Because they are heavily impregnated with fungicides.
Think about it, oranges at home in a warm kitchen eventually go mouldy. This can't be tolerated commercially and in transport, so nearly all oranges (and bananas) are sprayed with various types of fungicides before being crated off and shipped around the world.
Orthophenylphenol, Thiabendazol, Imazalil -- are among the most common brand names of fungicides sprayed on oranges.
You can usually find in the larger supermarkets special "unsprayed" oranges. Their peel is suitable for making marmalade. Also, in parts of the world where orange trees are common, you might be able to just pick some off a tree that will be safe enough -- assuming the tree owner hasn't used heavy doses of pesticides.
If at all doubt, don't nibble on orange peel -- even if it is supposed to be a terrific source of vitamin C. You can get more Vit. C from potatoes, after all.
Think about it, oranges at home in a warm kitchen eventually go mouldy. This can't be tolerated commercially and in transport, so nearly all oranges (and bananas) are sprayed with various types of fungicides before being crated off and shipped around the world.
Orthophenylphenol, Thiabendazol, Imazalil -- are among the most common brand names of fungicides sprayed on oranges.
You can usually find in the larger supermarkets special "unsprayed" oranges. Their peel is suitable for making marmalade. Also, in parts of the world where orange trees are common, you might be able to just pick some off a tree that will be safe enough -- assuming the tree owner hasn't used heavy doses of pesticides.
If at all doubt, don't nibble on orange peel -- even if it is supposed to be a terrific source of vitamin C. You can get more Vit. C from potatoes, after all.