London, Jun 17 (IANS): Despite global efforts to improve air quality, half of the world's population is exposed to increasing air pollution, and according to the WHO, more than four million deaths annually can be attributed to outdoor air pollution.

Vast swathes of the world's population are experiencing increased air pollution, says the study conducted in association with the World Health Organisation (WHO).

Air pollution constitutes major, and in many areas increasing, threat to public health, according to the study published in the journal Climate and Atmospheric Science.

"While long-term policies to reduce air pollution have been effective in many regions, notably in Europe and the US, there are regions that have dangerously high levels of air pollution," said study researcher Gavin Shaddick from the University of Exeter in the UK.

In some regions air pollution was five times higher than WHO guidelines, and in some countries it was still increasing, Shaddick added.

Major sources of fine particulate matter air pollution includes inefficient use of energy by households, industry, agriculture and transport sectors, and coal-fired power plants. In some regions, sand and desert dust, waste burning and deforestation add to it.

Although air pollution affects high- and low-income countries alike, low- and middle-income countries experience the largest burden with the highest concentrations seen in central, eastern, southern and south-eastern Asia.

For the study, the research team examined global air quality trends between 2010 and 2016 against the backdrop of efforts to reduce air pollution, both through short- and long-term policies.

The team used ground monitoring data together with information from satellite retrievals of aerosol optical depth, chemical transport models and other sources to provide yearly air quality profiles for countries, regions and the world.

This methodology constitutes a major advance in the ability to track progress towards the air quality-related indicators of the United Nation's Sustainable Development Goals and to expand the evidence base of air pollution's impact on health.

"Although precise quantification of outcomes of specific policies is difficult, coupling the evidence for effective interventions with global, regional and local trends can provide essential information for the evidence base that is key in informing and monitoring future policies," the authors wrote.