5000 atomic bombs worth of cesium-137 in spent fuel pool 4 http://majiasblog.blogspot.com/2012/05/how-much-radiation-is-at-issue-cesium.html
Perhaps up to 12,000 atomic bombs worth of cesium-137 in spent fuel pool 3 (the one that probably exploded). Even more in the common spent fuel pool.
How toxic is cesium-137?
Caesium. Wikipedia. http://en.wikipedia.org/wiki/Caesium#Health_and_safety_hazards
[excerpted] "Experiments with dogs showed that a single dose of 3.8 millicuries (140 MBq, 4.1 μg of caesium-137) per kilogram is lethal within three weeks; smaller amounts may cause infertility and cancer. The International Atomic Energy Agency and other sources have warned that radioactive materials, such as caesium-137, could be used in radiological dispersion devices, or "dirty bombs".[107 ]
105: Redman, H. C.; McClellan, R. O.; Jones, R. K.; Boecker, B. B.; Chiffelle, T. L.; Pickrell, J. A.; Rypka, E. W. (1972). "Toxicity of 137-CsCl in the Beagle. Early Biological Effects". Radiation Research 50 (3): 629–648. doi:10.2307/3573559. JSTOR 3573559. PMID 5030090.
106: "Chinese 'find' radioactive ball". BBC News. 2009-03-27. Retrieved 2010-01-25. http://news.bbc.co.uk/2/hi/asia-pacific/7967285.stm
107: Charbonneau, Louis (2003-03-12). "IAEA director warns of "dirty bomb" risk". Washington Post (Reuters): p. A15. Retrieved 2010-04-28.
MAJIA HERE: Even cesium-133, which is not regarded as toxic, has been shown to be toxic for people if too much is ingested. Furthermore, it is toxic to plants.
"Cesium-Induced Torsades de Pointes" N Engl J Med 2002; 346:383-384January 31, 2002 http://www.nejm.org/doi/full/10.1056/NEJM200201313460523
Cesium toxicity for plants: