What are formed felt storage bins made of?

Usually polyester, and often a portion of it is recycled polyester from water bottles. It is molded into a shape with heat, and dyed. While polyester can be relatively safe and chemically inert, it’s usually only the case with clothing or blankets made with it— they’ll have OEKO TEX certification confirming no toxic dyes or additives were used in processing. Moreover, I like to avoid recycled polyester unless it has GRS certification, since plastic water bottles can come from unknown origin and have lots of unsavory chemicals of the past mixed into them. Since boxes are not considered close-contact items, manufacturers usually don’t bother with this level of safety or OEKO TEX or GRS certification. I think that’s generally fine— I use some plastic storage bins because I touch them once a year at Christmas to get decorations out — but if you’re looking for something safer or that you touch often, this is just good background information to understand. I think felt often looks “natural” and safe, so it’s useful to know that it’s dyed plastic.

Sometimes, felted storage bins are made with wool! These will generally cost more— just be sure to check the label and not assume “felt” means “wool.”