You may be hearing more these days in television commercials and in the pages of "green" publications about ecological footprints. In other words: What does it cost the planet for you to live the way you do?

"The ecological footprint is a resource management tool that measures how much land and water area a human population requires to produce the resources it consumes and to absorb its wastes under prevailing technology," according to the Web site www.ecofoot.net.

Calculations under this concept come up with the number of acres of land it takes to meet your needs.

I took the test at www.earthday.net (also available at www.ecofoot.org). It asks about how much you drive, how much you fly, how far you live from work, and so on. At first glance I did great: my ecological footprint was 12 acres, just half of the U.S. average of 24 acres per person. Then there was this line: "Worldwide, there exist 4.5 biologically productive acres per person. If everyone lived like you, we would need 2.8 planets."

The theory is that most of the things we consume are products provided by nature, or can be calculated as if they were. Or, they require resources to deal with their waste. If you do things that consume a lot of resources (drive a lot in a fuel-inefficient car, live in a house that uses a lot of electricity, eat foods transported long distances), your footprint is bigger.

The calculation also can be applied to communities, cities and even nations.

The city of Santa Monica, Calif., calculated the change in its ecological footprint from 1990 to 2000, and found that as a city, it's improving. You can read the report at www.regionalprogress.org/santamonicafootprint.pdf.

The United States has the highest per capita ecological footprint, with Europe, Russia, Australia and New Zealand not far behind. Third World nations have much smaller footprints.

Then there's another calculation: How many useful acres does your country have per person. The United States by this measure is overextended, but so is China and Bangladesh, whose residents use .05 percent the resources of the average American.

For a nation-by-nation ranking, see www.ecouncil.ac.cr/rio/focus/report/english/footprint/ranking.htm.

What can you do about your footprint? The Sierra Club has some ideas at www.sierraclub.org/footprint. Eat locally grown food, dry clothes on a clothesline and use your car less. It adds up.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.