (of reasons not directly linked to the Chernobyl disaster; mostly sepsis), where they also reported a higher incidence of pancreatic tumors, Bandazhevsky found a concentration of 137Cs 3.9 times higher than in their livers (1359 vs 347 Bq/kg, equivalent to 36 and 9.3 nCi/kg in these organs, 600 Bq/kg = 16 nCi/kg in the body according to measurements), thus demonstrating that pancreatic tissue is a strong accumulator and secretor in the intestine of radioactive cesium.

[24] Accidental ingestion of caesium-137 can be treated with Prussian blue (FeIII4[FeII(CN)6]3), which binds to it chemically and reduces the biological half-life to 30 days.

[26] By observing the characteristic gamma rays emitted by this isotope, one can determine whether the contents of a given sealed container were made before or after the first atomic bomb explosion (Trinity test, 16 July 1945), which spread some of it into the atmosphere, quickly distributing trace amounts of it around the globe.

[33] A 2013 paper in Scientific Reports found that for a forest site 50 km from the stricken plant, 137Cs concentrations were high in leaf litter, fungi and detritivores, but low in herbivores.

The caesium precipitated with ferric ferrocyanide (Prussian blue) would be the only waste requiring special burial sites.

It is believed that the capsule, originally a part of a measurement device, was lost in the late 1970s and ended up mixed with gravel used to construct the building in 1980.

By the time the capsule was discovered, 6 residents of the building had died, 4 from leukemia and 17 more receiving varying doses of radiation.

[40] The 1994 Tammiku incident involved the theft of radioactive material from a nuclear waste storage facility in Männiku, Saku Parish, Harju County, Estonia.

One of the brothers received a 4,000 rad whole-body dose from a caesium-137 source that had been released from a damaged container, succumbing to radiation poisoning 12 days later.

They were eventually traced back to training sources left abandoned, forgotten, and unlabeled after the dissolution of the Soviet Union.

One was a caesium-137 pellet in a pocket of a shared jacket that released about 130,000 times the level of background radiation at 1 meter distance.

An investigation by the agency traced the source to a building from which STUK and a radioactive waste treatment company operate.

[48] Public health authorities in Western Australia issued an emergency alert for a stretch of road measuring about 1,400 km after a capsule containing caesium-137 was lost in transport on 25 January 2023.

According to volunteers of the dosimetric control group, the dosimeter at the NP site showed up to 800 microsieverts, which is 1600 times the safe value.

Employees of the Ministry of Emergency Situations fenced off the area of 30 by 30 meters (98 by 98 ft), where they found a capsule with caesium from a defectoscope.