Small-scale artisanal mining of gold is another source of dangerous child labour in poor rural areas in certain parts of the world. This form of mining uses labour-intensive and low-tech methods. It is informal sector of the economy. Human Rights Watch group estimates that about 12 percent of global gold production comes from artisanal mines. In west Africa, in countries such as Mali - the third largest exporter of gold in Africa - between 20,000 and 40,000 children work in artisanal mining. Locally known as orpaillage, children as young as 6 years old work with their families. These children and families suffer chronic exposure to toxic chemicals including mercury, and do hazardous work such as digging shafts and working underground, pulling up, carrying and crushing the ore. The poor work practices harm the long term health of children, as well as release hundreds of tons of mercury every year into local rivers, ground water and lakes. Gold is important to the economy of Mali and Ghana. For Mali, it is the second largest earner of its export revenue. For many poor families with children, it is the primary and sometimes the only source of income.