The market failure in the case of air pollution is what economists call an “externality”: those who pollute do not bear the entire cost. The market failure in the case of health care is what economists call “adverse selection”: insurers may not provide insurance, especially to patients with pre-existing conditions, if they fear that healthy customers have already taken themselves out of the risk pool.
在空气污染方面的市场失灵就是经济学家所说的“外部性”：那些污染者并未承担所有的成本。而在医疗保险方面的市场失灵则可归之为“逆向选择”：如果承保者有理由担心医疗消费者自身已经处于风险之下，那么他们就可能不会提供保障，尤其是对那些已经患病的病人。