In environments with ultra-low humidity, workers may have excessive body water loss due to evaporation through the skin, which can lead to dehydration. Before the development of clinical symptoms and signs, concentrated urine may be applied as an early indicator of dehydration. We used urine specific gravity (USG) as a biomarker to evaluate the hydration status of workers working in such an environment. We collected the urine samples from workers at a lithium battery plant during their annual health examination, and the relative humidity of some working areas called "dry rooms" in the plant was 1.5 +/- 1%. We recruited workers in those dry rooms as the exposure group (N=50) and defined the remaining workers, including administration office workers, as the comparison group (N=122). The prevalence of abnormally concentrated urine (USG>1.030) and related factors were compared between these two groups. While the exposure group were younger and had shorter employment durations compared to the comparison group (p<0.05), they had a higher prevalence of abnormally concentrated urine (p<0.01). After adjusting for age, gender, employment duration, and body surface area, we found that working in the dry rooms was associated with an odds ratio of 11.9 (95% confidence interval: 2.5 to 56.9) of having abnormally concentrated urine. Therefore, USG is a good biomarker for evaluating the hydration status of workers working in ultra-low humidity environments, who need proper protection and adequate fluid supply to prevent excess water loss and its adverse health effects.