Lead poisoning can cause symptoms that range from a vague feeling of ill health (malaise) to vomiting, stupor and convulsions.  Recent studies suggest that long-term exposure to lead may cause hypertension. Lead also affects behavior and academic performance in school.  Extremely high levels of lead may cause headaches, abdominal pain, loss of appetite, constipation and decreased activity.  The main route of poisoning is through the digestive system though lead can enter the body through the respiratory system.

Mercury poisoning is most likely to occur as a result of eating methyl mercury-contaminated fish.  Large marine (ocean) fish such as tuna, swordfish and shark are most likely to have elevated levels of methyl mercury because they eat other fish who may be contaminated.  Symptoms of mercury poisoning vary widely, depending on the source and exposure however the nervous system and kidneys are usually affected.

Arsenic poisoning may occur not only through ingestion (eating or drinking it), but also by direct contact (touching it) or by inhalation (breathing it).   A major use of arsenic is as a wood preservative.  It is often found mixed with zinc and other metals during mining operations and is also a common ingredient in pesticides and other poisons.  Some of the common health effects of arsenic poisoning are nausea, vomiting, stomach pain, diarrhea, dizziness, weakness, excess salivation and listlessness.  Arsenic can also cause Blackfoot disease, skin cancer, bladder cancer and lung cancer.

Nitrates themselves are not toxic to people, but infant's digestive systems are much more likely to contain bacteria that can transform nitrates in to nitrites. Nitrites change the oxygen-carrying component of blood called hemoglobin into methemoglobin, a substance that does not carry oxygen.  The result is methemoglobinemia or blue-baby syndrome where the infant turns blue and is deprived of oxygen.   It is often fatal.  The most common way for babies to ingest nitrates is through being fed formula that has been mixed with fertilizer-contaminated well water.