Abstract To evaluate the health risk of vacuum ultraviolet (VUV) photolysis of naphthalene (NP) in indoor air, intermediates were detected by gas chromatograph–mass spectrometry and proton transfer reaction-mass spectrometry. Results showed that 13 volatile organic compounds (VOCs) in gas phase and five semi-volatile organic compounds (SVOCs) in oil phase were the main intermediates. VUV photolysis pathways of NP can be divided into five stages including functionalization, partition, condensation, fragmentation, and mineralization. Initially NP was converted into several SVOCs via functionalization by oxidative radicals. SVOCs with high boiling points and polarity groups would partition between aerosol and gas phase. Certain amount of SVOCs in aerosol phase were transformed to oily substances by condensation, which can be washed out by conventional gas washing technique like wet scrubber easily. A majority of SVOCs in gas phase were converted to VOCs by fragmentation, which can be further mineralized into CO2. The accumulation of VOCs, especially highly harmful aldehydes, resulted in an increase of health risk influence index (η) to 150 after VUV irradiation of 2.81min, while the mineralization of VOCs led to a sharp decline of η to 28 after VUV irradiation of 7.01min. It can be concluded that the mineralization of VOCs is a key factor to alleviate the health risk of photolysis. The results will guide a safe and economical application of VUV photolysis technology in indoor air purification.