dioxins (PCDD)

Dioxins — formally polychlorinated dibenzo-p-dioxins (PCDDs) — are a family of 75 congeners of tricyclic aromatic compounds with 1-8 chlorine substituents. They are highly toxic, persistent, bioaccumulative byproducts formed unintentionally during combustion of chlorinated or brominated organic materials at sub-optimal temperatures. The dioxin family is the most toxic synthetic chemical class known — the most potent congener, 2,3,7,8-TCDD, is acutely lethal to laboratory animals at sub-microgram doses per kg body weight, and is the contaminant that caused the 1976 Seveso accident in Italy and the chloracne and birth defects from Agent Orange exposure in Vietnam.

The formation mechanism is well-characterised. Dioxins form during combustion of chlorine-containing materials (PVC plastic, chlorinated solvents, paper bleached with chlorine, brominated flame retardants, polychlorinated biphenyls) when temperature drops through the 300-450°C window with sufficient residence time and oxygen — a regime called de novo synthesis on fly ash particles. Open burning of e-waste, mixed-waste incineration without proper gas cleaning, and uncontrolled tyre or plastic pyrolysis are the dominant anthropogenic sources. Once formed, dioxins are extremely persistent (soil half-life 10-25 years), highly lipophilic (log Kow 6-8.5), and bioaccumulate up food chains by factors of 10⁴-10⁶ — concentrating in fatty tissues of livestock, dairy and fish, then in humans through diet.

The toxicity acts through binding to the aryl hydrocarbon receptor (AhR), a cytoplasmic transcription factor that regulates xenobiotic metabolism. Chronic AhR activation by dioxins disrupts thyroid function, reproduction, immune response, and induces cancer (IARC Group 1 for 2,3,7,8-TCDD). Toxicity of mixtures is measured as toxic equivalent (TEQ) using WHO 2005 toxic equivalency factors (TEFs) — each congener weighted by its potency relative to 2,3,7,8-TCDD (TEF = 1.0). Daily tolerable intake recommended by JECFA is 1-4 picograms WHO-TEQ per kg body weight per month.

For Indian e-waste, plastic and tyre recyclers, dioxin control is the principal reason high-temperature controlled-combustion infrastructure is mandated. Cement-kiln co-processing at 1,400-1,800°C with >5-second residence time at temperature, alkaline ash matrix and rapid post-combustion quench reliably destroys dioxins to non-detectable levels. Dedicated hazardous-waste incinerators operate at 1,100-1,200°C with 2-second residence and activated-carbon injection downstream. Plastic and tyre pyrolysis under inert atmosphere with controlled gas-cleaning trains has lower dioxin yields than open or under-oxygenated combustion but still requires activated-carbon adsorbent and HCl scrubbing. The recurring pattern in informal Indian e-waste recycling is open burning of cable insulation and PCB substrates to recover copper — generating dioxin emissions thousands of times above any regulatory limit. CPCB monitoring under the Stockholm Convention's National Implementation Plan estimates India's anthropogenic dioxin emissions at 350-500 g TEQ per year, with informal recycling accounting for 15-25% — making formal recycler scale-up itself a public-health priority beyond resource recovery.