The correct answers are:
- burning gasoline at a refinery
- releasing fumes at a chemical plant
- emitting particles from a dump truck
- releasing radon gas from a generator
- emitting carbon monoxide from a heater
The outdoor air pollution can be caused by both natural processes and human activities. The human activities though seem to have surpassed the natural processes and the air pollution they cause. The biggest outdoor air polluters are the industry, the combustion of fossil fuels by the vehicles, and the solid fuel burning. There's also natural polluters, like the windblown dust, the biogenic processes of the vegetation, bush-fires, volcanic activities, though all fall in the shadow of the human caused polluters in the past century.
