Science Questions

Why do living tissues turn black after being exposed to critically low temperatures? Derek via email

I think it's the same process that makes your skin turn green or black when you get a severe bruise. That's caused by haemoglobin and iron products in your blood oxidising. If an area of skin is very damaged, then it has become dead tissue. All the blood that's left in there will break down in response to the air. When something gets frostbite, you get ice crystals inside the cells. These are very jagged and spiky and pop all the cells in the tissue. So it's not just the surface that gets damaged, it's every bit of tissue inside the frozen area. It's like you hitting your hand with a mallet internally. So it's just a jellified mess inside and all the enzymes on the inside escape to the outside. Some of those are quite nasty and can break things down, but they're usually ok because they're locked away in a safe part of the cell. Once they've escaped, they start degrading tissue. That chews the tissue to pieces and you get this nasty black mess, which are the oxidation products. The iron in your blood goes black because your hand is essentially going rusty.

June 2006