This global map shows the concentration of nitrogen dioxide in the troposphere as detected by the Ozone Monitoring Instrument aboard the Aura satellite, averaged over 2014. (NASA)

BERLIN — When the Eiffel Tower nearly disappeared in a thick haze of smog last year, French authorities decided it was time to act. They limited car traffic in Paris as a last-resort measure. But many argued that much more should have been done much earlier.

Many researchers, who had warned of dangerous pollution levels for years, complained that authorities and politicians started to care only when levels became seriously threatening.

To document pollution levels over a longer period of time, NASA scientists created a series of maps that show the "human fingerprint on global air quality." The images offer insights into air pollution in nearly 200 cities around the world, as well as in neighboring areas.

The good news first: The scientists concluded that the United States, Europe and Japan have greatly improved their air quality. But pollution has worsened in parts of China, India and the Middle East.

The researchers focused their analysis on nitrogen dioxide, which is commonly emitted by cars and industrial plants and is used an indicator of air quality, according to NASA.

To illustrate to what extent pollution has increased or decreased, the NASA researchers measured concentrations between 2005 and 2014. Blue areas indicate a decrease in pollution; orange indicates an increase.

The trend map of the United States shows the large decreases in nitrogen dioxide concentrations from 2005 to 2014. Only decreases are highlighted in this map.

To say that air pollution has decreased in some areas, however, does not imply that perfect levels have been reached. This map, for instance, shows air pollution in the United States in 2014. There are still lots of red areas.

Nitrogen dioxide concentrations across the United States, averaged over 2014.

In western Europe, air quality has generally improved since 2014.

The trend map of Europe shows the change in nitrogen dioxide concentrations from 2005 to 2014.

Cities and areas that experienced a particularly positive trend are London, Paris and Madrid, as well as northern Italy, which is the country's economic powerhouse. In eastern Europe, however, air quality either stagnated or deteriorated in most countries.

Last year, scientists with the International Institute for Applied Systems Analysis predicted that air pollution would increase in many European towns and cities — far exceeding limits by the European Union and the World Health Organization. "Air pollution has a major impact on human health, contributing to lung and heart disease," the scientists warned.

London, for instance, is predicted to have much less harmless levels of pollution by 2030, but eastern Europe's deterioration will continue.

(Source: International Institute for Applied Systems Analysis/Modified by Gene Thorp for The Washington Post)

Modern-day East Asia and Europe are hardly comparable. Much of East Asia's air quality (with some exceptions, including Japan and Hong Kong) has deteriorated in recent years, although pollution was already a major problem by 2005.

The trend map of East Asia shows the change in nitrogen dioxide concentrations from 2005 to 2014.

The same is true for much of the Middle East. Air quality improved in Syria — but that development is most likely due to the ongoing war, which has forced millions to flee.

The trend map of the Middle East shows the change in nitrogen dioxide concentrations from 2005 to 2014.

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