Here’s a suggestion for filling in the blanks:

1. **actively**
2. **release**
3. **pollute**

So the completed passage would read:

"We are all (1) **actively** destroying the earth. The seas and the rivers are too dirty to swim in. There is so much smoke in the air that it is unhealthy to live in many of the world’s cities. In one well known city, (2) **release** poisonous gases from (3) **pollute** the air so much that people have to wear masks."