The average U.S. home contains approximately 45 bulbs in 30 different light fixtures....

People breathe it out, plants breathe it in: a perfect balance, at least in theory. This colorless gas is produced during respiration, organic decomposition, and combustion. Plants absorb CO2 from the air in photosynthesis, and humans, too, have put it to good use: in refrigeration, carbonation of beverages, fire extinguishers, and dry ice (in its solid state). It's only at high levels that CO2 creates trouble: Carbon dioxide is one of two by-products of fossil fuel combustion (the other being water), and its levels in the atmosphere have increased more than 35 percent over the past century and a half, making it one of the most significant greenhouse gases and a major contributor to global warming.

Landfills prohibit the breakdown of biodegradable materials such as vegetation and newsprint due...