Michael: Maybe you’ve seen those “BPA-Free” stickers on plastic water bottles before. Having them labeled that way makes it seem like a dangerous chemical, but you can find BPA in all sorts of things. DVDs, shatter-resistant eyeglasses, baby bottles... it’s even in resin that lines some cans of food, and in thermal paper receipts that you get at the store.

Most people have detectable levels of it in their urine, and that hasn’t caused the collapse of civilization, although 2016 may have come close. Anyway, how bad can BPA actually be?

Even though there’s been a fair amount of research on BPA, we still don’t know if it’s completely safe. BPA is short for bisphenol A, and it’s one of the most common ingredients in a type of hard, clear plastic called polycarbonate.

You can react it with another chemical, like carbonyl chloride, to form long chains, and those chains form a really durable plastic. Those chains are pretty chemically inactive, but the trouble comes when unreacted BPA molecules stick around in the plastic. When you heat it up, like by putting a plastic container in the microwave, those leftover molecules can become free.

And that is what might be a problem. See, BPA is what’s known as an endocrine-disrupting chemical. It’s a molecule that interferes with how natural hormones – like estrogen, androgen, and thyroid hormones – work in the body.

Animal studies have linked BPA to all sorts of problems, like decreased fertility, impaired fetal development, obesity, and cardiovascular diseases. But, like with any health research, you can’t just translate those findings to humans.

For one thing, laboratory conditions don’t mimic the way we interact with BPA very well, in terms of dosage or exposure. Plus, not all those research results have been reproducible. Lots of studies have found BPA is bad, but they don’t always agree on how or why. Right now, the US Food and Drug Administration sets the safe level for BPA exposure at half a microgram per kilogram of body weight per day.

But some scientists are saying that limit is still too high. Because BPA is a hormone disruptor, it might act differently from other toxic substances. Usually, more of the bad stuff means more bad news for your body. A tiny bit of cyanide is okay, but a lot of cyanide is not. But chemicals that target the endocrine system have been known to break that rule.

According to some research, a low dose of some chemicals might be worse than a high dose, because if the body suddenly detects a high dose, it knows something’s up and won’t respond.

So while BPA isn’t definitely terrible for humans, it might not hurt if manufacturers phase it out, especially when it comes to things like baby bottles. As long as we study the new replacement plastics just as carefully.

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