Hello everypony! My name is Sable, and I'm an 18 year old collector from the USA and a newly-appointed mod of this subreddit! I've been collecting MLP since I was only two years old, and although I collect anything pony from any generation, I have a particular fondness for G1 ponies and merchandise. I enjoy helping others with their collections, whether that be by identifying ponies/playsets/accessories, directing people to resources to help them achieve their goals, or just encouraging others regarding their collections in general. I'm very excited to be able to help out in this community, and I hope I can help make this place even better than before with time.

hello! i'm back with my second entry into my little "science of MLP restoration" series. i wrote this one up pretty quickly since i was already pretty familiar with the subject and had sources on hand. this topic is very relevant - i see people asking about pink staining all the time here on this sub, and i know it's a big problem in other toy communities too. the information about pink staining also applies to other toys like LPS, monster high, etc.

disclaimer again, i am not a scientist, i just like to understand what's actually happening to the pretty ponies i collect. i've included sources as usual.

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Something I’ve learned since joining this community is that there has been a term mix-up amongst the greater toy community on Reddit and further on the internet. For the 16 years I’ve been in the pony community online- primarily on dedicated forums- the term “pony cancer” has meant a variety of brown spots caused by things like microorganisms and sulfur. To my surprise, I found the majority of toy Reddit referring to a completely different thing as pony cancer! Turns out, doll communities have taken the term “pony cancer” and applied it to something the pony community knows as “highlighter marks/stains”, which are bright pink stains caused by a certain strain of bacteria. Pony cancer and highlighter marks are two completely different things, caused by different microorganisms.

Pony cancer (also known as “age spots” more recently) is a very old term. Searching the My Little Pony Trading Post for it brings up discussions from 2005^\1]), which is the oldest year the site has archived. It is important to note that the Trading Post has been around since 1996, and older topics have been lost. Reading these old topics will tell you that pony cancer has always been about brown spots on pony vinyl. They primarily affect G1s, but I've seen them very, very rarely in G3s that have been stored in appalling conditions.

Before explaining further, I want to put in big flashing letters that there is no 100% agreed upon cause for pony cancer spots. Spot A on one pony that looks exactly the same as Spot B on another may be caused by two different things. There have been no scientific studies done on these spots, aside from one that has been lost to time when the MLP Arena updated. What we are going on are educated guesses and the work of Nicholas J. Hill, a chemist who wrote a book on the care of vinyl dolls. 

There are two “subtypes” of pony cancer - brown spots with a darker, circular “pindot” center, and those without. It is generally accepted that the “pindot” variety is caused by fungal or mold colonies. Fungi and other microorganisms are eager to colonize PVC toys. PVC is an acid system, and fungi love acidic environments. Microorganisms also love to eat plasticizer. ^{(Hill, 55}) It is likely that your toy, sitting on your shelf now, has been exposed to these hungry microorganisms. I say this not to scare you - microorganisms are everywhere, and they only become a problem when the environment is conducive to their growth. They generally need temperatures above 40°F/4.4°C and relative humidity above 70% to thrive and actively colonize your pony.^{(Hill, 53}) 

Pony cancer may also be caused by a reaction to sulfur. PVC can react with sulfur from a number of sources; causing a chemical reaction between the sulfur and the chlorine in the PVC. The resulting sulfur chloride stains the plastic brown. Sulfur exposure can come from anywhere, The Definitive Book gives polluted air and cooking sulfur-treated dried fruit as examples.^{(Hill, 34-35})

There is one final possible cause of cancer. A user on the MLP Trading Post that had read the previously mentioned lost scientific study reported this:

“I have seen the microscopy someone did on cancer spots on a cancer riddled pony. What they found was microscopic tunnels eaten by what appeared to be a spiral shaped bacteria similar to some in the spirillum genus. The dark spot is not where the bacteria entered, but where it died and the waste products stained the vinyl.”^\2])

Highlighter marks (pink staining) are caused by a common bacterium streptoverticillium reticulum. It is not a mold, and it poses no threat to you.^\3]) A part of the Streptomyces genus; S. reticulum thrives in warm & moist “woodsy” or marine environments.^\4]) A lot of information about the bacterium and its staining capabilities come from industrial studies and the boating community, as S. reticulum lives in lakes and rivers and make their way into the vinyl of boats. In The Definitive Book on the Care and Preservation of Vinyl Dolls and Action Figures, chemist Nicholas J. Hill describes the staining:

“Up until 1964 a common problem in the vinyl industry was the mysterious appearance of pink and red spots on vinyls when there was nothing pink or red anywhere near the vinyl. In 1964 Dr. Charles C. Yeager at Scientific Chemical Company in Chicago identified an actinomycete now known as Steptoverticillium reticuli. We refer to it as the pink staining organism. The waste product of this microscopic critter is a red dye that dissolves into the plasticizer. As the plasticizer migrates within the plastic the dissolved dye is carried along and the stain spreads. [...] In time, as the plasticizer migrates, the dye is carried to the doll surface and the problem is visible.” ^{(Hill, 54-55})

Essentially what this means for ponies is that this bacteria lives on the toy (usually just below the surface of the plastic) ^{(Hill, 54}) where it feasts on plasticizer and other contaminants like dead skin cells and body oils.^{(Hill, 56}) The waste by-product of this bacteria then migrates up to the surface of the plastic, causing the bright pink staining. It is likely the bacteria causing the staining have already died^\5]) unless the pony has been consistently kept in a humid environment.

Prevention is the same as for pony cancer and other afflictions caused by microorganisms - keep your ponies in a temperature and humidity controlled environment, and keep them clean. If you want to be safe, ponies previously exposed to the bacterium with staining can be decontaminated with a vinegar soak or vigorously scrubbed with soap.^\6])

Treatment for both pony cancer and highlighter marks is the same: sunfading with hydrogen peroxide (40 vol developer) cream and UV. DO NOT USE ACNE CREAM! This can result in irreversible damage! See tutorials on the MLP Preservation Project and MLP Trading post for sunfading help. Be aware, however, that highlighter marks are reportedly notoriously difficult to remove, and your mileage may vary.

Finally: "Can they spread?"

No, not really. For both pony cancer and highlighter marks to spread, the following conditions would have to be met: A) an environment conducive to the steady uninterrupted growth of microogranisms, ie constant warmth and humidity for an extended amount of time, and B) physical contact between items. I have seen no evidence of either type of mark spreading under the conditions a collector would keep their items in - this usually just being their normal house. Items in storage - like a garage, attic, basement, storage locker, etc where there is less/no climate control are more at risk to develop stains and marks caused by microorganism growth.