Of all the quantitative measures of economic well-being, energy consumption per capita may be the most expressive. Of the world's 25 countries defined as having high incomes, all consume at least 100 gigajoules of commercial energy (the equivalent of about 3.5 metric tons of coal) per person per year, with some, such as the United States and Canada, having consumption rates in the 300-gigajoule range (the equivalent of more than 10 metric tons of coal per person per year). With the exception of the oil-rich Persian Gulf states, where consumption figures include the costly "burning off" of excess energy in the form of natural gas, most of the highest consuming countries are in the Northern Hemisphere, concentrated in North America and Western Europe. At the other end of the scale are the low-income countries where consumption rates are often less than 1 percent of those of the United States and other high consumers. These figures do not, of course, include the consumption of noncommercial energy—the traditional fuels of firewood, animal dung, and other organic matter—widely used in the less developed parts of the world.