In addition, outdoor pollutants are also traced in indoor spaces, such as radon (a
natural trace component of soil and ground). The major source of indoor air pollution is
household combustion of coal or biomass for cooking and heating. It is estimated that more
than half of the world’s population relies on animal dung, wood, crop waste, or coal to meet
their most basic energy needs
like power plants, incinerators, landfills for waste deposition