Humans have added fertilizers to crops for years now. Unfortunately, the nitrates which are in fertilizers are absorbed into groundwater when it rains and become part of runoff which ultimately contaminates nearby bodies of water. This can cause enrichment which degrades the water as algae use these nitrates to multiply, then, die off and bacteria of decay use up oxygen decomposing the dead algae. Lack of oxygen can result in fish kills. Humans have added antibiotics to our chickens and cattle as well as growth hormones. The antibiotics have caused resistant strains of bacteria to emerge that can sometimes be deadly to people if food is handled or cooked improperly. The hormones which cause our animals to grow larger and cows to produce more milk, become incorporated into our own bodies and interact with our cellular activities. The effect of these hormones on the population are currently being studied. We also add preservatives to prevent food from spoiling. The effect of these chemical agents in many cases is unknown and some are linked to causing cancer in laboratory animals. Sometimes our food supply is irradiated to kill off fungi, mold, bacteria which can spoil food.