Determining the health effect of pollutants present at such low levels is extremely difficult. Toxicity is the inherent ability to impair health. Toxic substances can cause cancer, birth defects, and other illnesses. The severity of the effect usually depends on the dose, defined as the weight of the contaminant consumed per day divided by body weight. Dosages are usually measured in milligrams per kilogram of body weight per day.
Acute toxicity refers to a contaminant's ability to cause immediately detectable health problems.
Chronic toxicity refers to a contaminant's ability to cause health problems years after a long-term exposure. Keep in mind that some contaminants that cause no detectable health problems at low doses can cause death at high doses. Nonlethal acute toxicity is easier to diagnose and treat than chronic toxicity since the ensuing health problems are generally brief and reversible. When the exposure to the chemical ceases, so do the effects, provided that the dosage has not been too high. Symptoms of acute toxicity can include diarrhea, nausea, convulsions, blurred vision, and difficulty in breathing.