[gozocabs]

Kawah-Putih crater lake is said to have high sulphur content.

[talaniman]

Not for the tourists as far as I know, but for scientists definitely. They sell insurance on the tours, if that makes you feel better.

https://www.livescience.com/54012-th...acid-lake.html

Though both car batteries and Kawah Ijen's lake are primarily composed of the same sulfuric acid, Kawah Ijen's sulfur is tapped naturally from its volcanic host. This sulfur is sourced from a magma reservoir somewhere beneath the 650-foot-deep, opaque aquamarine waters. The magma is continuously "exsolving," or venting, a brew of carbon dioxide, sulfur gases and other volatiles. The sulfur gases are mostly "scrubbed" by the circulating water of the hydrothermal system, which is linked to the lake above. There, bubbles of sulfur gas combine with hydrogen ions to produce sulfuric acid. [

Stranger than Fiction: Volcanic Eruption Creates Deadly Acid Lake

]

I would surely get details from the people who run those tour guides. Probably not the best vacation spot for those with respiratory conditions.