Remember classroom field trips to the library? Those were your most eco-friendly days of...

It packs one less O, and a lot more toxicity, than its cousin CO2. Colorless and odorless, this gas can be lethal at high levels indoors, where it's generated by residential wood burners and furnaces. It is essential to have fuel-burning appliances and chimneys professionally maintained and checked out regularly, and a good idea to install a detector to warn you if levels get high. (In fact, CO detectors are mandatory in some locales.)

Outside, while CO is produced naturally-by volcanic eruptions, for example-it is also a major pollutant. It results from the incomplete burning of carbon in fuel; the biggest source is motor-vehicle exhaust, although boats, airplanes, and construction equipment also contribute. Cigarette smoke and industrial processes, such as chemical manufacturing, are other sources. Federal air-quality standards and mandatory reductions in motor vehicle emissions have reined in the problem since the 1970s, but the increase in the number of cars on the road worldwide has started to undermine that progress. Carbon monoxide in the air can lead to a number of serious health problems, including cardiovascular difficulties. It also contributes to smog (ground-level ozone), which is linked to serious respiratory problems.

Many catalogs that arrive in people's mailboxes across the nation end up going straight to the...