Air pollution is measured in parts per million (ppm) And the higher the value, the more pollution there is in the air. Stephen is completing research for his senior project and records the amount of pollution in the air each day for two weeks. His results are show and the higher the value, the more pollution there is in the air. Stephen is completing research for his senior project and records the amount of pollution in the air each day for two weeks. His results are show in this stem and leaf plot below.