EPA based their predictions of seedling biomass loss on estimate of ozone exposures in 2001. Exposures were expressed as W126, a weighted 3-month seasonal sum of all one-hr average ozone concentrations expressed in parts per million-hours (ppm-hr). Levels above 13 ppm-hr can result in significant impacts to vegetation. In 2001, a fairly typical ozone year, a large portion of California had seasonal W126 above 31 ppm-hr, while broader multistate regions in the east and west had W126 above 13 ppm-hr. For more information on EPA's analysis, see the 2006 Staff Paper (pdf, 6 MB) developed for the review of the ozone standard.