I assume you've been using the bin Fuggle - in which case - it's probably too late to start worrying now!!
They do recommend not using any wood stains /preservatives etc on bins but most ready made bins are made of treated timber anyway so I'm not sure what they have in them that's different. If you're worried about it could you perhaps make a lining for it out of a breathable material or even plastic with some holes for ventilation. I've done something similar with mine using an old cover for outdoor furniture.
I will do something similar to make a lid for it too. Hope that helps