The presence of humans alone is enough to influence the microclimate of a cave. Carbon dioxide and water are expelled by groups of cave visitors, and as caves are closed systems, these gases accumulate. Banbury and de Freitas found that carbon dioxide builds up in caves over time, and can reach levels that are dangerous for humans. It was also found by Sarbu and Lascu that increased condensation due to increased water vapour has been shown to degrade speleothems. The temperature of a cave can also be greatly altered by the energy given off as body heat and from lights present in the cave system.