
79The Role of Greenhouse Gases
ozone, nitrogen dioxide, carbon monoxide, and lead. All of these pollutants
are tied either directly or indirectly to the combustion of fossil fuels. Even
though major efforts are under way today to clean polluted air over cities,
many cities worldwide still lack a healthy air quality. An inventory com-
pleted by the European Environment Agency determined that 70–80 per-
cent of 105 European cities surveyed exceeded World Health Organization
(WHO) air quality standards for at least one pollutant. In the United States,
an estimated 80 million people live in areas that do not meet U.S. air quality
standards, which ...