Today, we know that some gases such as O3, carbon monoxide, nitric oxide and carbon dioxide can have dual actions, behaving either as useful or as harmful agents.

• 现在，我们知道像超氧O3，一氧化碳，氮的氧化物和二氧化碳这样的气体，在使用上或许有害但皆可表现出其功效。