fallout-4
According to this wiki entry radiation has the following effect:
The rate is 1% of HP per 10 rads;
But what does +2 RADS (as an example, see on the screenshot) stand for? Does it mean, that you get +2 RADS every second and so you would die after (10/2)*100 sec? (Example provides that the radiation doesn't change in this time and that you are full-life in the beginning of this experiment.)
I haven't found anything about units regarding this topic.
Your settlers at home can die. Note that once a 'settler' has been assigned to a supply route, they will be a 'provisioner' the next time they are loaded.
Provisioners who are away from a settlement can't die. The can get knocked down for a few seconds like other essential NPCs, but they get right back up. Currently, the closest I've observed this is at the Rotten Landfill (east and slightly north of the drive in).
It may be that:
a) Provisioners are just essential always
b) Provisioners who are outside a certain range of a settlement are essential, but can be killed when close to it.
or c) Provisioners who are outside a certain range of their home settlement and/or destination are essential, but can be killed when close to them.
Clearly, more testing is required, but given what I know about how the essential flag works, it's a pretty safe bet that provisioners are just essential all the time, and therefore can't be killed.
The best solution I found to this is to use fast travel to travel to the nearest thing you've discovered. Or perhaps the second nearest. The point is to use it as a way to pass time, but not to make you travel far away from what you're doing.
So far for me, that makes the storm pass and no rads are absorbed.
Per your comment about being hit by these storms in gun fights, this won't help in those cases since you need to be a little ways away from hostiles in order to fast travel. But if you can break off from the fight (become hidden) or end it quickly, the fast travel will still be of use.
Also, as kalina mentioned in their answer, a radiation suit will help. There's a free Hazmat Suit with 1000 Radiation resistance in the Cambridge Polymer Lab. Here's the CPL's location:
I didn't think to take a screenshot of the Hazmat suit's specific location. The Hazmat Suit itself is on the first floor, in a room across from Ericka's lab(where you mix ingredients plus an isotope), and sits on top of a shelf in the corner. Below is where I think the hazmat suit was, but again I forgot to screenshot before picking it up.
Having such a suit will allow you to do some exploring in the open world during a rad storm, but as mentioned in the other answer it comes at the cost of your armor stats.
Best Answer
The rate is 1% of HP per 10 rads, meaning you lose 1% of your max health per 10 radiation you take. This is indicated by the red bar that grows from the right side of your health bar when you incur radiation. If your 100 health max character takes 200 radiation, your max health is now 80 until you lower your radiation.Using this formula, and confirmed by the max value on the "radiation" indicator on the right side of the Pip-Boy, 1000 radiation is lethal. 1000 radiation would reduce your max hp by 100%.
When you are receiving radiation, you'll get the indicator by your health bar. +2 rads would indicate you are taking 2 radiation per second. The time it would take to kill you while receiving x rads per second would be:
(1000 - CurrentRadiationLevel)/xseconds.