Smog, once an urban annoyance, is now recognized as a health risk, and the automobile has been pinpointed as the principal culprit.

• ??烟尘，原先只不过是一桩使城市居民感到烦恼的事，现在则被公认为危害健康的因素。汽车已被确认为损害健康的罪魁祸首。