Can't get pollution level down below 40?

How do you do this?

Place gardens or flowers near your buildings.

Gardens and flowers work. So do fountains, statues, cabanas and gazebos. The items in the Ecology tab seem to help mitigate pollution also. It is a balance between the things you have that pollute and the things you have that dampen the pollution. Sometimes overdeveloping (having more available than your goals say you need) will send you over the edge in terms of pollution and it will be hard to get back. ALSO, big one: pick up the trash (rubbish)! It can take you down several pollution points. You ahve to stay on it though...Good luck!

I found that having more housing than food or fun helps keep pollution down as well. Im stuck on getting 150 tourist, but Im almost there.