It's not harmless, but neither are the vast majority of the products you can buy off the shelf.

Microwaving plastic is terrible. Half the stuff you can buy from home Depot has to have a California reproductive health/cancer warning on it. We actively pump chemicals into the air, water, our bodies, etc. with very little thought to the downstream consequences.

What makes pot special? Alcohol, by nearly every measure, is worse, but we all recognize that banning it doesn't actually help anything, and criminalizing things based on a maybe health impact is insane.