Hot and fast or cold and slow
Effects on biological tissue from radioactive materials are not simple to evaluate for a lot of reasons. The rules and guidelines on exposure and intake currently in place are the result of a lot of empirical data and lab testing plus a fudge factor on top i.e. the maximum exposure permitted might be 10% or less of a dosage that produces any noticeable effect at all in trials. There's also the As Low As Reasonably Achievable (ALARA) limits which presuppose people shouldn't be exposed to any amounts of man-made radioactive materials at all. Radiation from natural (organic, free-range, cruelty-free, vegan-friendly) isotopes such as potassium-40, carbon-14 and others are tacitly ignored in these limit settings since there's nothing anyone can do to avoid them.
Radioactive isotopes with a short half-life are very "hot" so a small amount will do a lot of damage but it stops being a problem after a few hours, days or weeks. I-131 is one example of a problematic isotope in an accident, with a half-life of 8.5 days so it's "hot" plus it concentrates in the human thyroid gland like regular iodine. That fast decay rate means that a few weeks after any release it stops being a problem.
Very-long lived radioactive materials like U-235 (half-life 700 million years) and U-238 (4 billion years) aren't particularly dangerous in terms of the radiation they produce. They'll be around for a long time but they're not "hot" even in large quantities. It's the medium-life isotopes that escape an accident site that are really a problem -- Cs-137 and Sr-90 are the usual suspects here with half-lives of about 30 years.