Aside from even major disasters, the risk of dirty bombs used in terrorist attacks is a real one, despite one not having been used yet. One, relatively small, dirty bomb could easily blanket a small town, or have its effects spread across an entire city.
Luckily, for most, it's easy to know when radiation levels reach unsafe amounts through the use of Geiger counters, though they're not always convenient to carry around.
There are products such as the Nuke Alert keychain which are quite nifty, but often outside of the price range for most people (still a wise investment, however, if you can afford the oulay). These will sit idly on your keys but start chirping at you when radiation levels begin to rise.
As a more cost-efficient alternative, however, you can get yourself a nifty little Radiation Alert Dosimeter Sticker to keep on your person. It sticks easily to the inside of your wallet or phone case or car for you to be able to quickly read.
If you live in Australia, a great place to pick these up is Survival Gear Australia, a company run out of Melbourne. At only $15 a piece, it's hard to justify not having one on you. It won't chirp at you like the keychain, but if you're smart about where you put it, you'll get to regularly see it and see what's happening around you.
The cool thing about these is that the process is entirely chemical, so they require no batteries at all, though each one does only have a service life of around 2 years, depending on your climate (though that equates to roughly 2c per day for peace of mind)
They are very easy to read, and with a quick glance you can know what sort of radiation levels you're dealing with:
- 5 or 10 RAD Start GETTING AWAY FROM WHERE YOU ARE.
- 25 to 50 Rad go See your Doctor Soon.
- 100 Rad or More, Get to a Hospital that is treating Radiation Patients.
So I can't recommend highly enough that you go and grab some of these. Put them everywhere, and remember, be prepared, not scared.