They cause mutations. Results of damage to human genes may vary from clubfoot, harelip, reduced immune defence against diseases and minor heart defects to lethal malformations in the foetus. Several substances may mimick e.g. the oestrogen hormone, which is the major component of the contraceptive pill. Carcinogens are substances that may lead to cancer in living tissues. Several synthetic compounds and compounds from oil, like PAH, hydrocarbons and soot are potential carcinogens.
Some toxic contaminants act as allergens.
E.g. substances like dioxins and nickel are known to cause allergic reactions. Some persons become hypersensitive to certain substances and react by rashes, sneezing, asthma, etc. E.g. fish that crowd together in large schools when threatened by predators may forget this instinctual behaviour when intoxicated by pollutants, and thereby make themselves more vulnerable. Several strains of pests or weeds are now more or less unaffected by pesticides or herbicides. Some bacterias have developed resistance to certain antibiotics.