Every year millions of people visit a Japanese onsen without any noticeable health affects. In fact, most onsen regulars will tell you that onsen are healthy for body, mind and soul. That being said, there are some risks.
Commonly cited onsen dangers include:
1. Heat OverexposureMany onsen operators warn of the dangers of dehydration and heat exhaustion from staying in onsen too long.
It's not recommended to consume alcohol in onsen or to enter when you're intoxicated.
It's also recommended to limit your soaking time in onsen to a few minutes. You may not notice you're dizzy until you stand up.
2. SlippingOnsen are often made of rock and wood (that become slippery). It's easy enough to slip and fall (especially if you're a little dizzy from a hot soak).
3. Infectious DiseasesAny infectious disease that can be transmitted via hot water. This includes legionella bacteria and naegleria.
Many onsen aren't chlorinated. Onsen is meant to be as pure and natural as possible. Many onsen resorts replace water regularly.
You shouldn't visit onsen if you have a communicable disease or open cuts or sores on your body. Visitors to onsen must clean with soap and water before entering the bath.
4. AvalancheAvalanche is a danger in snowy mountainous areas. Several onsen resorts have been hit by deadly avalanches over the years.
5. Too HotOnsen water may come out of the ground at a very high temperature that's dangerous. Burns do happen. Always check the temperature of a natural onsen before jumping in.
Well managed onsen resorts will cool the water before adding it to the bath. However, it's still a good idea to check. Many resorts rely on ancient equipment to manage temperatures.
6. High Blood PressureIf you have a medical condition it's important to ask your doctor before soaking in hot water. For example, hot baths can increase your blood pressure.