Now, you are hardly ever going to be in the position where this could be a potential issue, but it is still a pretty cool fact which is why it is included.
If you are ever given the chance to eat the liver of a polar bear, then turn it down. While a small piece of it is not going to cause you any harm, eating only 30g could result in you dying and it is hardly worth taking that risk if you are honest with yourself.
The reason why it is deadly is because the liver of the polar bear is very high in Vitamin A, and a dose of Vitamin A that is above a certain level will end up being fatal for us humans. However, the exact level that is fatal does vary between people, which is why some people would be able to eat up to 90g of polar bear liver as the liver itself would also have different levels of the vitamin in it. Would you want to take the risk anyway or would you prefer to just avoid eating it entirely?