I've always wondered if I was missing the boat by not using water crystals, also known as water-retaining crystals or hydrogels, in my container gardens. Lots of people recommend them and claim that if they mix water crystals in with their potting soil, they have to water much less often. Water crystals are also often advertised as being "green," because they supposedly conserve water.
There are two types of water crystals, those made that are starch based (Quench) and the more common type which are made from polyacrylamide (Soil Moist and WaterCrystals).
Safety of Water Crystals
There is disagreement as to the environmental impact and toxicity of polyacrylamide. But some studies, suggest that polyacrylamide, which most water crystals are made of, is a carcinogen.
While the jury is still out on the safety of the polyacrylamide water crystals, the question may be moot because it appears that they don't work particularly well anyhow. According to Fine Gardening Magazine, neither kind of water crystal makes an appreciable difference in the amount of water your soil will hold.
Studies run by Jeff Gillman, author of the Fine Gardening piece, as well as the book, "The Truth about Garden Remedies," showed that the starch-based hyrdogels, like Quench, provided some benefit in prolonging water retention in container gardens, but only "an extra day at most between waterings." But in his studies, the "plants potted with polyacrylamide gels did not fare better than plants without."
Also, according to the University of Minnesota, "Major health effects of acrylamide are skin irritation such as redness and peeling of the skin of palms and neuropathy regarding the central nervous system and the peripheral nervous system. Acute and subacute intoxication with a large dose by ingestion water drink contaminated with acrylamide can cause severe symptoms of the central nervous system and polyneuropathy may appear later (17). Long term exposure to acrylamide produces a motor and sensory polyneuropathy that is insidious and distal in onset (12). Although severe exposure may result in permanent sequelae, affected humans recovered within several months to one year after cessation of exposure (13, 17)."
So I think the bottom line is, save your money and get out your hose.