The disguise
It’s called a paper cup, it feels like cardboard, and it often wears a recycling arrow. But paper alone can’t hold 90 °C liquid — it would sag in seconds. Something is doing the waterproofing.
The plastic
Almost every takeaway cup is paperboard fused to a thin film of polyethylene (LDPE) on the inside. The lid is usually polystyrene or PP, positioned directly in the steam path.
The evidence
NIST held 100 °C water in LDPE-lined cups for 20 minutes and measured trillions of nanoparticles per litre, mostly 30–80 nm (ES&T, 2022) lab. The Australian numbers arrived this month: UQ’s QAEHS measured PE-lined takeaway cups releasing about 2.7 million particles per millilitre of hot drink — call it ~675 million in a 250 mL flat white (ACS Food Science & Technology, 2026) lab.
The tell — and the fix
The tell: a smooth, slightly shiny interior, and the fact that the cup survives an hour of coffee without going soggy. The fix costs nothing ongoing: my ceramic travel cup — the one that literally says “I Am Not a Paper Cup” — or steel, or three minutes drinking from an actual cup at the café. And before reaching for the “compostable” cup instead, read the PLA page: the eco-labelled lining shed more than the PE it replaced.