Black market fuel made from stolen oil in rudimentary “bush” refineries hidden deep in the creeks and swamps of the Niger delta is less polluting than the highly toxic diesel and petrol that Europe exports to Nigeria, new laboratory analysis has found.

Shell, Exxon, Chevron and other major oil companies extract and export up to 2m barrels a day of high quality, low sulphur “Bonny Light” crude from the Niger delta. But very little of this oil is refined in the country because its four state-owned refineries are dysfunctional or have closed.

Instead, international dealers export to Nigeria around 900,000 tonnes a year of low-grade, “dirty” fuel, made in Dutch, Belgian and other European refineries, and hundreds of small-scale artisanal refineries produce large quantities of illegal fuel from oil stolen from the network of oil pipelines that criss-cross the Niger delta.

The net result, says international resource watchdog group Stakeholder Democracy Network (SDN) in a new report, is that Nigeria has some of the worst air pollution in the world, with dense clouds of choking soot hanging over gridlocked cities leading to a rise in serious health conditions as well as damaged vehicles.

The extreme toxicity of the “official” fuel exported from Europe surprised researchers who took samples of diesel sold in government-licensed filling stations in Port Harcourt and Lagos. They found that on average the fuel exceeded EU pollution limits by as much as 204 times, and by 43 times the level for gasoline.