The tricky bit is the distinction between situations where there being a problem would be a rather extraordinary turn of events; and ones where, while by no means proven, the notion is plausible enough that you’d really want to see it disproven.
It’s not a toxicology-exclusive problem: much of science relies on locating hypotheses worth dedicating our limited testing resources to while trying not to exclude ideas that would be of value prematurely or get bogged down in fruitless investigations; but it is certainly an issue for toxicology; and one that is extra pointy because people die if you screw it up.
In this case; concern over dust is a fairly plausible thing because the occupational hazard literature is loaded with ghastly reports of lung damage caused by exposure to dust forms of materials that are otherwise highly innocuous(wood, silicon dioxide); with exposure to dusts made of things that are themselves toxic generally being even less pleasant. It is also well known, though not yet as researched as we would like, that nanoparticles and very fine dust often have quite different toxicological behavior than the bulk forms of the material do(often this different behavior is why the nanoparticle is interesting; but it still means that bulk toxicology data are deeply unhelpful).
It’s perfectly possible that polyethylene is unexciting, even if finely divided and inhaled; the lungs can and do cope with some flavors of dust without incident; but there is nothing improbable about dust hazards.
A ‘wifi causes cancer’ theory; by contrast, requires carcinogenic effects from low intensity non-ionizing radiation. Not impossible; but something we have failed to observe in decades of futzing with radio waves of all sorts of lengths and intensities.
