The Philippine Department of Environment and Natural Resources (DENR) conducted a national emissions inventory
of air pollution sources in 1990. Updates to the inventory were made in 2001-2004 and then in 2006. The Philippine
Clean Air Act of 1999 (Republic Act 8749) requires an emissions inventory to be conducted once every three years.
The emissions inventory includes criteria pollutants such as particulate matter (PM), sulfur oxides, nitrogen oxides,
carbon monoxide, and volatile organic compounds. Based on the 2006 National Emissions Inventory, the transport
sector is the major source of air pollution in many regions of the country. It was estimated that 65% of the pollutants
came from mobile sources, 21% from stationary sources, and the remaining 14% from area sources.10 Motor vehicles
and open burning are identified by several cities as major sources of air pollution (Table 2.2.1).