I've heard of ground-level ozone but can you explain what it is exactly ?   Yes , it's one of the main air pollutants . Factories don't emit it directly into the air . It's actually formed when nitrogen oxides and VOC emissions come into contact with heat and sunlight .   Sorry ? What does VOC stand for ?   Oh , Vous are ' volatile organic compounds ' . That's a technical term for solvents or other chemicals found in things like paint . 