I’m a laymen in that regard, so my answer need not be perfect. But it may make you understand something.
We live at a time, when literally everything has been polluted to an uncertain degree. The Oceans are said to be pretty much polluted, as well as the woods or any other area on Earth.
When you catch a fish in the Sea, how can you know if its meat doesn’t contain anything that may be dangerous to your health?
They catch fish at teratons and then sell it on the market. They can’t ‘examine’ every single one of them searching for traces of metal or plastic in their meat. So they say, fish CAN contain traces of this or that.
If you kill an animal for food, the bullet that killed it can release small quantities of something into their blood, and if the animal doesn’t die quickly, the stuff can enter their tissue or some of their organs, say their heart. And if you then eat the animal’s heart, you will swallow some dangerous stuff.
Whatever, the quantities of dangerous stuff inside an animal’s meat are extremely low, so in 99,999999999% nobody gets harmed. It’s just that some people whose immune system is very weak MAY be affected by it.