Medical X‑ray Radiation Drives Chemodiversity of Indoor Organic Aerosols

While ultraviolet-driven photochemistry influences organic aerosols in typical indoor environments, the impact of high-energy medical X-raysdespite their greater energy and direct health relevanceremains unexplored in radiotherapy rooms. Here, we employed Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to characterize the organic molecular compositions of fine particles (PM_2.5) in three radiotherapy rooms and adjacent waiting areas. Results revealed a 1.3-2.3-fold increase in molecular formulas under X-ray exposure, driven by fragmentation-oligomerization cycles. Diurnal oxygen-to-carbon (O/C) ratios varied from 0.31 to 0.35 in the daytime and from 0.53 to 0.61 at night in three radiotherapy rooms, which indicate a light-modulated oxidation. Critically, PAH precursors, dominated by CHO/CHON species with double bond equivalence (DBE) ≥ 10, were enriched by 1.31-2.83-fold in radiotherapy environments. These compounds correlated strongly with oxidative stress biomarkers, implying potential health risks. Mechanistically, fragmentation and oligomerization prevail, likely enhancing gas-phase oxidation and particle-phase dimerization. Our findings necessitate air purification targeting reactive organics in medical radiation facilities to mitigate exposure risks.