Israeli researchers say using a trio of NASA satellites has allowed them to measure levels of air pollution over the world's largest cities.

On-the-ground monitoring stations do not always provide the most accurate picture of global smog created by traffic, industry and other human activities, they said.

Using eight years of data collected by the satellites, the researchers at Tel Aviv University tracked pollution trends for 189 cities where the population exceeds 2 million.

More than 50 of these metropolitan areas, including New York, Tokyo and Mumbai, have populations that exceed 5 million.

The researchers used data gathered by three aerosol-monitoring satellites, called MODIS-Terra, MODIS-Aqua, and MISR, which NASA launched from 2000 through 2002.

Northeastern China, India, the Middle East and Central Africa are currently experiencing the most increases in air pollution, the researchers said.

Among the cleanest cities were Houston, with a 31 percent decrease over the time period; Curitiba, Brazil, with a 26 percent decrease; and Stockholm, Sweden, with a 23 percent decrease, they said.

Research leader Pinhas Alpert said he believes this satellite-based monitoring method will hold countries accountable for their emissions and encourage more environmentally friendly practices.