Indonesia is the largest producer of palm oil. Production takes place primarily on the islands of Sumatra and Borneo, where fires are set every year to clear away forest for palm oil plantations.
Palm oil is a globally-traded commodity that's used in more than half of all consumer goods, such as lipsticks, processed foods and body lotions. American demand for palm oil has tripled over the last half a decade, driving palm oil cultivation deeper into Indonesia's rainforests and turning this crop into an existential threat to Orangutans.
A WWF study reported that the orangutan population has fallen by more than 50% in Sumatra since 1993, and at least 1,000 have disappeared in each of the last two years. Orangutans bear the brunt of forest fires on two fronts: the smoke, which causes their health to deteriorate, and the loss of their habitat. The fires drive them out of the forests and into villages, where they rarely receive a warm welcome.
P.S. The organisation Friends of the Earth has a fantastic fact sheet about palm oil that explains the pretty dire situation in great detail. Check it out here.