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I can't make heads or tails of the information on the net — although there seems to be plenty of it, it seems contradictory.

Supposedly, calcium carbonate is used in antacids—yet there are warnings about ingestion and contact with skin.

I need to handle it in pure powder form. I'm mainly concerned about skin contact, should it be avoided? (Am I getting confused between two or more forms of calcium carbonate?

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Looking at the Material Safety Data Sheet (MSDS), most of the issues with it seem to stem from the physical consequences of it being in fine dry powder form. This means that if you get it on your skin, it can get into cracks and pores and easily dry out your skin to the point of cracking, and will cause irritation or worse if you get it in your eyes, for example. It's not toxic if you get it on you, though -- rubbing a lump of limestone against your skin isn't going to do anything except abrade it.

So, yes, if you're working with the powder, wear gloves. And safety glasses. And long sleeves. (See also: lab coat.)

If in doubt, follow the MSDS.

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It's essentially chalk. I'm not aware of any longitudinal studies of gymnasts' long term exposure to powdered calcium carbonate, but my sense is that the worst that can happen is your skin can get dry and irritated. If you have a part time job as a hand model, you will want to be especially careful to avoid skin contact.

If you're in a lab, of course, it goes without saying that you should take necessary safety precautions such as proper eyewear and skin protection as well as a lab coat.

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So far the skin questions have been answered nicely. Here's the ingestion:

used in antacids—yet there are warnings about ingestion

  • half of the problem is the acid neutralizing capacity: your stomach may like to keep up its acidic pH.

  • The other half is of course how that neutralization happens:
    $\ce{CaCO3 + 2 H+ \rightarrow Ca^2+ + H2O + CO2 \uparrow}$
    You'll produce lots of carbondioxide gas in your stomach

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Think of the base as being weakly caustic. This applies not only to antacids based on calcium carbonate but also those based on magnesium oxide, another popular antacid agent.

Calcium carbonate is a compound that stands between full-fledged alkalies and weak metal-hydroxide bases, much like magnesium oxide discussed here. Such compounds are not very soluble in water as such, but can react in the presence of weak acids such as the fatty acids in your skin. Such reactions, which are possible with magnesia-based antacids as well, may lead to the quoted dryness and irritation.

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