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I have been told that eating food from a can that has been damaged (e.g. from falling) is dangerous, even if the damage is just a dent. Nobody is able to explain me why though so I am not convinced. My reasoning would be that as long as no air can get inside the can, the food is safe.

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Could you please define damaged - mechanically deformed or no longer hermetic? In the first case there are no good reasons to be inedible. In the second there is risk of contamination and spoilage. – Daniel Iankov Aug 24 '11 at 13:44
I understand that some cans are lined with BPA. I don't see any answers yet that discuss the possibility of a damaged BPA lining contaminating the food. – Highly Irregular Feb 9 '12 at 0:51

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Damage to a can, including dents, can be a sign of Botulism contamination. Even a tiny amount of botulinum toxin can be lethal.

NEVER USE food from containers that show possible "botulism" warnings: leaking, bulging, or badly dented cans; cracked jars or jars with loose or bulging lids; canned food with a foul odor; or any container that spurts liquid when opening. DON'T TASTE SUCH FOOD! Even a minuscule amount of botulinum toxin can be deadly.

However, dents by themselves are not necessarily dangerous. While they can make the can more susceptible to rust, which can allow in air, which can then result in bacterial contamination, so long as the can is still sealed a recent dent is not likely to be a problem.

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+1 The with a new dent is the dent may have created tiny openings you cannot see that make the can no longer safe. If you drop a can you are about to use and dent it, it is probably safe to open up and eat. But if you store it over time bacteria can potentially get into the tiny openings and contaminate the food. It may be unlikely but with the risks you point out being so high is it really worth it. – Chad Aug 24 '11 at 14:02
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I’m baffled by this warning: On the one hand, the FSIS is probably a very reputable organisation. On the other hand, C. botulinum is an anaerobic bacterium … as a consequence, leaking (= open) cans are a clear indication that there is no danger from botulism, since the oxygen would have killed any bacteria, and nor does the bacterium produce gas that may cause such leakage by pressure damaging the can. – Konrad Rudolph♦ Aug 24 '11 at 15:26
@kon: what's dangerous about C. botulinum is the toxins it produces. They may stay in the can even after the bacterium is long gone. – Sklivvz♦ Aug 24 '11 at 15:33
@Sklivvz True. But I fail to see how a leakage is indication of such a contamination. For other bacteria, this is cogent (leakage = container no longer provides isolation). For C. botulinum, however, it isn’t. – Konrad Rudolph♦ Aug 24 '11 at 15:36
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@Konrad the spores of C. botulinum can survive in oxygen environments, I believe. Once the spores reach suitable anaerobic areas (which can include food systems or even wounds where oxygen is seemingly abundant), they begin to grow and form the toxins. A liquid environment can form a barrier to oxygen, allowing the bacteria to grow within the food itself (but the area directly exposed to air would not be habitable to the bacteria). – Beofett Aug 24 '11 at 15:56
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The point about sealed but distorted cans is that an anaerobic bacterium (such as Clostridium botulinum) may have produced gases inside the can, and it may have been the pressure of these gases which have caused the distortion to the can.

So the problem here is not the dent allowing contamination (you should in any case avoid cans where the seal has been broken) but the possibility that the contamination caused dents.

Hence the advice in this U.S. Department of Agriculture endorsed leaflet to check for bloating and bulges, to test cans to see if they "pop" back after applying pressure to the dent, and most importantly to avoid those which spray or explode on opening. In other cases, it says "eating from dented cans is not unsafe".

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