Offsets online To learn more about carbon offsets and companies that provide them, start with these Web sites: Carbon Catalog, www.carboncatalog.org Offers a directory of more than 80 offset providers, rating each one based on the transparency and quality of the projects they support. The site also includes a glossary of terms. The United Nations Framework Convention on Climate Change, www.v-c-s.org Contains in-depth information on climate change, greenhouse gas emissions, and offsets. Consumers' Guide to Retail Carbon Offset Providers, www.cleanair-coolplanet.org Includes a ranking of carbon offsetters. The site also features an explanation of the offsets market, and a list of questions to ask providers before you buy.

n the 142 years since German biologist Ernst Haeckel introduced the word "ecology" to the world, the American government has tried many solutions to fix the environmental damage caused by its citizens. From Congress protecting woodlands with the Forest Reserve Act in 1891, to shielding the atmosphere with the Air Pollution Control Act of 1955, to safeguarding the water with the Clean Air Act of 1970, the efforts have all tried to limit man's encroachment on the natural world, and clean poisons from what we eat, drink, and breathe. But lately, Americans - and American companies - are looking for things they can personally do to save the environment. And the latest craze, it seems, is carbon offsets.

Carbon offsets, which are sometimes referred to as Renewable Energy Credits, became popular after the 1997 United Nations Framework Convention on Climate Change meeting in Kyoto, Japan. Carbon dioxide (CO²) emissions in our atmosphere are reaching their highest levels in 650,000 years and helping raise the earth's average temperature to levels so dangerous, the planet needs to lower its CO² emissions by 50 to 85 percent by 2050 according to the Intergovernmental Panel on Climate Change. If not, as the study, "Global Warming and Extinctions of Endemic Species from Biodiversity Hotspots" from the University of Toronto later warned in 2006, nearly 40 percent of the world's plant and vertebrate life in some areas might become extinct.

The solution: carbon offsets, where you pay a given company money that it invests in programs that offset the CO² you emit. For example, to offset 70 metric tons of CO² (the typical amount a small business buys at www.Carbonfund.org), you would pay Carbonfund approximately $650, which would fund the planting of about 15.9 acres of pine or fir trees in one of its reforestation programs.

Simple, right?

Not always. Carbon offsets are now a nearly $100 million (and rapidly growing) business, with approximately 170 companies worldwide offering some form of carbon-offset service, according to the "State of the Voluntary Carbon Markets 2007" report from London-based New Carbon Finance and Washington, DC-headquartered The Ecosystem Marketplace. They can vary widely in terms of prices, services, and certifications. In fact, when Clean Air-Cool Planet, a nonprofit organization in Portsmouth, NH, that focuses on solving global warming, commissioned an evaluation of 30 of these companies based on such criteria as the effectiveness of the offsets, the transparency of their operations, and education of the public on climate change, nearly 75 percent scored less than five on a scale of one to 10.

And even when companies are legitimate, not everyone is singing offsets' praises as a cure for what ails us. "Just because you donate to Mothers Against Drunk Driving doesn't mean you can drive drunk. Similarly, you can't just buy some carbon offsets and then tell people you've gone Green," says Scott Schenker, a vice president at Auburn Hills, MI-based event-marketing company The George P. Johnson Co. But for now, most industry experts and Green activists alike agree that while not a cure-all for our planet's carbon woes, purchasing carbon offsets, and thereby supporting eco-friendly causes such as reforestation and renewable energy sources, is better than doing nothing at all.

At almost every trade show, you'll find at least a handful of exhibitors using the admittedly imperfect offsets in a variety of effective ways, purchasing them to offset their exhibit's carbon footprint, for instance, or buying offsets to use as in-booth giveaways to offset attendees' travel to and from shows.

But no matter what exhibitors do with their offsets, there are a few things they need to know to tell the good offsetters from the bad and the ugly. Here, then, are five important factors to consider to help you offset without getting upset.

5. Not all offset companies are legitimate.

A 2007 investigation by the Financial Times of London found some companies took money for carbon-offset projects that either never took place or were less effective than advertised. To protect the integrity of offsets - which are, by most accounts, a better-than-nothing alternative to the emit-it-and-forget-it status quo - The World Wildlife Fund and Business for Social Responsibility stress using services that offer one or more third-party verification programs from groups such as the Environmental Resources Trust Inc. (www.ert.net); Green-e (www.green-e.org); and The Climate, Community, and Biodiversity Alliance (www.climate-standards.org). The Voluntary Carbon Standard (www.v-c-s.org) and The Gold Standard VER Registry (goldstandard.apx.com) are also highly regarded third-party verification services.