I have 26 Dwellers. I had all there health full, Rads were all the way gone. I went to bed lastnight, didnt check my Vault, woke up and checked it b4 work. When I checked it all the Dwellers had half their health bar gone and replaced with radiation poisoning. Is there a way to stop that from happening. I am blowing thru Rad-Away faster than I care to.

You need more water.

Make sure your resources are far into the green before you close the vault. You also want plenty of reserves, with the tic mark on each resource meter at about the midpoint. Build unoccupied production rooms for storage, if you have to.

Starvation causes health deterioration.

Dehydration causes radiation poisoning. Too quickly, IMHO.

Radiation poison will only take you to 50% health and is caused by lack of good water. Once you have a good water supply the poison will very slowly go away.