Ethynylestradiol binds to humic and fulvic acids by H-bonding and π-π interactions. Photosensitization by HS depends on reactive species and extent of binding. Triplet-state HS induces ethynylestradiol degradation by abstracting electrons. Chemicals with high affinity for HS can inhibit ethynylestradiol photodegradation. Ethynylestradiol degradation products are toxic to wheat, rice and Ormosia plants.