> This means even tens of kilometers away, the radiation can actually be higher than near the disaster. (Because, huh?, 137Cs sticks to the sand grains).
Radiation is a funky and tricky topic and frequently results in confusing cases like this. Let me try to explain (trying to keep things simple, so there's more nuance than what I'm writing).
Bq is a measurement of activity. That means the number of decays per second. That actually doesn't tell you the strength of that decay, the particle type, or the effective dosage one would receive. BUT it is still an incredibly important number. It will give you an indication of the half-life of the material and how much there is. A high Bq suggests low half life while a low Bq means a long half-life. OR a high Bq can tell you that there's A LOT of a material. This should make logical sense because Bq is telling you how fast it is decaying, and the more of a material there is the longer it takes to decay. Frequently if a material is known people will refer to its Bq as a way to tell someone else how much of the material there is. (yes, this can get confusing and there are two things it can be used to tell you. You have to pay careful attention. Context matters. In our case of the link you provided it tells you how much of the material there is.)
You'll also see Sieverts[0] listed commonly. This is a SI unit that tells you the effective dosage (REM is the equivalent). That means it considers the weighting that happens because of different particle types as well as where it impacts the body (you're in more danger if you swallow radioactive material than if you touch it with your hand). Which is important for maps like this one [1]. This is easier to understand.
On the other hand, there is grays (rads) is pretty similar to Sv but does not include particle weighting. In fact Sv is derived from Grays. They call this absorbed dosage.
So knowing this you should be able to conclude that the most dangerous material is something that has a high activity and has a high particle weighting.
For some comparisons, 137Cs has a half life of 30 years and decays with a beta- of 0.5Mev and a gamma at 0.7MeV. 235U on the other hand has a half life of 730Myr and decays an alpha particle at 4.8MeV.
[0] https://en.wikipedia.org/wiki/Sievert (There's a break down and link to the other units discussed here at the bottom).
[1] https://jciv.iidj.net/map/