The emission of air pollution from traffic in our cities is the last step for a fuel that produces air pollution at every stage of production, often starting with flaring at a distant oil well. The World Bank estimates that the 16,000 flares worldwide produce around 350m tonnes of CO 2 each year.

Black carbon from sooty flames adds to the problems, especially across the northern hemisphere where it darkens arctic and mountain snow encouraging melting. The flared gas is also a wasted resource.

Despite increases globally in 2016 some areas have improved. Iraq is the latest country to join a World Bank initiative to eliminate routine flaring by 2030. Nigeria reduced flaring by 18% between 2013 and 2015. In 2017 it dropped from the second largest flaring nation to seventh place.

Global media attention has focused on soot covering the city of Port Harcourt, but more than 20 million people across the Niger Delta are exposed to flaring pollution. This is mainly from onshore wells and refineries rather than the off-shore industry.

The impacts are not just local to the well or refinery. Dominant south westerly winds carry pollution from flaring areas to those with little oil industry. Air pollution across hundreds of square kilometres is above World Health Organisation guidelines for nitrogen dioxide and ozone.

These pollutants also cause acid rain across sensitive savanna, mangrove, rainforest and freshwater habits and directly attack crops. Twenty seven percent of the pollution came from just 14% of flares, suggesting targeted improvement could yield big results in Nigeria. But more action is needed worldwide.

•The caption to this article was amended on 25 May 2017