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