Are dented cans dangerous?
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Potentially yes. Avoid them. Bulging cans are ABSOLUTELY POSITIVELY dangerous.
I am not an expert or scientist, but something I heard at my local organic shop might be interesting here, though it could be complete urban legend. According to this particular employee, ironically, this is why companies began lining cans with BPA - to prevent dents in cans from actually becoming a pinhole which could let in bacteria. Clearly, a bulging can is no good, but I wonder how much there is to the BPA lining theory.
Cans are lined with a plastic coating to preserve flavor and to prevent acidic foods from eating through them from the inside out, not to preclude leaks due to dents. The practice started with the first beer cans back in the 30's.
When a can is severely dented on a seam, either on the body of the can or on a lid, there is a danger of failure at the seam which could allow bacteria to enter. Sharp dents anywhere on the can are also problematic as they can result in pinhole ruptures. Severe rust (rust that can't be wiped away) is also dangerous.
Minor, smooth dents do not present any danger whatsoever. Some cans today don't have a bottom seam similar to a Coke can (look at a tuna can for example). Many of these cans receive minor dents from normal handling due to the lack of reinforcement at the bottom edge -- not a problem if it's minor, still an issue if it's sharp. It takes a lot to actually rupture a can. Throw a can of soda onto the floor and see how much damage it can withstand without rupturing.
As always, bulging cans or cans that squirt when you open them represent extreme danger from botulism (carbonated beverages an obvious exception).