I lived in Mexico and visited many countries in Latin America and I can tell you that Costa Rica was by far the cleanest one and also the one with the highest protection of the environment and their incredible natural beauty and biodiversity. Almost 1/3 of the country consists of national park and natural reserves.

As for the heat... I visited Costa Rica in September and it felt much colder and more refreshing there than the scorching Yucatan where we lived back then ;)