It is crucial to understand the various biological effects induced by terahertz (THz) electromagnetic waves with the rapid development of electronic and photonic devices operating in the THz frequency region. The presence of sweat glands plays an important role in THz wave interactions with human skin. We investigated the morphological features of sweat ducts using optical coherence tomography (OCT) to further understand such phenomena. We observed remarkable features of the ducts, such as their clear helical structure. The intersubject and intrasubject variations in the diameter of sweat ducts were considerably smaller than the variations in other structural parameters, such as length and number of turns. Based on the sweat duct dimensions and THz dielectric properties of skin measured using terahertz time-domain spectroscopy (THz-TDS), we calculated the resonating frequency of the sweat duct under the assumption of it functioning as a helical antenna. Here, we show that the resonance frequency in the axial mode of operation lies in the THz wave region with a centre frequency of 0.44 ± 0.07 THz. We expect that these findings will further our understanding of the various health consequences of the interaction of THz waves with human beings.

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Owing to the rapid development of electronic and photonic devices operating in terahertz (THz) frequency region, it is crucial to understand the non-thermal effects of these electromagnetic waves on human body. In the past, it was reported that human sweat duct works as a helical antenna in terahertz frequency region and it plays an important role in the interaction of THz wave with the skin. In order to further understand the effects of these waves, it is essential to know the morphological features of the sweat ducts. In this work, we analyzed the density and dimension of the ducts based on three dimensional images obtained with optical coherence tomography (OCT). We observed that the sweat ducts possess helical structure and the diameter of the ducts from different regions of the body show considerable resemblance. Based on this information, we numerically determined the resonating frequency of helical ducts which resulted in the terahertz frequency region spanning from 0.3 THz to 0.7 THz.