Drinking-water, including most bottled varieties, is likely to have contaminants. That doesn't mean all water is unsuitable to drink; some levels of contamination are considered safe.
Turbidity is a measure of how cloudy water is. Although it doesn't cause illness itself, it provides the means for microbial growth and can obstruct disinfection. High turbidity levels are often associated with high levels of microorganisms.
Bacteria, protozoa and viruses are among the oldest threats to drinkable water and are responsible for most waterborne diseases today.
From bacteria like fecal coliform and E coli, which indicate human or animal waste may be present, to parasites such as cryptosporidium and giardia lamblia (also found in human and animal feces), microorganisms can cause a lot of trouble. Common gastrointestinal illnesses caused by microorganisms are diarrhea and cramps, nausea, vomiting and related headaches. Infection can be severe or even fatal in people who have weakened immune systems.
To clean water disinfectants (often just more water contaminants themselves) are added during water treatment to get rid of microorganisms. High-end UV light purifiers can kill microorganisms, as can reverse osmosis. Some of the better activated carbon filters can reduce the prevalence of microorganisms in your drinking water.
A long list of organic chemicals, including benzene, PCBs and vinyl chloride, can enter drinking water, potentially causing damage that includes increased cancer risk; liver, kidney, stomach, nervous system and immune system problems; reproductive difficulties; cataracts; and anemia. The water-contaminating chemicals come from farm runoff of herbicides or pesticides, leach from storage tanks and landfills and are discharged from factories. Many are known to be volatile organic compounds. Some continue on in the environment, as residue from pesticides that have long since been banned (like DDT) and others are added to the water during waste treatment (like chlorine).
The ABCs of water contaminants come from inorganic chemical sources: arsenic, barium and chromium, along with lead, antimony, mercury, cyanide, asbestos, copper and more, are mostly metals and salts. Many occur naturally, but they can also be introduced through human activities. Lead can leach into water from lead pipes, and fluoride is added intentionally to assist in dental care. With any of these contaminants, amounts greater than certain, acceptable levels can have unfavorable side effects.
Illnesses caused by overexposure to these contaminants can be relatively tame, such as diarrhea and staining or pitting of the teeth, or very serious, such as liver, kidney, bone and heart damage, nervous system effects and cancer.
Many organic and inorganic water contaminants can be eradicated or reduced through reverse osmosis or distillation. Make sure to find out what contaminants a water filter can handle before you purchase it.