Naftalan oil
Hydrocarbons built on closed rings; their high share in naftalan oil is why that oil does not burn and is used in treatment.
Naphthenic hydrocarbons are a class of hydrocarbon whose molecules are built as closed rings rather than open chains. They occur in ordinary crude too, but in naftalan oil their share is considerably higher, and it is this that gives the substance its properties.
Why it matters
The light fractions that make a fuel — petrol and its neighbours — are practically absent from naftalan oil, so it does not ignite and there is no sense in extracting it to burn. The therapeutic effect is attributed to these same ring compounds: inflammation subsides, pain eases, the vessels near the surface of the skin widen.
What to watch for
Composition varies with the part of the field and the way the oil is prepared, so not everything with "naftalan" on the label is the same thing. Pharmacy preparations generally use the deresined form; a sanatorium bath uses oil that has been through its own preparation.