According to HowStuffWorks, humans who breathe 100 percent pure oxygen can experience several negative affects, including accumulation of fluid in the lungs, chest pain and slowed gas flow across the alveoli, which causes the person to breathe more to retain enough oxygen. Also, the total volume of exchangeable air in the lungs decreases by 17 percent, and mucous plugs collapse the alveoli.
Astronauts in the Gemini and Apollo programs breathed in 100 percent oxygen at low pressure for approximately two weeks without experiencing any problems; however, when 100 percent oxygen is breathed in under high pressure -- more than four times that of atmospheric pressure -- acute oxygen poisoning may occur. The symptoms of acute oxygen poisoning include nausea, dizziness, muscle twitches, blurred vision, seizures and/or convulsions. Such high oxygen pressures can be experienced by military scuba divers using rebreathing devices, divers who are being treated for the bends in hyperbaric chambers, or patients who are being treated for acute carbon monoxide poisoning.
HowStuffWorks states that in a study, guinea pigs were exposed to 100 percent oxygen at normal pressure for 48 hours. The results showed that the guinea pigs had fluid accumulation in the lungs and epithelial lining of the alveoli.