Ozone is the primary atmospheric pollutant causing injury to vegetation in California. Together with other photochemical oxidants, ozone already causes estimated economic losses in the range of several hundred million U.S. dollars a year in California. Recent studies indicate that photosynthesis and growth rates in valuable tree crops such as almonds, apricots, plums, prunes, pears and apples, are substantially reduced when the plants are exposed to increased ambient levels of ozone such as is found at the University of California's field station located near Fresno. In addition to agricultural concerns, as air quality deteriorates, skin cancers and eye and respiratory problems are expected to increase in Central Valley livestock, especially in light-colored cattle and sheep. While there have been technological improvements in gasoline efficiency for motor vehicles, more people are driving more cars more miles on more trips.
People travel farther to work and most people still drive to work alone.
Buses and trucks that produce a significant amount of pollution have not been held to the same environmental restrictions as automobiles. Although leaded gas is being phased out, generating dramatic declines in air levels of lead, automobile fuel has actually become more polluting. Gasoline refiners change the formulas to make up for the loss of octane, and the refined product is more likely to release smog-forming vapors into the air. In addition, auto pollution control devices, which have been installed in cars since the 1970s, were only required to work for 50,000 miles, while a car in the United States is usually driven for 100,000 miles or more. In 1995, a study called the Index of Environmental Trends was published, which examined several environmental quality indicators.