A Staff Report from the Straight Dope Science Advisory Board

What's the difference between hard water and soft water?

January 27, 2004

Dear Straight Dope:

I live in New Orleans, Louisiana and I shower daily. From what I hear, we're recipients of "hard water." On trips out of state and such I've been privy to "soft water." I do notice a difference in the "thin layer of residue" that Zest commercials refer to, but what really is the difference between these two types of water?

Dear Straight Dope:

We have a water softener in our house and I hate the way my skin feels after showering using soft water. I never feel quite clean. What specifically does a water softener do to the water and what properties for soaps should I be looking for, for use with soft water?

Hard water contains higher-than-ordinary levels of dissolved minerals (strictly speaking, dissolved positive metallic ions) such as magnesium and calcium. Water from underground aquifers - well water, for you civilians - has extended contact with soft calcium and magnesium-bearing rock and mineral deposits such as limestone, chalk, dolomite, and marble, and small amounts of minerals are dissolved into an ionic solution in the water. Often the dissolution process is facilitated by carbon dioxide in the water, which forms a weak carbonic acid. Surface water generally doesn't contain enough calcium and magnesium to be considered hard, although there can be exceptions.

Water hardness typically is expressed in terms of dissolved calcium carbonate - in units of mg/l, ppm, or grains/gallon. The U.S. Department of the Interior and the U.S. Geological Survey (USGS) define 60 mg/l or less as soft water. Water with over 120 mg/l is considered hard, and water in between is moderately hard.

Water hardness varies a great deal regionally and even locally depending upon water sources and geology. The USGS graph at http://water.usgs.gov/owq/chart.jpeg shows "mean values of hardness at 344 stations during the 1975 water year." The USGS map at http://water.usgs.gov/owq/map1.jpeg shows the distribution of water hardness throughout the United States.

The principal disadvantage of hard water is that the dissolved ions react with the chemicals in soap to create a sticky scum or curd. Most soaps are made from compounds of sodium and potassium such as sodium stearate. Sodium stearate reacts with calcium compounds in the water to produce calcium stearate. Sodium stearate dissolves in water, but calcium stearate doesn't, meaning you get less lather and more gunk. (I realize some soaps are formulated to produce ample lather even in hard water, but I'm generalizing here.)

The upshot is that hard water doesn't wash as well as soft water. Clothes and sheets washed in hard water can feel rough, even harsh to the touch The Culligan water softener company even claims, "A Purdue University study found that fabrics washed in hard water tend to wear out as much as 15 percent faster than fabrics washed in soft water." (They cite a paper entitled, "Benefits of Using Soft vs. Hard Water in Laundering Operations," Water Quality Research Council, Purdue University.) Dishes, especially glasses, can display water spotting, streaking, or a visible film. Skin washed in hard water is typically reported as feeling scratchy and dry as a result of the soap scum, and hair is often reported as being dull-looking and sticky.

Then again, it all depends on what you're used to. People who have been born and raised using hard water are accustomed to the feeling of their clothes, bedding, skin and hair after washing. That seems clean to them, while the end result of washing with soft water often doesn't.

Why would soft water make you feel "not quite clean"? One reason is that when soap and mineral scum are absent, your natural body oils make your skin feel, well, oilier, which some people interpret as being slimy. For myself, washing with soft water makes my skin and hair feel dirtier, and no matter how much I tell myself that it's really cleaner, I can't help feeling I need to wash again. I've noticed that showering in soft water can make the floor of the shower stall so slippery that it increases the risk of falling if you're not careful. Washing your hair in rainwater (which is very soft) seems to produce the same "slimy" effect.

What's healthier, hard water or soft? I've seen arguments both ways. Some companies selling water softeners claim health benefits for soft water, saying that removing soap scum reduces the amount of bacteria on the skin through improved cleaning, but I couldn't find any good studies confirming that. Hard water can be problematic in terms of taste, color, and deposits inside pipes and plumbing and on cookware and appliances such as water heaters and dishwashers. It's not generally considered a health hazard, though. The U.S. Environmental Protection Agency doesn't regulate water hardness, although its "secondary" standards do contain recommendations for color, odor, pH, and total dissolved solids, which are related to hardness in varying degrees. The secondary standards give a maximum total dissolved solids limit of 500 mg/l, which is lower than the calcium and magnesium levels in some U.S. water supplies. But the secondary standards are non-mandatory and "established only as guidelines to assist public water systems in managing their drinking water for aesthetic considerations, such as taste, color and odor."

There's reason to believe hard water is actually good for you. The UK's Drinking Water Inspectorate claims that "there is evidence of less heart disease in hard water areas than in soft water areas, although not all studies find this link." A study by Lacey and Shaper in 1984 concluded:

New results based on changes that have taken place in water hardness and in cardiovascular death rates between 1961 and 1971 in the county boroughs of England and Wales indicate a significant trend for men, in the direction of decreasing cardiovascular mortality with increasing hardness, but no trend for women. The trend in male mortality appears to be specific to cardiovascular disease. The results are similar to those of the earlier study and support the hypothesis of a weak causal relationship between the hardness of drinking water and mortality from cardiovascular disease.

A 1980 National Research Council report claims that drinking water with high dissolved calcium and magnesium content can be an important source of these minerals in the diet.

What's the best soap to use with soft water? I don't know of any solid research on the subject, but some people favor soaps with high levels of moisturizer, preferring the feel of the moisturizer to the "oily clean skin" feel. For what it's worth, friends with water softeners recommend Dove and Caress brand soap. Myself, I think my skin is soft enough with hard water - it's never stopped my Special Lady from running her hands all over my body every night.

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