Why is fast fashion considered an environmental problem?

The problem is volume rather than any single garment. Production has roughly doubled in two decades while the average number of times a garment is worn before disposal has fallen sharply. A system designed around rapid turnover produces impacts that scale with that turnover.

The impacts fall into several categories.

Water and chemicals dominate at the growing and dyeing stages. Conventional cotton is water intensive, and textile dyeing is a significant source of industrial water pollution where treatment is inadequate.

Emissions come mostly from materials and manufacturing. Synthetic fibres are petroleum derived, and production is concentrated in regions with carbon intensive electricity.

Microfibres shed from synthetic textiles during washing, pass through treatment plants and accumulate in waterways. This is a comparatively recent concern and is not solved by any current mainstream technology, though filters and fuller wash loads reduce shedding.

Disposal is the visible end. Most discarded clothing is not recycled into new clothing, because blended fibres are extremely difficult to separate. Much is incinerated, landfilled, or exported, and export markets have in places become overwhelmed.

The uncomfortable part of the analysis is that donating clothes is a weaker solution than it feels, since only a fraction is resold locally.

What measurably helps is the least commercially promoted option: buying less, buying better, wearing things longer and repairing them. Extending a garment's active life by even a modest period reduces its per-wear footprint proportionally, which no material innovation currently matches.

It is also worth separating the environmental argument from the labour one. They are distinct concerns that happen to share a supply chain, and a garment can be produced with lower emissions while still being made under poor conditions, or the reverse.