For example, a substantial share of today's renewable energy consumption comes from the use of wood and charcoal by households in the developing world, however, such energy use is sometimes associated with unsustainable forestry practices and health-threatening, heavy, indoor air pollution. However, incompatibility between the definition of the industrial sector used for energy statistics and the classifications used for GDP statistics severely limits the calculation of a number of indicators, such as energy intensity for different sectors. A separate note of caution must be mentioned when referring loenergy intensity figures: this indicator is an imperfect proxy for energy efficiency as it is affected by a number of factors unrelated to energy efficiency, such as climate, structure of the economy and nature of economic activities.