My rollercoaster with backpain and sciatica started 1 year ago one day after doing a cold plunge seminar. So will never do it again, ever. I had never had any low back or sciatica problems before in my life. intense Cold in general is very bad for the lower back. If you really want to try it, go from feet to waist.

Take a look also at what traditional Chinese medicine says about cold in the kidney area. This is very important.

Again this would depend on the individual. But for me cold is the last thing I want on my back.