Besides, the real danger is the isotopes that are used by the body or mimic things used by the body.
Yeah, most of which are short-lived (eg Iodine-131), or are poorly taken up even when ingested (Strontium-90 and Caesium-137), or have such huge half-lives (ie low activity) that you would need to eat a crap-ton of them to suffer any ill effect.
Take iodine tablets in the immediate aftermath, and avoid eating locally grown produce, and there's not really a problem at all. A dirty bomb in a city - where little food is grown - is a fairly minor problem, as long as people are sensible and don't lose their minds over tiny risks.
So it's a really major problem.