Regulators consider the health effects, the feasibility and combined cost of analyzing water for a contaminant and for treating water to remove the contaminant. Standards for drinking water contaminants do not guarantee that water with a contaminant level below the standard is risk-free. Neither do they mean that water with a higher level is unsafe. The standards also do not take into account the possible presence of other chemicals that may increase or decrease the toxicity of the contaminant. With contaminants such as pesticides in drinking water, health officials are almost always concerned about chronic effects, such as cancer or damage to the central nervous system.
Chronic effects result from exposure to a substance over a period of weeks or years.
Acute effects are usually seen within a short time after exposure to the toxic substance. Prior to 1940, most pesticides were inorganic compounds of arsenic, mercury, copper or lead. These compounds seldom are used today. These compounds change very slowly in the soil and do not actually break down. They seldom leach from the soil and, therefore, rarely appear in groundwater.