Altitude-associated transformation and apparent biotransfer reshape PAH derivative risks in alpine food webs.

High-altitude environments are traditionally regarded as cold traps for semi-volatile organic pollutants, yet the roles of biotransfer and secondary transformation in shaping contaminant fate remain unclear. Here, we investigated 16 parent polycyclic aromatic hydrocarbons (PAHs) and 27 oxygenated and halogenated derivatives (OPAHs and HPAHs) along a complete soil-forage-yak milk continuum across the Tibetan Plateau (3106-4707 m). Across the pastoral food chain, contaminant profiles underwent pronounced compositional shifts, with low-molecular-weight (2-3 ring) PAHs increasing from 13% of total PAHs in soil to 91% in milk. Regression analysis showed that apparent forage-to-milk transfer potentials increased 4.5-fold for OPAHs and 3.7-fold for HPAHs across the altitudinal gradient, indicating altitude-associated differences in apparent transfer patterns. Concurrently, derivative-to-parent diagnostic ratios increased with altitude, particularly for the anthracene-anthraquinone system (r≥0.65,P<0.01 across all matrices), consistent with altitude-associated changes in oxidative signatures. Together, these compositional and transfer patterns suggest an altitude-associated shift toward smaller and more transformed contaminant profiles. Although the absolute toxic burden in milk remained relatively stable across elevations, the toxicological contribution of derivative compounds increased significantly with altitude, accounting for a median of 22.6% and up to 41.4% of total benzo[a]pyrene toxicity equivalents at high elevations. These findings indicate that altitude-associated changes in contaminant composition and apparent transfer potential may contribute to differences in toxicological profiles across alpine food webs.