[satori99]

A quote from the article in my earlier comment;

    Some of the debris buried beneath the dome includes
    plutonium-239, a fissile isotope used in nuclear
    warheads which is one of the most toxic substances
    on earth. It has a radioactive half-life of 24,100
    years.

[kenned3]

By your logic, Bismuth-209 must be the most dangerous element on earth, with a half-life of 19 quintillion years.

Elements with longer half lives tend to be more 'stable'...