Air Quality

 

Cave Air Quality

Mammoth Cave and surrounding area caves have natural airflow patterns that have allowed for tourism, research, and exploration for centuries. Over this time, human activity has changed airflow in some areas. The park continues to monitor and mitigate these changes and their impacts in order to protect the ecosystem, cultural history, and visitors of Mammoth Cave.

 

On June 13, 2021, a large and unexpected storm come through the park which caused a quick drop in the temperature. This pressure change caused the airflow to reverse, pulling into the cave, resulting in a dense fog filling the Rotunda room inside the Historic Entrance of Mammoth Cave.

NPS Photo/ Rachel Kem

Cave Airflow and Temperature

Air within the Mammoth Cave system and other caves in the park is in a constant state of movement and exchange. This exchange is caused by weather and climactic conditions on the surface and is driven in the cave by a process known as convection.

Convection causes air to move between cave passages, rooms, and in and out of the cave itself. Convective air movements are driven by differences in air temperature, warmer air is less dense than cooler air causing it to be more buoyant. It’s this same buoyancy difference that allows hot air balloons to float in the surrounding, cooler air.

During winter, this convective movement of air into Mammoth Cave is particularly impactful. Relatively warmer air in the cave rises and escapes the cave through small cracks in the rock layers. This movement of relative warm air in the cave then causes cold air from the surface to flow into the cave via openings like the Historic Entrance, thereby cooling those passages and rooms. During the summer, as hot humid air on the surface rises in the atmosphere, cool air inside the cave is drawn out of openings, reversing the air movement patterns observed in the winter.

 

Air Movement as a Form of Navigation

Cave crickets that live within the dark passages of Mammoth Cave use their long antennae to feel slight changes in air flow as a way to navigate and identify ways out of the cave.

 

Human Impacts

Over the course of recent history, openings into Mammoth Cave and other caves in the park have seen several human modifications. These modifications have resulted in altered air temperature and airflow patterns in the caves.

Human modifications to the Historic Entrance area in the early 1800s caused changes to the air flow and temperature in much of the Historic Section of Mammoth Cave. As a result, this section of Mammoth Cave no longer supports the hibernation of a significant number of bats, as it once did. The altered air temperature and airflow patterns have also resulted in air mixing at inopportune locations to result in significant condensation that falls onto wooden artifacts. These artifacts can then become pock-marked from the impact of repeated water droplet strikes or can be damaged from mold, mildew, and rot.

 

Weather monitoring equipment in Houchins Narrows.

NPS Photo/ Tegan Sorensen

Monitoring

Park scientists continue to collect data on air temperature and airflow patterns in Mammoth Cave. A variety of cave meteorology data, including air temperature, relative humidity, and wind speed/direction is collected at multiple locations throughout the Historic Section of Mammoth Cave.

The overall goal of this monitoring is to understand the effects of human altered entrances to the cave system. This understanding is needed in order to prescribe changes that can return air temperature and airflow patterns to a state that will support cave life and artifact preservation.

Radon

Like most caves, the caves in the region are formed in rock layers that contain trace amounts of uranium. As the uranium breaks down a process called radioactive decay occurs. This process produces a gas known as radon gas, which is released from the rock into the cave passages. Radon gas can cause health risks, such as lung cancer when inhaled over a long enough period of time and in sufficient quantity.

The radon levels in the air of caves in the park is not sufficient to be of concern to visitors on cave tours, but the park’s employees, such as those who lead tours on a regular basis, are considered to be at some level of risk. As such, OSHA requires the monitoring of:

• radon levels in areas frequented by employees
• the amount of time that each employee spends in those areas (for employees who spend more than a few hours per week in the cave system).

The results of that monitoring are used to ensure that the health of employees is not put at risk in the execution of their duties.

 

Using Cave Air to Cool Buildings

In 1959, during construction of the old park visitor center, a ventilation shaft was constructed connecting to the cave. This shaft was designed to use the naturally cool cave air to air-condition the visitor center building and park administrative offices.  

 In the mid 1970s, with scientific advancements, the issue of radon in caves of the National Park System came to the forefront. When elevated levels of radon were discovered in the air at Mammoth Cave it became apparent that having people work all day in a building that was air-conditioned with cave air, resulting in their prolonged exposure to elevated radon levels, was a bad idea. 

 As a result, in July 1976 the cave ventilation shaft was sealed off from the visitor center to stop flow of cave air to the building. The location where the air shaft connects to the cave is seen on several cave tours today. 

 

Surface Air Quality

Air quality on the parks surface is actively monitored by the National Park Services Air Resources Division (ARD). The ARD monitors current and long-term trends of air quality throughout the National Park System. To learn more about the air quality on the surface of Mammoth Cave National Park, view the park air profile complied by the ARD.