General:
* Contamination
* Soiling
* Taint
* Defilement
* Impurity
* Adulteration
* Poisoning
* Contamination
Specific to different types:
* Air pollution: Smog, haze, fumes, exhaust
* Water pollution: Contamination, sewage, runoff, oil slick
* Noise pollution: Din, racket, clamor, cacophony
* Light pollution: Glare, illumination, artificial light
* Plastic pollution: Debris, litter, waste
More abstract:
* Degradation
* Desecration
* Blight
* Spoilage
The best synonym to use will depend on the specific situation and what you want to emphasize.