Significance Air pollution via phototoxic polycyclic aromatic hydrocarbons (PAHs) is a major risk factor for human health. While in vitro observations and in vivo correlations suggest a detrimental effect of PAHs at physiological concentrations, in vivo observations of the structural impact of PAHs are scarce. Here, we use transmission electron microscopy on human hair fibers containing known concentrations of 25 biomarkers of PAH exposure. We show an increased structural degradation of the hair fiber over time, when increased PAH concentrations are present. Moreover, we show that exposure to UV radiation explains part of the increased damage in more contaminated fibers. Our results point toward possible detrimental effects in other human tissues at physiological concentrations of PAHs.