The hidden toxicity of indigo: what sits behind a blue pair of jeans
- Jun 25
- 5 min read
A pair of blue jeans looks simple. The colour feels familiar, even harmless. But the deep blue of denim sits at the end of a long industrial chain, and the risks in that chain are not all carried by the finished garment. Most of them sit upstream: in chemical manufacturing, in dye preparation, in dyehouses, in wastewater streams and in the communities and ecosystems that live with the consequences.
That is the first important thing to understand about indigo toxicity. The problem is not only the blue molecule itself. It is the whole way conventional indigo is made, formulated, reduced, applied, washed and discharged. Toxicity enters the story at different points, in different forms and for different people.
The first of those points is production. Today, most synthetic indigo is still made through chemical routes that rely on aniline or aniline-based precursors. This matters because aniline is not a benign starting material. It is a high-production-volume industrial chemical, used in dyes and pigments and many other products, and it is classified by the International Agency for Research on Cancer as probably carcinogenic to humans. In other words, the toxicity story begins before indigo is even blue. It begins with the chemical feedstocks used to make it.
Even after indigo is manufactured, the story does not end cleanly. Commercial indigo is not always chemically pure. Scientific work has shown that synthetic indigo can contain residual aromatic amine contaminants, especially aniline and N-methylaniline. Commercially produced indigo may contain up to 0.6% aniline and 0.4% N-methylaniline by weight. Indigo itself shows a small mutagenic effect that is most likely linked to these contaminants. That is a crucial distinction. It suggests that part of the toxicity associated with conventional indigo is not simply the dye molecule, but the industrial baggage that comes with how it is produced.
Then comes the dyeing stage, which is where indigo becomes especially complicated. Indigo in its blue form is insoluble in water. To dye denim, manufacturers first have to reduce it into a soluble form, often called leuco-indigo, so that it can enter the fibre. For decades, the standard industrial way to do this has been with sodium dithionite, also known as sodium hydrosulfite, under strongly alkaline conditions created with sodium hydroxide.
Both chemicals come with their own hazards. Sodium dithionite is an unstable reducing agent. International Chemical Safety Card data describe it as irritating to the eyes and respiratory tract, harmful to aquatic life and capable of producing toxic sulfur oxides when it decomposes. It is also reactive enough that contact with moisture, acids, or incompatible substances can create serious handling risks. Sodium hydroxide is different, but no less concerning in a workplace context. It is highly corrosive. NIOSH lists inhalation, ingestion and skin or eye contact as exposure routes, and notes effects including irritation, burns and respiratory injury. In practical terms, this means conventional indigo dyeing depends on a chemistry that is not just inconvenient for the environment; it is also inherently demanding from a worker-safety perspective.
In denim mills, those risks do not stay neatly inside reaction vessels. Dyeing is a wet process, and wet processes create wastewater. That wastewater does not contain only colour. It carries alkalinity, reducing chemicals, decomposition products, auxiliaries, salts, suspended solids and residues from repeated dipping, oxidation, rinsing and washing. A recent review of denim wastewater describes it as poorly biodegradable and loaded with persistent pollutants that can generate toxic and carcinogenic compounds. The environmental issue is therefore not simply that wastewater is coloured blue. The deeper problem is that indigo dyeing wastewater can be chemically complex, difficult to treat and harmful if inadequately managed.
This is where the supply-chain dimension becomes impossible to ignore. The toxicity burden of indigo is unevenly distributed. Consumers see a finished pair of jeans. Workers in production hubs may see chemical dusts, alkaline baths, splashes, fumes and poorly controlled waste streams. Nearby waterways may receive effluents that are difficult to break down. Treatment plants may struggle with colour removal and contaminant load. Agricultural soils and groundwater can become part of the story when polluted water is reused or poorly managed. In other words, the visual simplicity of denim hides many risks.
None of this means that wearing jeans is the main health problem. In fact, the consumer end of the chain is more complicated and, in many cases, less dramatic than the upstream manufacturing stages. A scientific review of colourful textiles makes an important point here: ideal dyes would remain fully in the fabric during use, but many textile dyes are capable of leaching or wearing off, and exposure assessment is still limited by data gaps. For denim, the greater and better-documented concern is not ordinary wear by consumers, but the industrial chain that produces the fabric in the first place. That is where workers face direct chemical contact, and where the largest environmental releases occur.
This is also why the conversation around indigo toxicity has changed in recent years. The question is no longer only whether indigo can dye denim effectively. It is whether the current way of making and applying it is still acceptable in a world that is paying more attention to occupational safety, water pollution, chemical regulation and planetary limits. Once toxicity is viewed across the full value chain, conventional indigo stops looking like a single dye and starts looking like a system: one built on hazardous feedstocks, reactive reducing agents, corrosive process chemistry and waste streams that demand careful control.
That broader view also explains why so much innovation in denim is now focused on process redesign rather than colour replacement. The blue itself is not the enemy. The real challenge is the chemistry wrapped around it. Cleaner indigo systems aim to remove aniline-linked contamination, avoid toxic reducing agents, reduce alkalinity, lower wastewater burden and make dyeing safer for both workers and ecosystems. In that sense, the future of indigo is not just about making blue jeans more sustainable. It is about making the whole blue supply chain less toxic.
Indigo helped build one of the most recognizable products in modern fashion. But the closer we look at its journey, the clearer it becomes that the true cost of blue is often paid far from the wardrobe. If denim is going to remain a global icon, the next chapter cannot be only about style. It has to be about chemistry, exposure and responsibility across the whole chain.
References
Linke, J.A., Rayat, A. & Ward, J.M. Production of indigo by recombinant bacteria. Bioresour. Bioprocess. 10, 20 (2023). https://doi.org/10.1186/s40643-023-00626-7
Cordin M, Bechtold T, Pham T. Quantification of aniline and N-methylaniline in indigo. Sci Rep. 2021 Oct 26;11(1):21135. doi: 10.1038/s41598-021-00634-7. PMID: 34702925; PMCID: PMC8548543.
Castillo-Suárez, L.A., Sierra-Sánchez, A.G., Linares-Hernández, I. et al. A critical review of textile industry wastewater: green technologies for the removal of indigo dyes. Int. J. Environ. Sci. Technol. 20, 10553–10590 (2023). https://doi.org/10.1007/s13762-023-04810-2
Nicolai, S., Tralau, T., Luch, A. et al. A scientific review of colorful textiles. J Consum Prot Food Saf 16, 5–17 (2021). https://doi.org/10.1007/s00003-020-01301-1
