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Perhaps a year ago, I read a comment where the author said something along the lines of:
“I wish I wore sunscreen as a youth. I used to just slap on some petrolatum jelly before playing a long afternoon of tennis. Now I have…”
This comment received multiple responses, one of which included something like:
“I’m so sorry to hear that. And it’s too bad that petrolatum actually worsens the effects of the sun!”
I’ve always thought that this claim was unfounded and ridiculous. But reading that initial comment, I’ve witnessed quite a few people reiterate such a belief. By no means does that provide substantive; rather, it confirms a need to address such a subject.
How Did This Idea Arise?
While I cannot pinpoint the original explanatory source, proponents claim that this phenomenon occurs because cosmetic-grade petrolatum (white petrolatum) creates a shiny and uniform layer on the surface of the skin, which in-turn focuses and intensifies the UV rays of the sun—similar to how a magnifying lens can channel sunlight into a single beam of (literally) burning heat.
As elegant of an analogy this is, to predicate validity based on such a line of reasoning, would be erroneous. Here’s why:
A magnifying lens uses convexity to compress a given amount of light energy into a relatively smaller quantity of space, whose size is dependent on the convexity of the lens, and the distance between the lens and the “target.” Forced into a smaller quantity of space, this now denser, but still same amount of light energy will naturally increase in strength and potency.
Topical petrolatum obviously does not possess any of these attributes.
Once applied to the skin, the “uniform” layer of petrolatum cannot form and maintain a meaningful degree of convexity, as it hugs the skin’s every contour.
Furthermore, because the strength of the focused beam of light is dependent (to an endpoint distance) on how far the convex (magnifying) lens is in relation to the “target,” the layer of petrolatum would also have to be some distance away from the skin to have any increased strength. So unless you can somehow apply petrolatum without having it touch your skin (imagine a layer of petrolatum in the shape of your face, floating an inch away it), both the distance away and the increase in strength are zero
Let me put it more bluntly: the magnifying glass analogy is a horribly inaccurate representation of the claim that petrolatum enhances UV damage.
But Is This Claim True?
As with any skin care ingredient, let’s look at the research.
In a two-part in vivo (single-blind, placebo-controlled) clinical study that tested (in-part) the effects of petrolatum on UVA and UVB absorption, it was shown that petrolatum was able to confer mild blocking effects on both types of UV irradiation.
The two-part study also concluded that petrolatum was more adept at blocking UVB rays compared to UVA rays. This is supported by the fact that, when comparing the average minimal erythema doses (MEDs) achieved from applying a thin (0.1 mL/25 cm2) layer of petrolatum versus a thick (0.3 mL/25 cm2) layer of petrolatum, there was a statistically significant difference between the two groups in terms of UVB light. This distinction was not seen when experimenting with UVA light.
The conclusion that petrolatum actually inhibits a portion of damaging UVA and UVB light, was confirmed by another study, which also demonstrated that both olive oil and glycerin do not (directly) block UVB radiation.
***Note that, when I use the word “block” I am not inferring or explaining how petrolatum protects against UV light. Whether via regenerative absorption like organic sunscreens, scattering like inorganic sunscreens, or another process entirely; that mechanism of action is unknown, though it likely involves some type of degenerative absorption.
***Also note that, while the study tested both petrolatum and salicylic acid (SA) (a known UV blocker as previously discussed in the Hydroxy Acids Part III: Common Misconceptions post, the study was placebo-controlled for BOTH ingredients, meaning that the mild UV-blocking effects of petrolatum were not inaccurately derived from that of SA.
Petrolatum Protects Against UV Light
Despite the existing canon out there, it turns out that petrolatum actually reduces UV absorption by the skin. Keep in mind that in no way can petrolatum replace a good sunscreen (nor should it). Still, I personally think this conclusion is a gratifyingly ironic sort of repartee to the people who chose to use a certain bombastic analogy… Haha!
So if you should encounter another kook who tells you that Vaseline will cause skin cancer, send them my way! Do any of the daytime products in your routine contain petrolatum? What are you favorites?