Lead and mercury are toxic heavy metals.
Heavy metals can cause a wide variety of health problems and symptoms. Some examples (there are many others):
- Kidney damage
- Nervous system damage
- Reproductive system damage
- Higher blood pressure
- Headaches
- Lethargy
- Emotional changes
- Muscle weakness
It takes a longer term exposure or a larger short term dose to cause some of the more severe effects, but in those cases heavy metals present a very dangerous health risk.
Possible sources of lead contamination:
- Pipes from older buildings (especially before 1950)
- Paint containing lead
- Electrical solder
- Car batteries
- Fishing weights
- Bullets
Possible sources of Mercury contamination:
- Fluorescent light bulbs
- Mining
- Dental operations
- Manufacturing
- Waste incineration
Other Toxic Heavy Metals
Arsenic (which is actually a metalloid) and cadmium are also commonly grouped with lead and mercury. They can be introduced into water in similar ways such as industrial activity, mining, and waste incineration.
Many other elements and minerals can cause health hazards in our water. Wikipedia’s Metal Toxicity article lists many of those.