What it is
Polypropylene is the tough, heat-tolerant workhorse — it melts around 160 °C, which is why it’s the plastic labelled “microwave-safe”. That label means it keeps its shape. It says nothing about keeping its particles. Code “5”, usually translucent or opaque, and the material of every flip-top cap hinge that never snaps.
Where you meet it
Takeaway containers, electric kettles, baby bottles, bottle caps, yoghurt tubs, lunchboxes — and the heat-seal in most “paper” teabags.
What the research shows
PP is the most heat-tested polymer in the microplastics literature, and the flagship study is from my home town. UQ’s QAEHS found a brand-new plastic kettle’s first boil delivers ~3 billion nanoparticles into a single 250 mL cup, falling about 70% within ten boils and settling near ~205 million per cup by 150 uses (npj Emerging Contaminants, 2025, UQ News) lab. The strange twist: hard tap water’s mineral scale coats the plastic and suppresses release — the limescale Australians dutifully descale is a protective liner.
The rest of the heat evidence stacks the same way lab:
- PP baby bottles shed 0.6 million particles/L at 25 °C, 16.2 million at 70 °C, and 55 million at 95 °C during formula prep (Trinity College Dublin, Nature Food, 2020) — with infants in Oceania estimated among the world’s most exposed, at ~2.1 million particles a day.
- Microwaving PP containers released up to ~4 million microplastics and ~2 billion nanoplastics per cm² in three minutes (University of Nebraska–Lincoln, ES&T, 2023).
- PP teabags shed ~1.2 billion particles per millilitre of brew (UAB Barcelona, Chemosphere, 2024).
On harm: in the Nebraska work, 77% of cultured kidney cells died after three days at the highest particle concentrations lab — concentrations well above a realistic diet. The Barcelona team watched teabag particles enter intestinal cells and reach the nucleus lab. There are no PP-specific human outcome studies yet.
What I do
The plastic kettle was one of my first retirements — replaced with stainless steel, for about $67. Nothing gets microwaved in plastic; hot food is reheated in glass or ceramic. If a plastic kettle must stay: boil and discard the first ~10 fills, and let the scale build.
The honest bit
PP itself carries almost no additive baggage — no BPA, no phthalates, few plasticisers. Cold yoghurt in a PP tub is a non-issue. The entire case is heat × time × surface area, which is also why it’s so fixable: the kettle, the microwave and the teabag are three decisions, not a lifestyle.