Blanketing around 100 million people, primarily in the Gangetic Valley, the pollution levels are about five times larger than those typically found over Los Angeles, according to US-based researchers.

The discovery was made after analysing four years of data collected by the Multi-angle Imaging Spectro-Radiometer (MISR) onboard the Terra satellite. Lofted into orbit on December 18, 1999, Terra is the flagship of NASA's Earth Observing System Programme.

While high pollution levels were found over much of India, a concentrated pool of particles was discovered over Bihar, a largely rural area with a high population density. A large source contributing to pollution pool is the inefficient burning of a variety of bio-fuels during cooking and other domestic use.

Particles in the smoke remain close to the ground, trapped by valley walls, and unable to mix upward because of a high-pressure system that dominates the region during winter.

"The pollution pool must be having a tremendous impact on the local climate in Bihar and the health of the approximately 100 million people that reside within this pool. Our long-term goal is to better predict the occurrence of these pollution episodes and their impact on public health and local climate," says Mr Larry Di Girolamo, a co-investigator on the MISR mission. This study is the most comprehensive and detailed examination of industrial, smoke and other air pollution particles over the Indian subcontinent to date, and reveals how topography, meteorology and human activity help determine where these particles are concentrated.

MISR is the first instrument to make high-resolution, multi-angle radiometric measurements of Earth from space. By measuring reflected sunlight at nine angles, the amount of particulate matter, including that generated from man-made pollution, in the atmosphere can be determined.

The result is a pollution episode that can affect both human health and local climate, Mr Di Girolamo said. The airborne particles can damage delicate lung tissue, and by altering the radiative heating profile of the atmosphere, the particles may change temperature and precipitation patterns.

Prior to the MISR study, atmospheric models had predicted a `tongue' of pollution extending across the middle of India. The MISR observations, however, show the pollution lies much farther North.

These models are used to forecast pollution episodes and climate change. The role of airborne particles remains one of the largest uncertainties in atmospheric modelling.

In addition to modifying local climate, the particles can interact with clouds and change the cloud properties. This is particularly important, since clouds have the greatest radiative forcing on the climate system.