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By Patricia McLaughlin

On Christmas Eve day, as I was getting out of the car, I noticed an odd-looking thing on the ground: an oval plastic pillowy thing a little smaller than a credit card. It turned out to be the inner heel cushion from the Nike boots I was wearing. They looked fine when I put them on in the morning, but now the heel had separated into a top slice and a bottom, spitting out the air cushion that had been imprisoned inside.

An hour or so later, the other heel started flapping and disgorged its cushion. Next, the front of one sole started flapping like a clown shoe. All day, everywhere I went, I left a Hansel-and-Gretel-like trail of black crumbles as the bottoms of my shoes continued to disintegrate.

Not long after, I noticed deep crumbly fissures in the soles of a pair of Sanita clogs. Both soles had split along the stress line where they bend when you walk, and chunks were beginning to separate and crumble.

Then, last weekend, at the screening of a documentary film, I kept being distracted by the way the heels of my sleek Ecco pull-on boots kept sticking to the floor. Chewing gum? Spilled soda?

Nope, it turned out that the molded polyurethane soles of my boots were beginning to liquefy. Later that night, I noticed telltale black crumbles on the coffee table I had rested my feet on: fragments of spontaneously disintegrating sole. This surprised me because, although the boots weren't new, I'd only worn them a dozen or so times. I saved them for snow, and it doesn't snow usually snow that much in Philadelphia .

Gremlins? Witchcraft? Some mysterious shoe-toxic environmental condition?

No, the culprit is chemistry. I knew this because I had encountered spontaneously disintegrating shoes once before. Six years ago, on the first day of a vacation in Barcelona, the soles of my Ecco walking shoes had crumbled over a period of a couple of hours. The shoes had looked perfect when I put them on that morning. By the time I got back to the hotel, they had no soles.

When I got home, I interviewed Ecco's brand manager, who examined the shoes and diagnosed a chemical process called hydrolysis that causes molded polyurethane to crumble by breaking the bonds that hold its molecules together. He said it was "quite rare."

But now that it's happened to five different pairs of my shoes--the four pair I just mentioned plus my first pair of Eccos--I have to wonder.

Look up "crumbling soles" on Google, and you'll be amazed at how many people have had their soles suddenly crumble under them, leaving them shoeless and bereft in situations ranging from formal weddings to wilderness adventures.

Ecco comes in for pages and pages of angry postings, but they are by no means alone. Apparently all sorts of shoes with molded polyurethane soles--hunting boots, riding boots, walking shoes, comfort shoes, etc.-- can succumb to hydrolysis.

I spent hours trying to get through to somebody who knew something at Nike, Sanita and Ecco. As I write this, Ecco spokesman Dan Legor is still working on "determining who is the correct person" to answer my questions. The folks at Sanita are consulting with their Danish production team. Nobody at Nike has returned my calls.

But I'm beginning to think that this epidemic of crumbling soles has two distinct causes. One, as then-Ecco brand manager Dave Pompel explained to me six years ago, is a manufacturing defect that can cause a molded polyurethane sole to disintegrate without warning.

I suspect the second cause is an unnoticed, acknowledged collision of paradigms. Our expectations about shoes are formed by centuries of experience with soles made of leather, an organic material that is slowly and steadily worn down by friction when exposed to pavement--and, absent friction, will last pretty much forever.

Molded polyurethane foam soles can be amazingly comfortable but, unlike leather soles, they have a limited shelf life. According to footwear industry consultant Phillip Nutt, the shelf life of a direct-injected polyurethane sole should be in the range of four to five years. He says the material tends to "crumble into a sticky mess" when stored for "periods longer than four years or so."

Disuse apparently increases the risk that polyurethane soles will degrade--and that probably increases the indignation of consumers whose barely worn shoes disintegrate. If you wear your shoes every day and walk miles in them, you expect the soles to show wear. You don't expect shoes to be destroyed by months or years of sitting unworn in their box in your closet.

But that is what happens. It's striking that some of the disintegrated soles people complain about in online postings are hunting boots or hiking shoes, shoes that--like the Nike lace-up boots I only wore in the winter--spend months at a time unworn, waiting for the next hunting trip or whatever.

Maybe by next week I'll find someone at Nike or Ecco or Sanita or somewhere else who can explain exactly how and why such inactivity turns out to be sole-destroying--at least when it comes to injection-molded polyurethane.

Meanwhile, I think shoe manufacturers should consider establishing a freshness-dating system for polyurethane soles. You wouldn't buy a gallon of outdated milk. You probably shouldn't buy shoes with 8-year-old polyurethane soles, either.

But now, how would you know?

Patricia McLaughlin writes about fashion from her home in Fairmount.