Balls hold heat very well as long as you have a good lightbulb. The bulbs (I have 2) that come with the product are great but not very durable. 15W bulbs do a good job of heating the balls, but I've found 25W or more to work better. My husband has a lot of breathing problems and the salt stones helped him a lot. This is a bonus as you can pick them up and throw them at the sore spots. I woke up one night with a spasm so bad I thought I had to go to the hospital and my husband rolled one of the balls over it and it went away in a few minutes.
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