Simplify the following text for a Grade 9 native English-speaking person.
Original: An important example of a secondary pollutant is ground level ozone â '' one of the many secondary pollutants that make up photochemical smog . Some pollutants may be both primary and secondary : that is , they are both emitted directly and formed from other primary pollutants .
Simplified: Ozone and smog are secondary pollutants .