sentence: People also add to it.
sentences_before: Dust and dirt in the air come from natural sources such as volcanos, erosion and meteoric dust.
sentences_after: Some of this dirt is large, so it falls out within a few hours. Some is so small that it could stay in the air for years. This big dirt is still very small. It's about the width of a human hair. The big dirt does not stay in the air for years though because rain hits it and brings it to the surface.
concept_name: People
headings: Global warming Temperature changes Dust and dirt
categories: Energy Climate change Air pollution