The reference to “great pollutions” refers to moral turpitude, not modern air quality. Acts 15:20 uses the term similarly: “Abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.” The “pollutions” are contaminating evils caused either by idol worship or by secondary practices stemming from idol worship. The Book of Mormon defines these evils as “murders, and robbing, and lying, and deceivings, and whoredoms, and all manner of abominations.”