Across African cities home diesel generators are becoming more common. Generating their own electricity is the only option for many homes and businesses, and unreliable grid supplies mean backup generators are used widely. Inefficient diesel generators may produce far more carbon dioxide than large power plants and the exhaust is released right into densely populated neighbourhoods.

There is little hard data to assess the impact of backup generators – in 2015 the city of Paris had three times more air pollution measurement sites than the whole of Africa – but the poor air is obvious. Public campaigns such as those in the Nigerian city of Port Harcourt, are now calling for government action.

In the UK, more than 1,500 combined heat and power systems (CHP) now generate electricity and use the waste heat to warm buildings and heat water. They are mainly powered by natural gas, which is cleaner than the diesel generators used across Africa, but most CHP exhausts are not required to have clean-up systems. Two new reports have pointed to serious impacts on urban air pollution in the UK, too.