Several studies have attempted to determine the lower limit of atmospheric oxygen under which combustion can occur; however, none have been conducted within a fully controlled and realistic atmospheric environment. We performed experimental burns (using pine wood, moss, matches, paper, and a candle) at 20 degrees C in O2 concentrations ranging from 9 to 21% and at ambient and high CO2 (2000 parts per million) in a controlled environment room, which was equipped with a thermal imaging system and full atmospheric, temperature, and humidity control. Our data reveal that the lower O2 limit for combustion should be increased from 12 to 15%. These results, coupled with a record of Mesozoic paleowildfires, are incompatible with the prediction of prolonged intervals of low atmospheric O2 levels (10 to 12%) in the Mesozoic.