# I have a rash! What physically happened?

**URL:** <https://boards.straightdope.com/t/i-have-a-rash-what-physically-happened/955559>\
**Category:** Factual Questions\
**Created:** [December 6, 2021, 2:14am UTC](https://boards.straightdope.com/t/i-have-a-rash-what-physically-happened/955559 "2021-12-06T02:14:07Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![GreysonCarlisle](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@GreysonCarlisle](https://boards.straightdope.com/u/GreysonCarlisle)\
**Post date:** [December 6, 2021, 2:14am UTC](https://boards.straightdope.com/t/i-have-a-rash-what-physically-happened/955559/1 "2021-12-06T02:14:07Z")

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Note, that I’m not looking for medical advice, just for the physical process that occurred. And I won’t post a picture. That horror show is between me and my mirror (and my girlfriend).

So I wrestled with the dogs earlier, and one got me on the side with a stray claw. While checking it out, I was shocked to find a mottled rash–nice and dark, almost purple–on my lower back. It starts just below my shoulder blade and it stops exactly at my waistline. I’ve been using a heating pad on my back lately, and I’m pretty certain that’s what caused it.

I showed it to my girlfriend, expecting her to be horrified. Your heating pad she said, totally deadpan and unconcerned. Evidently, she gets them on her arms when she uses her hot rice-bags.

It’s smooth–not prickled like a heat rash–and like I said, it’s mottled. You can use your finger to draw on it, and the parts you touch will stay white for a short while. It’s kinda neat, now that I’m convinced it’s not blood poisoning.

My pad goes up to 160 F, and that’s pretty much where I keep it. My girlfriend’s explanation is that I’m cooking myself slowly, and that sounds like a reasonable explanation. So is that what’s happening? I’m baking the blood vessels? Killing the blood cells?

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**Author:** ![Jackmannii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackmannii/32/311_2.png) [@Jackmannii](https://boards.straightdope.com/u/Jackmannii)\
**Post date:** [December 6, 2021, 2:34am UTC](https://boards.straightdope.com/t/i-have-a-rash-what-physically-happened/955559/2 "2021-12-06T02:34:53Z")

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Erythema ab igne?

> **[Erythema Ab Igne from Heating Pad Use: A Report of Three Clinical Cases and a...](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6047836/)**
>
> Erythema ab igne is an asymptomatic cutaneous disorder characterized by erythematous reticulated hyperpigmentation resulting from chronic exposure to infrared radiation in the form of heat. We report three cases of erythema ab igne from chronic...

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**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\
**Post date:** [December 6, 2021, 2:40am UTC](https://boards.straightdope.com/t/i-have-a-rash-what-physically-happened/955559/3 "2021-12-06T02:40:08Z")

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> [@Jackmannii](#):
>
> Erythema ab igne? -

Whoever came up with that _had_ to be drunk.  
😉

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**Author:** ![GreysonCarlisle](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@GreysonCarlisle](https://boards.straightdope.com/u/GreysonCarlisle)\
**Post date:** [December 6, 2021, 2:48am UTC](https://boards.straightdope.com/t/i-have-a-rash-what-physically-happened/955559/4 "2021-12-06T02:48:08Z")

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> [@Jackmannii](#):
>
> Erythema ab igne?

Yup, that looks like it.

If I’m reading this right

> [@](#):
>
> Histopathology often demonstrates epidermal keratinocyte atypia, elastosis in the dermis and occasionally liquefaction degeneration of the basal layer, as well as melanin incontinence and hemosiderin in the dermis.

the heat is changing the skin cells, themselves, and moving around some iron-rich proteins. Interesting.

Thanks!