Here are the combined sentences along with their classifications:

1. **Combined Sentence:** The air is polluted, and people often have breath problems.
**Type:** Compound

2. **Combined Sentence:** Factories release harmful gases, and they release harmful gases.
**Type:** Compound (or you could combine it as: Factories release harmful gases.)

3. **Combined Sentence:** The water in the river is polluted, so fish cannot live in it.
**Type:** Compound

4. **Combined Sentence:** Mary wants to help the environment by participating in beach clean-ups.
**Type:** Complex (due to the subordinate clause)

If you need further explanations or variations, feel free to ask!