According to the environmental consulting firm Cleantech, which aggregated a series of studies, devices such as the iPad and Kindle are better for the environment than printed books, reports the "Washington Post" (http://macosg.me/2/rd).

How so? Cleantech says a single book generates about 7.5 kilograms (almost 17 pounds) of carbon dioxide equivalents. That's the value of all its greenhouse gas emissions expressed in terms of the impact of carbon dioxide. This figure includes production, transport and either recycling or disposal.

According to Cleantech, the iPad generates 130 kilograms of carbon dioxide equivalents during its lifetime, according to company estimates. Amazon has not released numbers for the Kindle, but Cleantech and other analysts put it at 168 kilograms. Those analyses don't indicate how much additional carbon is generated per book read (as a result of the energy required to host the e-bookstore's servers and power the screen while you read), but they do include the full cost of manufacture, which likely accounts for the lion's share of emissions.

The iPad uses just three watts of electricity while you're reading, far less than most light bulbs. "If we can trust those numbers, then, the iPad pays for its CO2 emissions about one-third of the way through your 18th book," says the "Post." "You'd need to get halfway into your 23rd book on Kindle to get out of the environmental red."