In the 1950s, it seemed that urban air was improving, as gas and electric heating took the place of coal fires. Now, air quality appears to be getting worse, as cars and other machines release more and worse pollutants into the atmosphere. Parks can make a small but valuable contribution to the problem. All green plants take in carbon dioxide and give out oxygen. Deciduous plants collect particulate matter on their leaves and carry it to the ground when the leaves fall. Trees can provide shelter and shade, cooling the air in summer and fending off cold winds in winter. This is air conditioning by natural means.