If powder comes out of a mushroom does that mean it can hurt you?

Answer 1

The powder that sometimes comes out of the gills of mushrooms are the spores. A spore is a reproductive structure similar to a seed. Because the spores are so tiny they can be carried easily on the wind or on animals fur, like pollen is carried by bees. The spores of a common edible 'button' mushroom wont do you any harm in small quantities. It is advisable to wash the gills of a mushroom before you eat it, not because you shouldn't eat or breathe the spores but because dirt and bacteria often get in there. If you are cooking the mushrooms then this isn't so much of an issue.