Most people have a slight hyperacidity in their body tissue. Harmful substances can weaken your immune system, trigger pains in your joints or muscles, give you a headache, lead to problems with your digestion, or cause any one of a number of chronic illnesses. Using the sauna regularly helps to wash out harmful acids. “Sweating out” poisonous trace elements such as lead, cadmium and nickel in the sauna is a positive side effect.