I looked at them pretty close and I cant see how rain could make it in their in enough quantity to fire the mechinism. You have to remember the device is in front of you. The firing mechinism is located up inside. It would take a alot of rain and an extended time frame to get water to that point and keep it their. Could rain set it off of course it is water the likely hood near zero. If you subject the coat to situations that are very wet. You need to check the firing mech. more often. Better yet take the cap to make your auto inflate vest a manual one and put it on. Just hope you are concious to pull the rip cord. I like the idea of these units because it is something I will actually wear as apposed to a bulky life jacket.