After entering the cabin he experienced symptoms of rapid pulse and breathing, followed by headaches and dizziness. The following summer two other Forest Service employees reported similar symptoms when entering utility vaults in the Horseshoe area. During the remainder of 1990 some trees appeared to be dying and a larger area of tree death was observed in 1991. By 1992 the area of tree death had reached a size of 15 acres. In 1994 the area had expanded to 28 acres.
Carbon dioxide is heavier than air and tends to collect in low and enclosed spaces.
All of the reports of breathing difficulty have occurred in poorly ventilated spaces not open to the public, The U,S, Geological Survey and the Forest Service have been collecting air samples in the area in both tents and restrooms and have found the levels of carbon dioxide to be well within public health standards. There are plans to intensify the monitoring of the gas. Since carbon dioxide gas dilutes quite quickly once it exits the soil, persons spending time in the open air at Horseshoe should experience normal levels'of carbon dioxide. Trees with potential of striking a campsite, picnic site, car, or restroom are considered hazard trees and must be felled. Hazard trees include only 7-9 acres of the total 30 acres of dead trees.