Their total number nowadays is unknown due to heavy assimilation and lack of censuses based on ethnicity, but are known to be significant. Due to the same assimilation however, no sizeable number speaks the Circassian language anymore. Most of the large Circassian migrational waves towards mainland Iran stem from the Safavid and Qajar era, however a certain amount also stem from the relatively recent arrivals that migrated as the Circassians were displaced from the Caucasus in the 19th century. A Black African population exists due to historical slavery. A substantial number of Russians arrived in the early 20th century as refugees from the Russian revolution, but their number has dwindled following the Iran crisis of 1946 and the Iranian Revolution. In the 20th to 21st centuries, there has been limited immigration to Iran from Turkey, Iraqis (especially huge numbers during the 1970s known as Moaveds), Afghanistan (mostly arriving as refugees in 1978), Lebanese (especially in Qom, though a Lebanese community has been present in the nation for centuries), Indians (mostly arriving temporarily during the 1950s to 1970s, typically working as doctors, engineers, and teachers), Koreans (mostly in the 1970s as labour migrants), China (mostly since the 2000s working in engineering or business projects), and Pakistan, partly due to labour migrants and partly to Balochi ties across the Iranian-Pakistani border. About 200,000 Iraqis arrived as refugees in 2003, mostly living in refugee camps near the border; an unknown number of these has since returned to Iraq. Over the same period, there has also been substantial emigration from Iran, especially since the Iranian revolution (see Iranian diaspora, Human capital flight from Iran, Jewish exodus from Iran), especially to the United States, Canada, Germany, Israel, and Sweden.