A spa resort near Croatia's capital Zagreb uses a derivative extracted from crude oil deposits to treat skin diseases and arthritis. The treatments in Ivanic Grad are attracting a growing number of patients from home and abroad.

The Naftalan clinic is one of only two places in the world where naphthalene oil, an organic compound extracted from crude oil, is used for treatment of hard-to-cure skin diseases and rheumatic ailments. The dark oily substance slowly seeps into a row of bathtubs as patients undress and prepare to submerge up to their shoulders.

One of them is Piotr Khlebtsov, a businessman from Moscow suffering from psoriasis, a disease that creates red flaky patches on the skin.

"It's my eighth visit here, at the Naftalan hospital. The therapy has been very helpful for me. The daily therapeutic bath lasts for 15 minutes, and after that one needs to rest in their room for about an hour, so that the body recovers," said Khlebtsov.

The Russian patient said he is happy with the results so far.

"This whole area was a red patch, which has become smaller and smaller over time, and is reduced now to just two red dots. And I'm praying to God that this is the year when it goes away completely," he said.

The Naftalan clinic opened its doors in 1989. Its managers say the idea came from Azerbaijan, where this type of medicinal oil has been in use for some time in a place also called Naftalan.

"Engineers from here, who had travelled to Azerbaijan for work, realized that the oil used there could be the same kind as the one extracted here, which was already used there to treat some diseases. So they took samples, tested them for chemical composition and concluded that this was the same type of medicinal oil that they had over there," said Milica Birk, Deputy Manager of the Naftalan hospital.

But the Croatian team says it is using a more refined, distilled oil derivative, unlike Azerbaijan's facility, which, they say, uses crude oil.

"In medicine we should not really use crude oil, which has some substances that could be harmful. So we had to find a way to produce a distilled version, which still has all the medicinal properties, while compounds which are not desired were removed. Through cooperation between the Croatian oil company experts and our physicians here, we came up with this final product," explained Birk.

Ivanic Grad is located only 25 kilometers from the Zagreb airport. The price of the treatment is a little over $100 a day, which many patients consider low, considering the benefits.

The clinic has recently launched a naphthalene-based line of creams, shampoos, soaps and oils, which can be used for home treatments.