Both thick and thin films of pGEMR-3Zf- plasmid DNA deposited on a tantalum foil were exposed to soft X rays (effective energy of 14.8 keV) for various times in air under a relative humidity of 45% (Gamma approximately 6, where Gamma is the number of water molecules per nucleotide) and 84% (Gamma approximately 21), respectively. For a thick film, the DNA damage was induced chiefly by X-ray photons. For a thin film of DNA, X-ray-induced secondary electrons emitted from the tantalum result in a substantial increase in DNA damages. Different forms of plasmid DNA were separated and quantified by agarose gel electrophoresis and laser scanning. The exposure curves for the formation of nicked circular (single-strand break, SSB), linear (double-strand break, DSB), and interduplex crosslink forms 1 and 2 were obtained for both thick and thin films of DNA. The secondary electron enhancement factor for SSBs, DSBs and crosslinks of the thin film of DNA were derived to be 3.8 +/- 0.5, 2.9 +/- 0.7 and 7 +/- 3 at Gamma approximately 6 and 6.0 +/- 0.8, 7 +/- 1 and 3.9 +/- 0.9 at Gamma approximately 21, respectively. This study provides a molecular basis for understanding the enhanced biological effects at interfaces during diagnostic X-ray examination and radiotherapy.