An environmental consulting firm has tried to determine just how environmentally friendly e-book readers and tablets such as Apple's iPad and Amazon's Kindle are.

The Cleantech Group, billed as a leading research, advisory and networking resource for the global clean technology industry, calculates a single book generates almost 17 pounds, or 7.5 kilograms, of carbon dioxide equivalents.

That figure, based on a aggregated a series of studies, includes production, transport and either recycling or disposal reports The Washington Post.

In contrast, Cleantech estimates Apple's iPad generates a hefty 286 pounds or 130 kilograms of carbon dioxide equivalents during its lifetime. Meanwhile, Amazon's Kindle generates around 370 pounds or 168 kilograms of carbon dioxide equivalents according to estimates.

But the report indicates that, on average, the carbon emitted in the lifecycle of a Kindle is fully offset after the first year of use.

The Washington Post's Brian Palmer writes: "If we can trust those numbers, then, the iPad pays for its CO2 emissions about one-third of the way through your 18th book. You'd need to get halfway into your 23rd book on Kindle to get out of the environmental red."

Palmer adds: "Forrester Research estimates that the average user purchases three books per month. At that rate, you could earn back your iPad's carbon dioxide in just six months."

With regards water consumption, the New York Times reports it takes seven gallons to produce the average printed book, compared to "less than two cups of water" to create a typical e-book. However, the newspaper estimates 79 gallons of water are needed to produce a e-book reader. "So you come out on top, water-wise, after reading about a dozen books," Palmer adds.

Despite the green credentials promoted by companies such as Apple and Amazon, when it comes to toxic chemicals both traditional books and e-readers aren't free of potential risks. Palmer claims e-book readers require the mining of non-renewable minerals, such as columbite-tantalite, as well as relying on lithium batteries, while the printing process can release volatile organic compounds.

Brian Palmer's full article can be found here.