The best way to reduce human fertility (without child abandonment and forced abortions and other human rights horrors) is to educate the population and make them wealthy... at least to somewhat middle class standards. Then fertility drops to replacement level or near it, sometimes even below. This has been validated countless times in many cultures and seems to be a general pattern in human behavior.

China is slowly doing this, and will do so better if it can get its political corruption issues under control.

China's pollution is definitely a problem, but that stems mostly from corruption, lack of concern, and heavy industry that is largely done in service to customers not in China. The one child policy would not impact this much.