and amount of injury caused by pollutants when exposed singly. Plant exposures to mixtures of SO2+O3 and SO2+NO2 have reportedly decreased injury thresholds. Injury produced by exposure to gaseous mixtures may be more severe than the sum of the injury caused by exposure to individual pollutants (a synergistic response). Antagonistic responses are generally observed when injury caused by pollutants applied singly is severe; in these cases
the effect of mixtures is to reduce the severity of injury. Visible symptoms produced by gaseous mixtures are usually characteristic of a single pollutant. 6.1.2 Economic Losses Numerous cases of significant pollutant-caused plant injury were reported in the 20th century. Excluding the photochemical oxidant problem in southern California and some areas of the northeast
most reports of air pollution-induced plant injury have been associated with point sources. With the exception of primary metal smelters
economic losses associated with point sources have often been insignificant. Many offending point sources in the U.S. have since been controlled. Nevertheless
because of photochemical oxidants such as O3 and PAN
significant air pollution injury to vegetation is still widespread; associated economic losses
however
are often undetermined. Scientists generally agree that O3 causes 90% or more of the air pollution injury to crops in the U.S. This recognition has led to the establishment of a National Crop Loss Assessment Network (NCLAN). From summaries of O3 monitoring data; determination of the O3 sensitivity of major crop plants such as corn
soy beans
wheat
cotton
grain
sorghum
and barley; and economic data
NCLAN-participating scientists have modeled the economic impact of O3 on U.S. agriculture. They have estimated that a 25% reduction in ambient tropospheric O3 would result in a $1.71 billion annual increase in agricultural production; a 40% reduction would result in a $2.52 billion annual increase. Quantification of economic loss is a difficult task. It requires surveys of suspected air pollution injury
confirmation of the causal factor
and estimation of the dollar costs. Loss estimates may be confounded by a number of factors. For example
presence of visible injury on plants may not be translatable into economic loss. In many cases
plants with severe foliar injury may quickly recover
replacing injured leaves with younger healthy ones with no apparent lasting effects. In other instances
even a slight amount of injury can decrease the marketability of a crop. To deal with this reality
plant scientists use the concept of plant damage rather than plant injury when it relates to potential economic losses. It is implicit in the damage concept that economic losses have resulted. Plant injury does not have this connotation. It is reasonable to expect that when a phytotoxic pollution problem continues for a period of time
economic losses will decline
as no grower can sustain such losses indefinitely. He or she may discontinue agricultural production altogether or utilize pollution-resistant varieties or other pollutant-tolerant crops. Air quality 204
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