Actually it does have an effect on humans. There was a comment on the WP article about that.
What I would guess that the 2001 article was saying is that if it explodes high in the atmosphere, it dissipates and it doesn't have a large impact, because it is distributed over a very large area, so the concentration would be small.
But if it explodes close to the ground, it impacts a smaller area, but could impact it more severely, with higher radiation levels.
This is a guess on my part, but I think it makes sense.
"And how small is small?"
From a link from Davea's posted link:
http://www.rti.chuvashia.com/alazan5.htm It gives the dimensions:
1.4 m (about 4.5 feet long)
Diameter: 96 mm (about 4 inches)
8.8 kg (about 20 lbs)
Range: 1.2 km -- about 3/4 mile
It doesn't say the weight of the payload, i.e. radioactive material in this case, that it can carry.
>>>>Why would the radiation have no effect if exploded in the air, but consequences similar to a small nuclear bomb if on the ground?
Dirty bombs have three components to worry about:
blast
fragmentation
latent ionizing radiation.
The blast will be effective in a very, very short area around the explosion. Explosions on TV and movies use zip-lock bags filled with gasoline and something called detonation cord (det cord is about twice the size of plastic coated clothes line). They put lots of these little baggies of gas wrapped twice in det cord in a long string everywhere. On cue from the director they touch off the det cord exploding it in a long line at well over 20,000 feet a second vaporizing the gas then igniting it. This looks cool, but doesn't do any real work to speak of. They show it in slow motion and from various angles over and over again to milk it for full effect. A real explosion from a real bomb and the casualties from the blast will be long since finished in one second. The blast wave looks like a big ice-cream cone so it's force goes up and away, remember, explosives are generally lazy and will always take the path of least resistance which is up and away from you.
Fragments are parts of the bomb, what it was packed in, and anything laying loose around it. These fragments (called shrapnel incorrectly by most people) will loose speed and power at an amazing rate. Just because you're near it doesn't mean you're going to be hurt. Consider the bombers in Israel blowing themselves up with large charges and lots of intentional fragments in crowded places. The vast majority of people there aren't hurt. 99% of the whole fragment danger period will last one or two seconds literally.
Now we're down to the "Dirty," part. The anthrax in the Hart building in Washington, DC was passive; meaning the decontamination crews had to go find it. The good news for us is the contamination will lead cleanup crews (using a Geiger counter) to itself like a beacon. The government will probably be passing out badges like the ones the girl at the dentist's office wears. They signal when you've had a minor exposure to ionizing radiation (did you know you get exposed to this stuff naturally every day?). If your badge changes color, you've had a minor exposure that will have no discernible effect on you. The clean up crews will check the places where the most badges change the fastest after an initial clean up period. If you're exposed during the initial attack a warm shower and washing your clothes will erase the danger. You need long term exposure to a lot of this stuff to have it harm you twenty years from now. Even if you're near a dirty bomb you are still more likely to be killed by a drunk driver.
Dirty bombs are "The," non-event of the year. It's a terror weapon, designed to let everyone's imagination scare the bejeepers out of them with their own worst fears. When we get scared, when we make new laws, when we alter our lives; the bad people win. Right now those boys are giggling and laughing at how we're reacting to their threats. If you want to help in the war on terror, only worry about things that have happened in your immediate area. If you can throw a rock and have it hit the danger, then be concerned (leave). Don't give them the satisfaction of being scared until we've got something right here-right now to be scared of.
http://goexcelglobal.com/NJ_DefenseForce