In 2007 the use of coal and biomass (collectively referred to as solid fuels) for domestic purposes was nearly ubiquitous in rural households but declining in urban homes. At that time, estimates put the number of premature deaths due to indoor air pollution at 420,000 per year, which is even higher than due to outdoor air pollution, estimated at around 300,000 deaths per year. The specific mechanisms for death cited have been respiratory illnesses, lung cancer, Chronic Obstructive Pulmonary Disease (COPD), weakening of the immune system, and reduction in lung function.