Saunas from China have been know to be high in EMF. EMF comes from electronic devices, microwaves, phones, powerlines, x-rays. Some of the heaters in the poor quality saunas produce very high levels of EMF. Side Effects of EMF include headaches, tremors, dizziness, memory loss, loss of concentration, and sleep disturbances
Source: https://www.healthline.com/health/emf#levels