Sony RNPK-4 Help Manual page 41

Check the condition of your skin about once an hour. If you notice any abnormalities, stop using the unit immediately.
If the unit feels too hot, stop using it immediately.
Do not use the unit when sleeping.
Do not use the unit in bedding or near heaters such as stoves.
Pay special attention when someone with poor circulation or impaired skin sensation uses the unit.
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