European countries, which rely heavily on diesel-fueled vehicles, remain far behind the United States in their efforts to reduce harmful air pollution, according to a report to be issued Thursday by the World Health Organization.

The report, which compiled air quality readings from 3,000 cities in 103 countries, found that more than 80 percent of people in those cities were exposed to pollution exceeding the limits set by W.H.O. guidelines, above which air quality is considered to be unhealthy. And in poorer countries, 98 percent of cities with more than 100,000 inhabitants were out of compliance with the health organization’s guidelines.

Lower levels of pollution were far more prevalent in North America and higher-income European countries than in most other places, especially countries like India, Pakistan and China.

But in Europe, a higher percentage of cities exceeded the limits set by the W.H.O. than in North America.