1. What proportion of the world’s population is breathing unsafe air?
A week in 12 questions
A man drives his miniature steam engine out of the main arena of the Yorkshire Traction Engine Rally held in the grounds of Scampston Hall, in Malton, near York, northern England on September 4, 2021. - The steam fair takes place on September 4th and 5th and features numerous working steam engines alongside displays of military and classic vehicles. (Photo by OLI SCARFF / AFP) (Photo by OLI SCARFF/AFP via Getty Images)