Generally speaking, hot tub rash, or Folliculitis, is due to improper sanitizing of your spa water. The cause of the rash is a bacterium, Pseudomonas aeruginosa, that gets in your skin pores causing inflammation (or a rash). Pseudomonas aeruginosa is actually rather common and found in water, soil, etc. However, if it’s allowed to concentrate in spa water due to insufficient sanitizer, you can wind up with hot tub rash.