Fluoride Findings
A report by the National Research Council (NRC) is cited as suggesting negative effects of fluoride in “Second Thoughts about Fluoride,” by Dan Fagin. But the NRC notes that its report was not initiated because of concerns about the low levels of fluoride used in community water fluoridation, nor did it examine that issue. Instead the report is part of a routine review by the Environmental Protection Agency to address whether the higher levels of naturally occurring fluoride currently allowed in drinking water pose a health risk. The EPA is evaluating the report.

The article rightly points out that enamel fluorosis only has a health impact in the severest cases, yet Fagin incorrectly refers to it as a “disease.” It is rather a disruption in enamel formation that affects the way teeth look. The American Dental Association (ADA) offers information on reducing the risk of fluorosis at www.ada.org.

The ADA’s support for fluoridation is based on more than six decades of research, thousands of studies, and the experience of more than 170 million Americans. We welcome additional peer-reviewed scientific studies that will add to the body of knowledge on the use of fluoride.

Mark Feldman
President, American Dental Association

Cosmic Growth Spurt
In “Making Space for Time” [News Scan], Scott Dodd explains that cosmic microwave background radiation shows that 380,000 years after its birth, the universe was filled with hot gas. He then writes,

“Eventually the early cosmos underwent inflation....” This statement is misleading. It implies that the exponential expansion of the universe called inflation occurred hundreds of thousands of years after the big bang. According to inflationary cosmology, inflation occurred around 10^–35 second after the big bang.

Mark Egdall
Hollywood, Fla.

Sunlit Path?
A Solar Grand Plan,” by Ken Zweibel, James Mason and Vasilis Fthenakis, calls for the conversion of 30,000 square miles of pristine desert into photovoltaic farms. A better alternative exists: utilize rooftops. Although this strategy will not take advantage of the concentrated sunshine of the Southwest and will not be as efficient, it will distribute power generation across all time zones and weather conditions, without paving over additional land.

Mathieu Federspiel
via e-mail

Has anyone looked into the effects of installing 30,000 square miles of low-albedo surface material? Solar panels, by design, have a much lower albedo than most flat ground in the Southwest. How would their greater heat absorption affect the local environment?

Talon Swanson

THE AUTHORS REPLY: In regard to the first letter, a common and valid criticism of our solar plan is that we undermodeled distributed energy systems, such as rooftop photovoltaics (PVs) and solar hot-water systems. If the price of residential systems is drastically reduced and local storage is provided, dispersed installations can play a much larger role than our article describes.

As to the second letter, locally we would experience differences in temperature and air movements because of albedo change. Although studies on this effect have not yet been conducted for large PV plants, observations and global models suggest some tentative conclusions. Tom Hansen, manager of Tucson Electric Power Company’s PV plant in Springerville, Ariz., has measured a two- to three-degree-Fahrenheit increase at the center of the PV field and a wind vortex from its periphery toward its center. An area of 50,000 square kilometers would receive about 3 x 10^14 watt-hours of solar energy daily. With a 20 percent albedo differential between desert and PV surfaces, this would amount to a net excess of 6 x 10^13 watt-hours per day. Similar albedo changes have also been caused by the major cities of the Southwest with no apparent effects. One should also consider that albedo heating will, nationally, be counterbalanced by avoidance of the heating caused by thermoelectric plants. Greg Nemet of the University of Wisconsin–Madison has studied global net radiative forcing by supplying 50 percent of the world’s energy with PVs, taking into account the albedo effect, and concludes that they are one of the most effective solutions to anthropogenic global warming.

Nevertheless, the potential for local effects deserves detailed studies, and it is conceivable that buffer, nonsolar zones around large arrays may be advised. Such arrays would not be built near large populations, so local heating would likely be inconsequential.

For additional commentary of this article, visit the discussion in our community.

Political Science
In “Congress Fails Science” [Perspectives], the editors propose that Congress is habitually inattentive to science and that this irresoluteness has persisted despite the shift of legislative power in 2007 to the Democrats. But like college students, Congress usually concludes most of its work in the last week or two of each session. Had the editors waited one month, they might have noticed that the moribund energy bill they cite has actually passed, as has the increase in fuel economy standards. Although Congress can be slow and indecisive, it was designed that way to minimize precipitous action. Do the editors really expect it, in less than a year and facing a dead-certain presidential veto, to pass a bill that would result in historically sweeping changes to our energy and environmental policies, to our economy and probably to our lifestyles? If nothing has happened in three to four years, they can get on their high horse. But now I would be concerned about the soundness of any major proposal that passed in a few months.

Bob Palmer
Gainesville, Fla.

Editor's Note: This story was originally printed with the title "Letters"