For public companies, going green is hot. Not hot like global warming, but hot for scoring public relations points and, sometimes, profits.

And even though there's a lot of chest thumping on Earth Day, some companies are making long-term changes in the way they do business — in hopes of saving money, conserving resources, and appeasing environmental activists and consumers demanding change.

Sandy Springs-based United Parcel Service, for example, is on the ground floor in developing a new hybrid hydraulic truck that generates electricity when it brakes.

Other environmental programs at Atlanta-based companies like Coca-Cola and Home Depot run the gamut.

• Coca-Cola has set a goal of returning to communities and nature an equivalent amount of the water used in its beverages and their production. It has committed $20 million to the World Wildlife Foundation to help conserve freshwater resources around the globe. Coke also has set the goal to recycle or reuse 100 percent of the bottles and cans it sells in the United States.

• Home Depot, which changed its wood-buying practices after environmental activists targeted the company in the late 1990s, introduced a new line of environmentally friendly products in 2007.

In their debut year, "Eco Options" products rang up $3 billion in sales, according to Ron Jarvis, Home Depot's vice president of environmental innovation.

To be sure, green programs are in the early stages for most companies and rarely represent the lion's share of sales. For example, Home Depot's annual sales in 2007 were $77.3 billion. Eco Options represented less than 4 percent of that total.

Of UPS' fleet of 93,637 cars, vans, semi trucks and motorcycles, only 2 percent run on alternative fuels. UPS has spent $15 million on new fuel technologies, however, and is working on its third alternative fuel concept, the hydraulic hybrid, in partnership with the Environmental Protection Agency, Eaton Corp., International Truck and Engine and the U.S. Army.

Last year, UPS helped test a delivery truck outfitted with the system in Detroit. The vehicle has a small diesel combustion engine and hydraulic components, which get recharged upon braking. In the road test, the vehicle used 40 percent to 50 percent less diesel fuel than a conventional engine, said UPS spokeswoman Heather Robinson.

But being environmentally friendly can sometimes cost more.

A gas-powered UPS delivery truck costs about $48,000 to build, said company spokesman Norman Black. A compressed-natural-gas vehicle costs $66,000. The company saves about $1 per gallon buying CNG fuel, he said.

— Staff writer Joe Guy Collier contributed to this article.