Chlorine & Chloramine
Many cities disinfect water with chlorine.This is basically bleach folks. While this process is vital to ensure clean drinking water free of microorganisms that can make many of us very sick, chlorine carries its own riskincluding potentially causing bladder and rectal cancer. Many municipalities are investigating other means for cleaning water and are switching to chloramine. Chloramines are most commonly formed when ammonia is added to chlorine to treat drinking water. Unlike chlorine which can dissipate as it travels through pipes, chloramine remains active in the water (and on your body). This helps to kill bacteria more effectively in the water, however chloramine also carries risks including bladder cancer, linkages to liver, kidney, central nervous system problems, and reproductive effects, and anemia. (source)
While I realize that we do need to be protected from some serious microorganisms and that we are extremely lucky to live in a country with clean drinking water, I personally do not want my family drinking and bathing in what is effectively bleach and ammonia on a daily basis.
Fluoride
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Pesticides, Pharmaceuticals and Heavy Metals
There can be many trace chemicals and pharmaceuticals contaminating your water – things like prescription drugs, lead, heavy metals, pesticides, and even asbestos. According to a National Academy of Sciences report, the residues of 39 pesticides and their degradation products have been detected in the groundwater of thirty-four states and Canadian provinces. We try so hard by eating organic fruits and vegetables to avoid consuming pesticides and they are most likely in our water!
source: https://www.homemademommy.net/2013/08/why-you-need-a-water-filter-in-your-home.html