Hey Mr. Green, Please explain how carbon dioxide emissions are measured. I find it hard to believe that 2,000 pounds of CO2 are emitted per passenger for a round-trip flight from Los Angeles to New York City. —Heath in Calabasas, California

Your perplexity probably stems from the way the units are stated. As I explain below, it only takes 100 gallons of fossil fuel to make 2,000 pounds of CO₂. So for the approximately 5,000-mile cross-country round-trip, an airliner averages about 50 miles per passenger per gallon--way better than you'd get driving all by your lonesome in most cars. Plus, you're only subjected to two airline meals, as opposed to a dozen fast-food stops.

Now for the math. Atomic behavior isn't always intuitive, as the chemistry of combustion reveals. A gallon of gas or jet fuel weighs 6 to 6.5 pounds, about 5.5 pounds of which is carbon. When an atom of carbon burns, it joins up with two atoms of oxygen to produce CO₂. Although carbon seems like the heavier part of this equation--it's a hefty ingredient in everything from coal to tennis rackets--oxygen has 1.33 times the atomic weight. Therefore, the two oxygen atoms in carbon dioxide are 2.66 times heavier than the original carbon atom. So your initial 5.5 pounds of carbon combine with 14.6 pounds of oxygen to give you 20 pounds of CO₂. Call it intelligent design or confusing design or unfathomable design, but that's the way it is.

Hey Mr. Green, Should I turn off my computer at night or put it to "sleep"? —Dave in Bozeman, Montana

Unless you have software or network hookups that require your PC to be on constantly, turn that sucker off. The sleep mode cuts energy use by 70 percent, but using the off switch reduces it even more--and turning off the power strip stanches the flow entirely. (Please don't confuse power saving with screen savers; though the latter may feature a heartwarming picture, they do not save energy.) Since computers in the business sector alone waste more than $1 billion worth of electricity a year, it's surprising that more fuss isn't made about these simple steps.

It's also rumored that turning a computer on and off repeatedly will cause it to die prematurely. This is simply not true. Even if it were, planned obsolescence would likely kill off your machine first. And when your desktop PC is put to sleep--permanently--consider replacing it with a laptop, which uses only about a fifth as much power.

Hey Mr. Green, What should I do with recycled paper? I was told it can't be recycled again. —Patricia in Dallas

One of the amazing things I've discovered in this gig is the power of urban legends. Among the most persistent are: (1) that recyclers are unscrupulous tricksters who never actually ship cans back to manufacturers; (2) that it takes as much power to fire up a fluorescent light as it does to keep it on for a day; (3) that gas taxes pay for all highway construction and maintenance costs; and, the most enduring fable of all, (4) that automakers invented--and have been hiding for half a century--a device that can boost mileage to 100 miles a gallon. All 100 percent wrong, just like whoever told you this tall tale. Recycled paper is generally indistinguishable from the virgin product and, though the fibers eventually get too short to reuse, it can be recycled again.

ON THE WEB Got a question on any environmental topic? Send it to Mr. Green via our Web site at sierraclub.org/mrgreen.

Illustration by Melinda Beck