The pollutants often occur with high levels of other toxins such as lead, zinc, and ground-level ozone, a component of smog formed by chemical reactions between car exhausts and sunlight.

• 这些污染物常常与其他有害物质，如铅、锌和地面臭氧一起出现。地面臭氧是汽车排气与太阳光一起发生光化学反应形成的一种烟雾成分。