The disguise
“Compostable.” “Plant-based.” “Made from corn.” Every word steers you away from the one that matters: plastic.
The plastic
PLA — polylactic acid — is a genuine synthetic polymer; a bioplastic is still a plastic. Its feedstock is fermented plant starch instead of oil, which changes where it comes from, not what it is. Two properties do the damage: its glass-transition temperature is around 60 °C (your tea is 95 °C), and “compostable” means EN 13432-style industrial composting — ~58 °C, controlled, for weeks. In your cup, your home compost or landfill, that certification buys nothing.
The evidence
The decisive Australian measurement, published this month: UQ compared cup linings under hot liquid and found PLA-lined cups released ~4.3 million nanoparticles/mL versus ~2.7 million for PE-lined — roughly 12× more particle mass overall (ACS Food Science & Technology, 2026) lab. The swap marketed as the fix sheds more than the trap it replaced. A 2023 lab study had already pointed the same direction (Chemical Engineering Journal, 2023) lab.
A sourcing note, in the open: an early draft of my tea-bag post credited the “biodegradable cups shed nanoparticles” finding to University College Dublin, following a secondary write-up. The study is the University of Queensland work above; it was caught and fixed before publication. When I get an attribution wrong, this site’s policy is to say so rather than quietly edit.
The tell — and the fix
The tell: “compostable” without “certified home compostable + plastic-free”; a lining that feels exactly like plastic because it is one; the little green leaf logo doing heavy lifting. The fix: same as paper cups — steel, ceramic, glass. PLA’s honest niche is cold drinks, single use, in a city that actually runs industrial composting collection.