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.
That disparity was greatest in wealthier countries; more than 60 percent of European cities failed to meet the guidelines, compared with less than 20 percent in North America.
Cities in wealthier Asian countries like Japan, Korea and Singapore, the report found, also outpaced Europe, with more cities in compliance.
“The United States primarily has done an excellent job, moving from being a very dirty place in the 1950s to quite a clean place today,” said Dr. Carlos Dora, the health organization’s coordinator for its department of public health, environmental and social determinants of health.
Europe, he added, “has also moved from being extremely polluted,” but it has lagged — a delay that experts have speculated may result from factors that include wider use of fertilizer in urban areas, weaker environmental regulations and the popularity of diesel-powered engines.
Air pollution accounts for more than three million deaths annually from heart disease, stroke, lung cancer and respiratory illnesses, according to the W.H.O. The most dangerous pollutants are particles in the air that are composed of sulfate, nitrates, ammonia, soot, dust and other chemical compounds — a majority of them from human activities like the burning of fossil fuels — and that penetrate deep into the lungs. Pollution also poses a long-term health threat because many of the same emissions contribute to climate change.
The W.H.O. guidelines set limits for two types of particles: fine particles smaller than 2.5 microns in diameter — less than a fraction of the width of a human hair — known as PM 2.5; and slightly larger particles that are smaller than 10 microns in diameter, known as PM10. Both types are considered dangerous to human health at levels above the limits in the guidelines, which are slightly more conservative than those used by government agencies in the United States and other countries.
The new report, an update of the health organization’s database, contains more than double the number of air quality readings than a version released in 2014, and it reflects many of the same trends.
Michael Greenstone, a professor of economics at the University of Chicago who has studied air pollution, said the report released Thursday and others like it were “incredibly important.”
Cities in India, Pakistan, China and Southeast Asia had mean annual air quality readings that were five to 10 times higher than the limits set by the guidelines, the W.H.O. report found, and in some outlying cases 22 times higher or more.
Dr. Dora said that health experts suspected that many African countries would have similarly high readings, but very few cities in Africa monitor air quality.
Scientists know little about the effects of long-term exposure to such extreme concentrations of polluting particles in the air. But Dr. Greenstone said that his own research suggested that the impact was profound.
In one recent study, Dr. Greenstone compared the life expectancy of people who lived north or south of the Huai River in China.
Government policies, he said, provide free coal to cities north of the river for heating, leading to high levels of pollution, while cities south of the rivers are not allowed to operate any heating system, resulting in better air quality. The findings indicated that very high levels of pollution may reduce life expectancy by about two-thirds of a year for every 10 micrograms per cubic meter above the levels considered safe, he said.
The W.H.O. sets the limit for PM10 at an annual mean of 20 micrograms per cubic meter and for PM2.5 at 10 micrograms per cubic meter. Some cities in China register mean annual PM10 readings of 150 micrograms per cubic meter or more and PM2.5 readings in the 50s and 60s. Dr. Greenstone said his study had not yet been submitted for publication or peer review.
In its new report, the health organization also analyzed trends for a subset of 795 cities in 67 countries from 2008 to 2013. During that time, the report found, global air pollution increased by 8 percent, with the highest levels found in the poorest countries.
But Dr. Dora said that the news was not all bad. More cities were monitoring pollution levels, he said, and air quality had improved over that period in more than half the cities in wealthy countries and more than a third of the cities in poorer countries.
“That’s a very positive thing,” he said.
He added that cities in China, whose premier, Li Keqiang, said in 2014 that the government would “declare war” on environmental air pollution, were among those improving.
“They have increased massively their amount of monitoring,” Dr. Dora said. But he added, “It’s difficult to grow and grow and still rely on the manufacturing industry and also clean the air.”
Air quality has also continued to improve in the United States, where the airborne concentrations of one pollutant in particular, sulfate particles, has decreased by 3 percent a year, according to Dan Jacob, a professor of atmospheric chemistry at Harvard.
“Sulphur emissions in the U.S. are mainly from coal combustion,” Dr. Jacob said, “and we have been decreasing those emissions both deliberately and because we’ve been moving away from coal and towards natural gas.”