1.

Factories are burning extremely dangerous chemicals.

BY

Extremely dangerous chemicals are being burned BY factories.

2.

Smog has endangered people's lives in many large cities.

AS A RESULT OF

People's lives in many large cities have been endangered AS A RESULT OF smog.

3.

Global warming has damaged plants and wildlife.

THROUGH

Plants and wildlife have been damaged THROUGH global warming.

4.

Reduced rainfall is creating bigger and bigger deserts.

BECAUSE OF

Bigger and bigger deserts are being created BECAUSE OF reduced rainfall.