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.