title: A paradigm of fragile Earth in Priestley's bell jar.
abstract: BACKGROUND
  Photosynthesis maintains aerobic life on Earth, and Joseph Priestly first demonstrated this in his eighteenth-century bell jar experiments using mice and mint plants. In order to demonstrate the fragility of life on Earth, Priestley's experiment was recreated using a human subject placed within a modern-day bell jar.
  
  
  METHODS
  A single male subject was placed within a sealed, oxygen-depleted enclosure (12.4% oxygen), which contained 274 C3 and C4 plants for a total of 48 h. A combination of natural and artificial light was used to ensure continuous photosynthesis during the experiment. Atmospheric gas composition within the enclosure was recorded throughout the study, and physiological responses in the subject were monitored.
  
  
  RESULTS
  After 48 h, the oxygen concentration within the container had risen to 18.1%, and hypoxaemia in the subject was alleviated (arterial oxygen saturation rose from 86% at commencement of the experiment to 99% at its end). The concentration of carbon dioxide rose to a maximum of 0.66% during the experiment.
  
  
  CONCLUSIONS
  This simple but unique experiment highlights the importance of plant life within the Earth's ecosystem by demonstrating our dependence upon it to restore and sustain an oxygen concentration that supports aerobic metabolism. Without the presence of plants within the sealed enclosure, the concentration of oxygen would have fallen, and carbon dioxide concentration would have risen to a point at which human life could no longer be supported.
authors.affiliation: UCL Centre for Altitude, Space and Extreme Environment Medicine, Portex Unit, Institute of Child Health, 30 Guilford Street, London, WC1N 1EH, UK. daniel.martin@ucl.ac.uk.
authors.forename: Daniel
authors.initials: D
authors.lastname: Martin
authors.forename: Andrew
authors.initials: A
authors.lastname: Thompson
authors.forename: Iain
authors.initials: I
authors.lastname: Stewart
authors.forename: Edward
authors.initials: E
authors.lastname: Gilbert
authors.forename: Katrina
authors.initials: K
authors.lastname: Hope
authors.forename: Grace
authors.initials: G
authors.lastname: Kawai
authors.forename: Alistair
authors.initials: A
authors.lastname: Griffiths