Breathing pure oxygen can be harmful to the body. Here are some of the effects of breathing pure oxygen:

• Chemical reactions: Breathing pure oxygen sets off a series of runaway chemical reactions that can cause oxygen radicals to harm the fats, protein, and DNA in the body.

• Damage to the body: Breathing pure oxygen can damage the eyes, making it difficult to see properly, and the lungs, making it difficult to breathe normally.

• Oxygen toxicity: Breathing oxygen at higher than normal partial pressure leads to hyperoxia and can cause oxygen toxicity or oxygen poisoning. Extended exposure to above-normal oxygen partial pressures or shorter exposures to very high partial pressures can cause oxidative damage to cell membranes leading to the collapse of the alveoli in the lungs. Symptoms of oxygen toxicity include pleuritic chest pain, substernal heaviness, coughing, and dyspnea secondary to tracheobronchitis and absorptive atelectasis, which can lead to pulmonary edema.

• Brain damage: Inhaling straight oxygen can actually harm the brain. A UCLA brain-imaging study reveals that breathing pure oxygen can cause a drop in carbon dioxide, which narrows the blood vessels, preventing oxygen from reaching tissue in the brain and heart.

• Irreversible lung damage: Breathing oxygen at pressures of 0.5 bar or more (roughly two and a half times normal) for more than 16 hours can lead to irreversible lung damage and, eventually, death.

However, breathing pure oxygen can sometimes be necessary in certain situations, such as for astronauts and deep-sea scuba divers who work in very dangerous places. The length of time they breathe pure oxygen and how much they breathe is carefully controlled so they are not harmed. In hospital settings, 100% oxygen may be delivered, but even then, only on a short-term basis, preferably less than 12 hours.