INCOME INEQUALITY
In arguing that wealth inequality has not grown in the U.S. as much as is perceived in “The Myth of Income Inequality” [Skeptic], Michael Shermer ignores that poverty among Americans, particularly youths, is far worse than in other advanced nations. It ruins the lives and educational chances of at least a fifth of young Americans, which makes a mockery of his claim that the U.S. is still the land of equal opportunity.

BRUCE J. BIDDLE
University of Missouri

Shermer's statistics fail to support his argument. He first states that income (not wealth) has not changed much by comparing the relatively affluent 1979 with 2010, the aftermath of the Great Recession. The data he cites include government transfers such as welfare payments and unemployment as “income.” Such transfer payments go up dramatically in a recession.

He then asserts that the “pie” of national wealth got bigger between 2012 and 2013. Overall wealth went up then primarily because the stock market was making 30 percent gains after the Great Recession, and corporations were making record unreinvested profits. Who do you suppose benefited from that growth?

Shermer goes on to argue that movement into different income groups was higher within the top 1 percent and top 0.01 percent slices of income taxpayers between 1996 and 2005 than among the 20 percent slices of lower taxed groups. Ignoring dramatic changes in tax policy in those years, there is simply no way to expect that anything, taxpayers included, will remain within a narrow range as often as within a range 20 or 2,000 times larger.

HUGH WALKUP
Alexandria, Va.

Comparing the income of rich and poor people is misleading. The majority of the rich don't collect much as salaries or wages; they have other methods of wealth accumulation, such as capital gains, which are reported only when an asset is sold. You need to look elsewhere than IRS records to discover how much the rich get richer.

The Rich and the Rest of Us, by Tavis Smiley and Cornel West, asserts that from 1983 to 2009, the top 20 percent took more than 100 percent of the wealth gain, and the bottom 60 percent lost net worth.

RICHARD USCHOLD
via e-mail

SHERMER REPLIES: As noted in the numerous responses to my column, there are many ways to compute income inequality. I did not intend to deny that the rich have gotten richer more than the poor have gotten richer, only that the differences between rich and poor are not as great as most people think. In my new book, The Moral Arc, I go into much more detail on the nuances of the discussion, but here let me make the general point that the trend lines are moving in the right direction.

According to a 2002 analysis, in the year 1820, 94 percent of the world's population was in poverty, and 84 percent was in extreme poverty (defined as living on less than $2 and $1 a day, respectively, in 1985 purchasing power parity, or PPP). The World Bank reports that by 1981, the figure of those living under its threshold for poverty ($1.25 per day in 2005 PPP) had dropped to 52 percent, and by 2010 it had fallen to 21 percent. That's still far too many impoverished people, but several economists predict that the figure will be 0 percent by 2100 and possibly even by 2050. If that isn't progress, I don't know what is.

The Bill & Melinda Gates Foundation optimistically projects poverty's end by 2035 (http://annualletter.gatesfoundation.org/#section=myth-one), but of course the date very much depends on how actively we work toward that goal.

POTTY TO POTABILITY
In “Bottoms Up,” Olive Heffernan's article about converting sewage to tap water, one important factor was omitted: reliability. No set of equipment runs perfectly forever; unexpected problems always arise and often escalate to levels that could easily cause dangerous health problems.

With tap water as the product, extreme measures of automation and quality monitoring must be applied to handle all possible faults in the system and to shut it down at the first indication of a problem.

R. W. LOWRIE
Dade City, Fla.

Heffernan's description of a process by which sewage is treated to become potable tap water includes a step to remove salts called reverse osmosis. What's not obvious is why this is easier than treating seawater, which is even more plentiful.

GREG ARZOOMANIAN
Providence, R.I.

The article says purified wastewater contains substances such as hand cleanser “in such minute doses as to be harmless.” But if the water is continually recycled and these compounds are not broken down, might they become more concentrated over time? Also, can we really assume that they are harmless in minute doses?

TOM FITZ
Northland College

HEFFERNAN REPLIES: Lowrie is right that no system runs reliably forever, and monitoring at each stage of the process would be crucial for any potable reuse system. Part of the rationale for having a multiple-barrier or multiple-step system in San Diego is so that each step can be monitored, and the plant could be temporarily closed should any of those crucial barriers fail. The aim would be to provide a fail-proof system for delivering clean drinking water to the city.

In response to Arzoomanian: Converting sewage into potable tap water is not necessarily easier than treating seawater. There are a number of steps involved in creating potable water from sewage in addition to reverse osmosis. But purifying sewage is likely to be cheaper and is better for the environment, making it a potentially more attractive option than desalination. For a start, the product being treated is already available on site at a wastewater treatment plant, avoiding the energy and infrastructure required for pumping water from the sea. What is more, there is a large amount of waste brine involved in desalination, which needs to be disposed of. In comparison, sewage needs to be treated anyhow and, in the case of San Diego, would end up being disposed of at sea—and polluting coastal waters—if it was not converted to drinking water.

Regarding Fitz's question: Although it is not possible to completely eliminate exposure to small doses of contaminants such as pharmaceuticals and personal care products when treating sewage, the system used at the San Diego Advanced Water Purification Facility aims to reduce this risk to a very low level and to prevent the accumulation of those substances with numerous treatment steps. This has the added advantage of protecting the water supply should one of the approaches fail.

ERRATA
Because of an editing error, “Giant Bubbles of the Milky Way,” by Douglas Finkbeiner, Meng Su and Dmitry Malyshev, referred to M42 as a nearby galaxy with Fermi bubbles. M42 is a nebula. The authors meant to speak of the M82 galaxy.

“How to Curb an Epidemic,” by Annie Sneed [Advances], describes the application of the anti-HIV microbicide tenofovir gel, now in testing, as occurring before sex. It is instead applied both before and after sex.