What it is:

• Calcium is called an alkaline earth metal. It forms Ca++ salts, such as calcium chloride (CaCl2) and calcium carbonate (CaCO3), by losing two electrons. Its atomic number is 20 and its most common form has an atomic weight of 40 daltons.         

• Surprisingly, calcium metal is less dense and lighter than magnesium metal even though its atomic weight is much greater.

• Calcium salts and their water solutions are colorless, but the salts burn at very high temperatures with an orange-red flame, making the salts useful in fireworks.



• Calcium is the fifth most abundant element in the earth’s crust. It is commonly found as calcium carbonate (limestone, marble, chalk), which can be converted to calcium oxide (lime) by heating and then to calcium hydroxide (slaked lime) by controlled water addition. Mortar is a combination of lime and sand.

• In water, calcium and magnesium ions are the primary components of hard water.


Health effects:

• Calcium salts have very low toxicity.

• Calcium is an important essential dietary mineral and it is present in substantial amounts in dairy products as well as numerous vegetables like kale, soy beans, broccoli and nuts. Vitamin D3 is involved in the absorption of calcium from the diet and is therefore important for bone building and cardiac function.

• Calcium carbonate is also used in antacids.

• The recommended daily intake (RDI) for calcium ranges from about 1,000 mg for children to about 1,200 mg/day for adults. A person typically carries about 1,200 grams of calcium with about 99 percent in teeth and bone as a durable phosphate compound called hydroxyapatite. 

• Some calcium is also in circulation along with sodium, magnesium and potassium and other ions that all have important functions. Calcium and sodium interact dynamically in controlling cardiac function.

• High calcium intake seems to result in reduced risk of kidney stones.

• Calcium deficiency causes rickets in children and osteoporosis. There is some indication that excess calcium intake (e.g. ~ 2,000 mg/day) can be harmful.       

• Some recent studies have raised questions about the benefits and risks of consuming high dose calcium supplements. Results have been mixed with some studies indicating lower risks of some types of cancer, but others indicating slightly higher mortality.

• There is a long standing hypothesis that consumption of hard water reduces the risk of cardiovascular mortality. Literally hundreds of correlational studies have reported that benefit in the epidemiological literature. However, more sophisticated analyses have led to the conclusion that if there is a hard water cardiovascular benefit effect, it is most likely due to the presence of magnesium, and not hardness per se.



• There are no known direct health based regulations or guidelines for calcium or calcium hardness in drinking water, but excess hardness can cause heat transfer problems with hot water heaters and cooling systems and can interfere with operations and service life.


Water treatment:

• Water softening is, therefore, a common practice both in the home and in larger scale facilities. Precipitative lime softening with calcium hydroxide and lime-soda ash softening are used by municipal water plants that soften. Point-of-use (POU) softening with a cation exchange resin is a common practice. Point-of-entry (POE) reverse osmosis (RO) softening is another option in the home.


Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.