(おせん) Pollution OR contamination. If you want to say “environmental pollution,” you have to use 汚染, not 公害.   $$$
(こうがい) Pollution. Has a more broad meaning than the related word 汚染. Not just dirty smoggy air, but also noise pollution, the annoyance of someone talking on a cell phone on a train, and 情報公害 (じょうほうこうがい):“ad pollution” – the inescapable advertisements one sees everywhere.