To find out the harmful effects of tap water, complemented by a look at the hazards of hard water.
The hardness of water is determined by the amount of calcium and magnesium found within it.
Hazards of hard water include property damage (e.g., stained bathtubs, scales in the sink) and sometimes damage to personal property like clothing.
Hard water is also associated with causing appliances to break (limescale).
In a 2003, heavily-cited document, the World Health Organization noted the following: "There does not appear to be any convincing evidence that water hardness causes adverse health effects in humans. In contrast, the results of a number of epidemiological studies have suggested that water hardness may protect against disease. However, the available data are inadequate to prove any causal association."
However, other sources give pushback about this WHO statement and claim hard water does have adverse effects on health (e.g., dry skin, eczema, skin irritation, coarser hair texture, thinning hair/hair loss, and kidney stones).
Skinirritation appears to be the most noted negative effect of hard water.
Hard water is also noted to be less effective at cleaning because calcium and magnesium ions in the water react to chemicals in the soap resulting in scum and less cleansing potency.
The overall health effects of hard water are mixed. Some studies show correlation with childhood eczema, others show benefits of hard water being protective against gastric, colon, rectal, esophageal and ovarian cancers. Still, others correlate it with kidney stones, and no source definitely says it is bad for humans.
Only the project owner can select the next research path.