Earth Day

4. able to be maintained at a certain rate or level
5. a natural resource or source of energy that is not depleted by use
7. the natural home or environment of an organism
12. our surroundings
13. rubbish such as paper, cans, and bottles left lying in an open or public place
14. convert (waste) into reusable material
15. the production and discharge of something, especially gas or radiation
17. a biological community of interacting organisms and their physical environment
18. the branch of biology that deals with the relations of organisms to one another and to their physical surroundings
19. contaminate

1. reuse (discarded objects or material) in such a way as to create a product of higher quality or value than the original
2. produced without the use of artificial chemicals
3. a colourless unstable toxic gas with a pungent odour and powerful oxidizing properties, formed from oxygen by electrical discharges or ultraviolet light
4. relating to or determined by the sun
6. no longer in existence
8. the disposal of waste material by burying it
9. protect from harm or destruction
10. maintain in its original or existing state
11. the air in any particular place
16. decayed organic material used as a fertilizer for growing plants