High levels of carbon dioxide concentrations in caves can lead to reerosion of stalactites and health hazards to humans. For this reason, the international guidelines for show caves recommend atmospheric observations. Akiyoshi-do Cave, located in Yamaguchi prefecture, Japan, is one of the most visited show caves in the world and has been designated as a Ramsar Convention wetland, Special Natural Monument, and Japanese geopark. However, few studies have investigated the atmospheric environment in the cave and the physical and psychological effects of the atmospheric conditions on tourists. There has been insufficient discussion of tourists’ health during tours. Therefore, the purpose of this study was to clarify the actual microclimate inside Akiyoshi-do Cave and to understand the effects of that microclimate on the physical and psychological conditions of tourists. Fixed-point and moving-point atmospheric observations were conducted in the cave in August 2018 (summer) and February 2019 (winter), when the air temperature, relative humidity, and carbon dioxide concentrations were measured. In the summer survey, the temperature fluctuation was within 1°C and the relative humidity was greater than 94%, indicating that the influence of advection and mixing by the outside air was small. The carbon dioxide concentration exceeded 1,000 ppm and approached 2,000 ppm during the summer survey. Such high concentrations of carbon dioxide can cause headaches and other health hazards. The distance between tourists was calculated to be less than 0.45 m in the summer survey, indicating that they were exposed to the risk of intrusion into the intimate space by neighboring tourists, which causes discomfort. During the winter survey, the carbon dioxide concentration stabilized at around 500 ppm; tourists did not intrude into intimate spaces. Therefore, in addition to atmospheric conditions such as exceeding natural variability, human aspects such as physical and psychological health hazards should also be considered as an important factor when calculating the appropriate tourist capacity. Additions and revisions to the international guidelines on the need for atmospheric observation data-based tourist safety considerations will contribute to the improvement of future show cave management. Although this was the first study to quantify the differences in the atmospheric conditions in summer and winter in Akiyoshi-do Cave, continuous monitoring over several years is needed to gain sufficient information on the microclimatic conditions of the cave and their impacts on tourists’ health.

This study extracted the summits of world mountains located between 60°N and 60°S and evaluated their steepness based on the relief and slope around the summits using approximately 30-m digital elevation models (DEMs). The summit was extracted if the center point of a 1-km radius was the highest in the circle. The relief of the summit was calculated from the difference between the highest point (summit) and the lowest point of the 1-km circle. The average slope of the summit was determined for the 1-km circle from the 30-m grid slope angle raster data derived from the 30-m DEMs.

The summits of the world were classified into types I, II, or III; type I was further classified into types Ia–Id. Extremely steep summits, i.e., types Ia (relief higher than 2,000 m and average slope steeper than 50°) and Ib (relief higher than 1,500 m and average slope steeper than 45°), were mostly located in the Himalayas. Several Ib summits were distributed in the Alps, Tianshan, and Andes ranges. Steep summits, i.e., types Ic (relief higher than 1,000 m and average slope steeper than 40°) and Id (relief higher than 500 m and average slope steeper than 35°), were distributed in the mountains of the Alpine-Himalayan belt, Pacific Rim, and Central Asia and African Rift valleys. The distribution of type I summits indicated the importance of active tectonism in their steepness. High-relief and gentle-slope summits, i.e., type II (relief higher than 500 m and average slope less than 35°), were distributed in the mountains of the passive margins in addition to type I mountains. Low-relief and steep-slope summits, i.e., type III (relief lower than 500 m and average slope steeper than 35°), were mostly located in the tower karst region of southern East Asia, showing the significant specificity of tower karst landforms in this region.