The United States may have ranked last in the second annual National Geographic Greendex survey, but the study also shows that Americans are increasingly becoming more aware of how their purchasing decisions impact the environment.

The Greendex survey at www.nationalgeographic.com/greendex monitors consumer progress in ecologically sustainable consumption in 17 developed and developing nations.

The survey, with more than 17,000 global respondents, covered four broad categories, including energy use, transportation, food choices, and green versus conventional products. The Greendex Web page also includes a survey allowing consumers to rank their own sustainability.

The top-scoring consumers were found in India, Brazil and China. Americans ranked last. Canadians were next to last in the survey.

But the survey found some encouraging trends in energy use and sustainable purchasing, and a greater awareness of how purchasing decisions impact the environment.

"People are becoming more aware of the ways their food and energy decisions are related to the environment," said Jovana Ruzicic, spokesperson for the Environmental Working Group, a Washington, D.C. nonprofit that aims to protect human health and the environment. "Hopefully, we'll close the gap with other countries by making better environmental choices."

The U.S. came in last in the area of housing, due to larger residences, fewer residents per household and higher energy use, compared to developing nations. The finding was not an unexpected one. Gross domestic product per capita in the U.S. is 16 times greater than India's and eight times greater than China's GDP.

However, American consumers improved in energy use for heating and cooling, and increased the number of second-hand household products purchased.

In the food category, Americans also ranked near the bottom, and consumed less locally grown food than any other country in the survey. But Americans also showed the highest rate of improvement in this category.

While the global economic downturn may have impacted scores, the data suggest that greater environmental awareness remains a driving force behind consumer choices.