The best way to examine this claim is to find comparative studies that test both types of compounds. Fortunately, there are a few. In this 1979 study, multiple emollients were studied to see their effects on UVB transmission. Petrolatum was shown to reduce UVB transmission, while mineral oil had “minimal effects.” Then in this 1995 study, this same conclusion was confirmed when testing fewer emollients: “Thick application of petrolatum and emollient creams can reduce transmission of UVB. Mineral oil and a clear liquid emollient did not significantly affect transmission or erythemogenicity of UVB.”Finally, in this much more specialized and more in-depth study published in 2005 on mineral oil alone, it was shown that, “On the Vaseline oil pretreated side, significantly more plaques were cleared, especially in severe psoriasis. Scaling and infiltration were significantly improved. Application of Vaseline oil was more interesting in thick and scaly psoriasis probably because the oil penetrates the intercellular space allowing an optical matching effect which increases the UV transmission… We strongly recommend Vaseline oil pretreatment with UVB TL01 phototherapy in psoriasis, especially in severe psoriasis.”
Okay, we can definitely confirm that petrolatum does indeed reduce UVB transmission: the conclusion drawn in last week’s post. But does mineral oil behave similarly?Well, the two early studies concluded that mineral oil had minimal or non-significant effects on UVB absorption, which most likely means that the studies did not yield statistically significant figures with p-tests; probably <0.05. The more specialized and more recent study however, found that mineral oil did enhance UVB transmission due to the oil penetrating the intercellular space and allowing an “optical matching effect.” The authors went to conclude that mineral oil would be especially (think yielding statistically significantly results) effective for those with severe psoriasis. Upon clinical observation of patients with severe psoriasis, you’ll notice that the plaques are very dry and have tons of flaking skin everywhere. Microscopically, these layers of skin of the stratum corneum (SC) are in disarray: bent, twisted, and misaligned. Therefore, this arrangement will dilute the potency of UV light by reflecting it in all directions. What mineral oil does is slip between those layers of dry skin cells (think: INTER-cellular) and normalizes all the weird angles to allow the layers of skin to sit properly on top of each other; to match. It’s kind of like how a zipper will realign all the teeth after a pass. This new arrangement of skin will therefore, be characterized by (more) regular layers that resemble a brick wall, which will reduce the reflection of UV light as it can now pass through at a direct angle. Think of how a knife will glance off of a bullet-proof vest if it comes in at an angle, but will penetrate if stabbed directly. This is likely the “matching optical effect” described. So yes, mineral oil does indeed enhance penetration of UV light. And to answer the initial question posed: it does also appear that the difference in the sizes of the alkanes is responsible for the influence that mineral oil has on UV transmission. The smaller-sized alkanes of mineral oil allows the substance to better penetrate into the “intercellular space,” while those of petrolatum are larger and less able to do so. ***Note that, it is more the mechanical effect of mineral oil on the skin, rather than a direct chemical interactive effect between mineral oil and UV light, that is responsible.
But does that conclusion apply to the general population, or even to those with mild to moderate cases of psoriasis? Fortunately, it does not!You have to keep in mind that all of these studies were tested on patients with psoriasis, who have decreased susceptibility to UV transmission. And even in patients with mild to moderate cases as noted above, mineral oil had no significant effect. Mineral oil only had a significant effect on those with severe cases, due to the fact that it induced a change to a characteristic (flat layers of skin) that’s already present in “regular” skin. That significant increase was only a measure of the change in efficacy of mineral oil’s capacity to enhance UV irradiation when psoriatic skin mimicked normal skin. But as previously emphasized, the layers of skin are already flat in normal skin, so mineral oil would have no “optical matching effect.” Therefore, this unique finding cannot be applied to the general population, and was taken out of context by the reader. It is logical to conclude that mineral oil neither protects against nor enhances UV transmission in normal skin. Also, please take the time to tell me your experiences with water-based products versus oil-based ones in this discussion. It would be of great help to me!