However,in many cases it takes coal to produce the steam that make electricity.In addition,the number of factories has increased tremendously,and most of them are hugein comparison to those in former years.Furthermore,thousands of automobiles are now driven in cities.Their exhausts pour gases into the air continually.
Until about 1880,most factories used steam for power.They needed great quantities of coal to produce the steam.The smoke settled over the cities and polluted the air.Eventually most factories began to use electricity for power.It was thought this would solve the pollution problem.