This is a complex question to answer because only some of the haze-causing particles are directly released into the air. Many haze-causing particles are actually formed in the air when pollutant "precursor" gases undergo a chemical reaction in the sky. Haze-causing particles and their precursor gases come from both natural and manmade sources. Natural sources can include windblown dust, wildfires, "biogenic" emissions from trees and coastal emissions from the ocean. Manmade sources can include motor vehicles, electric utility and industrial fuel burning, and manufacturing operations. In addition, the formation of haze is greatly impacted by factors such as humidity and weather.