Dishwasher rinse-aid and fuel-cell car "exhaust".

My new dishwasher has receptacles for both soap and rinse-aid. rinse-aid safe for the environment, and do I need to use it in my dishwasher?--Britten Clark, Seattle, WA

If your region's water source is rich in magnesium and calcium salts ("hard" water), adding rinse-aid to your dishwasher along with the detergent may help prevent streaks an spotting on your glassware and dishes.

Rinse-aid--the ingredients of which are usually ethanol, citric acid, sodium, dyes and acrylic acid polymers--breaks dew the salts in hard water, thereby preventing the adhesion of soap clumps during the rinse cycle, leaving cleaner-looking results (although consuming food and drinks from streaked or spotted dishes and glassware is not a health hazard in its own right). The National Institutes of Health report that most rinse-aid is completely biodegradable, and while it is neither carcinogenic nor dangerous if used properly, it can cause eye and skin irritation following prolonged exposure.

While the use of rinse-aid is no environmental crime, those concerned about the consumption of resources might think twice about the need for it. Mainstream rinse-aid, like many dishwasher soaps, contains phosphates in its cleaning agents. Wastewater containing phosphates that escapes sewage treatment can cause excessive algae growth in waterways, which in turn pollutes drinking water and leads to marine "dead zones"--underwater environments deprived of oxygen and thus unable to support life.

It's easy to avoid rinse-aid and other household items with phosphates by seeking out products from any of several companies that only use plant-based ingredients. Earth Friendly Products, Ecover and Simply Clean, to name just a few, make environmentally friendly rinse-aid that can be found in most natural foods markets.

Also, even if your dishwasher is putting spots on glasses you need not fear drinking hard water from the tap. CONTACT: Earth Friendly Products, (800)335-326 www.ecos.com; Ecover, www.ecover.com; Simply Clean, www.simplyclean.ca; U.S. Environmental Protection Agency, www.epa.gov.

With all the talk of hydrogen-fueled vehicles, I can't help but wonder if millions of cars driving around spewing out water vapor--a well-known "greenhouse gas" itself--is any better than the carbon dioxide (CO2) emitted by traditional cars?--Kelly Grubby, Fleetwood, PA

Climate analysts do believe that water vapor in the atmosphere--mostly originating from natural evaporation from bodies of water--is already contributing significantly to climate change. According to the esteemed Intergovernmental Panel on Climate Change, atmospheric water vapor exacerbates warming caused by the emission of fossil fuels by as much as 50 percent. However, the additional water vapor that might be created by millions of fuel-cell vehicles running on hydrogen--while it may sound like a lot--would constitute only a drop in the bucket compared to that which naturally occurs.

According to Mississippi State University meteorologist Jeff Haby, who runs the Weather Prediction Website, the average concentration of water vapor in the atmosphere around the globe is presently between two and three percent, while CO2 levels are only at about .04 percent.

However, water vapor is far less efficient at trapping heat within Earth's atmosphere than CO2, the leading fossil-fuel-based greenhouse gas. Despite its prevalence, water vapor tends to concentrate locally and then get cycled through the meteorological system quickly (in the form of clouds and then rain). Meanwhile, CO2 lingers in the upper atmosphere for long periods of time and forms a dense barrier to the escape of heat. While water vapor can cause short-term day-to-day warming locally, CO2 is already raising the Earth's temperature both globally and permanently.

Industrial designer Robert Q. Riley argues, "Natural evaporation from lakes and rivers produces about 1,000 times more water vapor than would come from a transportation system that was totally powered by fuel cells." CONTACT: Intergovernmental Panel on Climate Change, www.ipcc.ch; Weather Prediction Website, www.theweatherprediction.com.

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