pollution

air ammonia atmosphere black carbon cfc cfcs change chemical chemicals chemistry city civilization clean climate contaminate contamination dioxide dioxins dirty dust earth eco ecological ecology ecosystem emission emissions energy environment environmental exhaust factories factory filthy fine fuel fuels fume fumes gas gases green harmful haze impact industrial industries industry methane metropolis nature nitrogen oxide particle particles planet pollutant pollutants pollute polluting pollution problem quality reduce reduction small smog smoke society sulphur sustainability sustainable town toxic unhealthy urban