Trees produce natural chemicals, called isoprene and terpene, that are thought to protect their leaves from too much heat and sunlight. When the chemicals react with nitrogen oxid.

• 正常情况下，树木是抵御污染的最佳卫士，但是当外界温度过高时，树木自身的防御机制实际上可能会令污染状况恶化。