Herbicides are chemicals used for weed control. The first chemical weed killer was common salt. Other inorganic substances like arsenates, copper sulphate and sulphuric acid have been tried, but unfortunately these materials also poison the soil and kill beneficial plants. Modern herbicides, although very effective in removing unwanted weeds, are potentially hazardous and the tons of weedkillers used each year in agriculture have deleterious side affects. Many of these chemicals persist in the soil - often affecting the subsequent year's crop - or find their way into domestic water sources. Some herbicides are suspect as carcinogens, mutagens, or they may interfere with embryo development. Older types, such as arsenic, have the added hazards of persistence.
As usually applied, herbicides can create serious problems with drift, killing plants not intended. Even on calm days, sprays of these materials can be detected surprising distances away, and damage may occur a mile or more from point of application. Danger can also arise from the use of lawn conditioners which contain herbicides such as 2,4-D, and 2,4,5-T in granular form. In hot weather these can volatilize and affect susceptible plants nearby.
There is a great number of commercial herbicides available. For example, there are 7,000 formulations of some 130 organic chemicals. Besides these, there are scores of inorganic preparations. Their dangerous properties are mainly those of injury to health and, in some instances, of inflammability.