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Blink and it’s gone. The ephemeral McRib sandwich appears at McDonald’s infrequently and only for a limited time. If you haven’t indulged in one yet, here’s what you’re missing: azodicarbonamide, ammonium sulfate and polysorbate 80 — those are just three of the 70 ingredients (34 in the bun alone) that go into the BBQ pork sandwich, according to the restaurant’s website.

These components are in small enough quantities to be innocuous. But it’s still a little disconcerting to know that, for example, azodicarbonamide, a flour-bleaching agent that is most commonly used in the manufacture of foamed plastics like in gym mats and the soles of shoes, is found in the McRib bun. The compound is banned in Europe and Australia as a food additive. (England’s Health and Safety Executive classified it as a “respiratory sensitizer” that potentially contributes to asthma through occupational exposure.) The U.S. limits azodicarbonamide to 45 parts per million in commercial flour products, based on analysis of lab testing.

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The McRib enjoys a bizarre cult following, in part because of its impermanence. Reports Brad Tuttle over at Moneyland:

First introduced in 1982, the sandwich first disappeared in 1985, but then has periodically resurfaced in McDonald’s in the U.S. and abroad. The McRib’s cult-like following has generated not only Facebook pages, but McRib Locator websites and a Twitter account. This fall, the McRib made news as McDonald’s re-introduced it once again — this time, making it available in all U.S. locations through Nov. 14. The obvious question is: if the McRib is so popular, why doesn’t McDonald’s sell it year-in, year-out, at all locations? The answer is that, sort of in the same way that some people are attracted to bad boys (or girls) who won’t commit, the elusiveness of the McRib is part of its appeal.

If the chemistry-lab ingredient list isn’t enough to put you off the McRib’s saucy allure, perhaps the nutrition information will: with 980 mg of sodium (more than half your recommended daily intake) and 10 g of saturated fat, the sandwich is, quite literally, not for the faint of heart.

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Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.