Those are pretty far down on the likelihood scale, though we have had instances of that, especially near river dikes in the last couple of years. We do not have a lot of dams here.

The more likely cause for such an alarm is chemical pollution from either an accidental release or a fire. Their old use was for air raids but those are even more unlikely (at the moment, of course this can change).

The actual instruction from the government is:

* Go inside

* Close doors and windows

* Listen to the emergency channel

* Know when it is safe to go outside again

Sirens are sunsetting, but still operational until 2020.