I've no idea why they chose teabags to study - one has to start somewhere, presumably - but I can answer the second question. The distinctive feature of plastic

I've no idea why they chose teabags to study - one has to start somewhere, presumably - but I can answer the second question. The distinctive feature of plastics is the synthetic polymers that they contain, which classically feature bonds between oxygen atoms. These are extremely difficult for any organic process to break apart. Wood, however, can even be digested in small quantities, so 'microwood' will just break down into its constituent parts in the human body. The body can cope with metals and indeed has evolved to require a small amount, for instance in hemoglobin. We aren't fully aware of the implications of microplastics on health, but the main cause for concern is that we have no easy way of getting them out (either naturally or medically) in the event that they are harmful.