Lagos became a major slaving port in the 1760s, despite its location, further up the Guinea Coast than other slaving hubs to the West. When the dominance of Portuguese and Brazilian merchants decline, British, French and Dutch traders moved in. European trading companies would purchase enslaved Africans captured by African middlemen, and transport them to the Americas to work on plantations. Over 350 years of the transatlantic slave trade, around 12.5 million people were forcibly exported from Lagos alone. All suffered, and many died, in the crossing.
European sailors too faced extraordinary mortality rates between the African coast and the Americas. The most common causes of death were yellow fever, malaria and dysentery. Liverpool crew musters of transatlantic voyages completed between 1770 and 1775 show that the death rate crew approximated 4.5% per 30 days. Nevertheless, crewmen had access to proper living quarters, food, water and fresh air. By contrast, the death toll among enslaved captives on the Middle Passage averaged as high as 14.2% per voyage.
In his discussion of the British Royal African Company, which dominated the British slave trade, Smith explores how the state financed the building of forts in Africa to support the slave trade and encourage British exports. He explains how the funds encouraged corruption as they were used to export inferior British bricks to the Guinea Coast leading to the need to re-build forts with local materials. (WN V.i.e.14, 739).