Holbeck and Leeds formed a continuous built-up area by 1858, with Hunslet nearly meeting them. In the latter half of the nineteenth century, population growth in Hunslet, Armley, and Wortley outstripped that of Leeds. When pollution became a problem, the wealthier residents left the industrial conurbation to live in Headingley, Potternewton and Chapel Allerton which led to a 50% increase in the population of Headingley and Burley from 1851 to 1861. The middle-class flight from the industrial areas led to development beyond the borough at Roundhay and Adel. The introduction of the electric tramway led to intensification of development in Headingley and Potternewton and expansion outside the borough into Roundhay.