Chemical pesticides and fertilizers came into common use during the 20th century both in home gardening and commercial agriculture. However, as synthetic substances, these products can cause problems if not used properly. When using these types of products, it is important to carefully follow the manufacturer's instructions. However, following these instructions may not abate all risks and dangers.
Pesticides were the basis for a number of nerve agents that were eventually used in war. German pesticide research resulted in the development of tabun and sarin nerve gas. Some modern pesticides are based on biological activity similar to banned chemical weapons. As such, all pesticides must be used with extreme caution. Whether an agent is lethal or not will depend on the level of exposure. High exposure can result in acute health problems or death. Fertilizers, although not as toxic, can also cause health problems if ingested or breathed in high enough quantities. The key to avoiding these dangers is to follow all safety instructions. However, chemical pesticides and fertilizers can be toxic to pets and other animals in much lower concentrations.
Some pesticides can have a long residual life. This can mean that foods sprayed with pesticides are still present on fruit and vegetables. Although the quantity of pesticides on fruit and vegetables is generally not considered harmful, there is very little credible research showing either that they are harmful or harmless. However, some pesticides break down into other byproducts, like 3,5,6-trichloro-2-pyridinol (TCP). The FDA has set a limit on the amount of TCP that can be present in food to prevent potential health issues. TCP is a probable carcinogen and an endocrine disruptor.
Residual exposure to chemical fertilizers usually occurs when a child or animal comes into contact with treated earth. Some fertilizers are ammonia based and can break down into products that may be lung irritants. In general, commercial agriculture uses ammonia in fertilization. However, most home-based chemical fertilizers will have few issues with residual exposure.
Environmental persistence of pesticides is measured in half-life. Half-life is the amount of time it takes for 50 percent of the pesticide to break down. Applying pesticides too often can cause them to build up in the environment. This happens when pesticides are applied faster than they can break down. Over-fertilizing with chemical fertilizers can cause both phosphorous and nitrogen runoff problems. Phosphorus is responsible for increased growth of algae and weeds in lakes, which changes the balance of nutrients in lakes and can adversely affect food chains. Excess nitrogen can move into lakes and into the water table and can result in contaminated water supplies. Drinking water with excessive nitrates can reduce the blood's ability to transport oxygen. Infants who drink water high in nitrates can appear bluish because of a lack of oxygen in the blood.