Carbon monoxide (CO) is a clear odorless gas that is lethal to humans in large concentrations. It’s one of the gasses that’s emitted from a car’s exhaust pipe. That’s one of the reasons you should not run you car inside a closed garage.
Carbon monoxide has another interesting property – it can be used on meat and fish to make them look more fresh: the CO molecules delay the discoloration of the flesh that would otherwise indicate it is starting to rot. There is also a small benefit in gassing – it kills some potential parasites. Tilapia, a relatively cheap and healthy fish, reacts very well to treatment with CO. It can be frozen, shipped halfway across the world, and then, when thawed, be put on display and appear “just caught”.
The amount of gas used is tiny, and it does not pose any direct health risk.The problems begin when you buy a fresh looking fish that really isn’t that fresh. Regardless, the European Union and China forbid CO treatment of food. And in the US? you know the answer…
Americans eat about 500 million pounds of tilapia each year. The US sources tilapia mostly from Latin America and from China. Almost all tilapia is farmed. The fish from Latin America are not gassed, the fish from China are. Yes, the Chinese government allows gassing for export, but not for local consumption.
You’re probably thinking, “next time I go shopping, I’ll choose the tilapia from south or central America”. The problem is that in most cases, the origin of the fish is not labeled. Since the amount of tilapia imported from China is about ten-fold what we get from Latin America, it is safe to assume you are getting tilapia+CO.