Seeing the (end) effects doesn't matter. Everyone sees pollution directly, whether the simple knowledge that the sea of cars on the freeway in front of them is burning gas, or just seeing litter all over the place. "Pollution" is a tangible thing everyone experiences.

Telling people there is a pollutions epidemic, that is something they can immediately relate too. It doesn't matter that the smoke stack they drive past every day is just water vapor, they hear "pollution epidemic", they see what they interpret as pollution, their brain tells them "yeah, this is real".

Remember this phrasing isn't for educated or knowledgeable people, it's for common folk who don't put much thought into anything.