These words refer to tainting or dirtying something or otherwise impairing its integrity, purity or effectiveness. Pollute now most commonly indicates a thoroughgoing physical befouling that renders something noisome or noxious to health or life.

• Some factories pollute our water supply by dumping untreated chemical wastes into streams and rivers; The fumes from furnaces and motor vehicles have polluted the air we breathe and shortened our life expectancy.

Recent usage of the word has focused so heavily on such instances that other uses are taken as metaphorical extensions of this meaning: political debate polluted by recriminations and unfounded allegations. Actually, an earlier use of the word referred directly to any defiling of sanctity or of physical or moral purity: temples polluted by barbarian invaders.

Contaminate refers to the spreading of harmful or undesirable impurities may be negligible or, as is the case for pollute , thoroughgoing and widespread. In addition, where pollute often indicates readily apparent or grossly visible impurities, contaminate is often the word of choice where the change is slow, devious, unsuspected or not noticeably by ordinary means: an order to burn all linen that may have come into contact with smallpox victims or have been otherwise contaminated ; official denials that the atmosphere had been seriously contaminated by radioactive fallout from the testing of hydrogen bombs. In other uses, the word is milder than pollute in that it can sometimes suggest a temporary tinge rather than an irrevocable stain.

Adulterate has special relevance to food products to which harmful, low-quality or low-cost substances have been deliberately and deceptively added in order to defraud the buyer: sausages adulterated with bread crumbs and food flavouring; milk adulterated with water. Theoretically, the word can apply to other products debased by additives, even where no intent to defraud exists, but such uses are rare. In its widest applications, the word can apply to any mixture of good and bad where the latter seems deliberately included by the producer in the hope that it will go unnoticed: academic curricula adulterated with courses on everything from social dancing to surfboard acrobatics.

The remaining words apply more generally than the foregoing and do not have specific applications to some concrete situation. Pervert indicates misdirecting something, leading it astray or turning it in a wrong direction, as away from the good, pure and moral, and towards what is depraved or evil: television personalities who pervert discussion programmes into freak shows for the airing of lunatic-fringe ideas; a beautiful novel that was wilfully perverted by Hollywood into a sex-and-sadism spectacular. As can be seen, the word suggests a grotesque or hideous transformation; this makes it even more emphatic than pollute in comparable uses, although here complete alteration is indicated, rather than the inclusion of additives: an attempt to pervert the truth by appealing to dishonest, ignorant and irresponsible bigotry. A specific sense of the word once referred to something that caused sexual deviation, but this use is more and more avoided because of its tone of judgement.

Vitiate is the most general word here, having no area of concrete reference. It applies widely to whatever can be seen as completely nullifying the value of something: an overweening arrogance that vitiates all his efforts to make friends. Sometimes the word is used only for impairment rather than complete destruction: an irresponsible act that to some extent vitiates our claim that we are seeking peaceful solutions to the crisis. More often the word suggests that one thing cancles out or invalidates another: pay rises vitiated by the rising cost of living.