Is onsen safe for breastfeeding mums?

I came across this post where the father blogged that: "The warm, therapeutic waters of an onsen (温泉、hot springs) , and even your own bath at home to be honest, are known to stimulate milk production. My wife claims that her best milk producing days ever (in terms of quantity) were in the few days we stayed at Beppu Onsen (別府温泉) six weeks after our son was born. " Sounds like onsen helped his wife with milk production. You can read more here: http://www.stippy.com/japan-life/first-time-gaijin-dad-3/