Haze over cities is a visual reminder of the air pollution we breathe. This is less obvious than the coal smoke induced gloom that Victorians suffered. In the 1880s central London had just 17% of the winter sunshine that was measured in the countryside. This had only improved to 45% by 1916–20.

However, even modern haze is still reducing sunlight in cities. Scientists find that in Delhi air pollution over the city is reducing the efficiency of solar panels by between 12% and 17%, depending on the type of photovoltaic technology. Losses are between 9% and 13% in Beijing, and around 2% in London and Los Angeles.

In the worst cities air pollution could affect the viability of solar energy. It also means money – Los Angeles’ expanding solar electricity sector could be losing about $6m to $9ma year by 2020.

During the 1920s medics thought that smog-induced lack of sunlight caused vitamin D deficiency and the rickets which plagued the UK’s urban poor. Sunlight therapy is no longer used for respiratory diseases but recently scientists at King’s College London have looked again at vitamin D and found that it might help protect lung cells from air pollution.