oxygen can be found in air & an animal requires oxygen for to breathe ->  an animal requires oxygen in air to breathe  an animal requires oxygen in air to breathe & air quality is a measure of how breathable air is ->  oxygen has a positive impact on air quality  a source of something produces that something & plants are a source of oxygen through photosynthesis ->  plants produce oxygen through photosynthesis  oxygen has a positive impact on air quality & plants produce oxygen through photosynthesis -> plants have a positive impact on air quality by producing oxygen through photosynthesis