20 Photochemical pollutants inflict damage on forest ecosystems and seriously impact the growth of certain crops such as corn, wheat and soybeans [Mackensie and El-Ashry 1988] and thin-leaved vegetables. Visible symptoms of "Waldsterben", or forest decline, first appeared in Europe in 1979-80 and within four years the symptoms had spread over large areas of the continent. It is associated with a high frequency of damage from secondary stress factors such as insects, needle, and root fungi, and climatic stress such as frost, wind, and snow damage. It now affects virtuaLly every tree species in Europe including the four most important conifers (spruce, fir, pine, larch) and six angiosperms (beech, birch, oak, ash, maple, alder). According to one analysis, damage in Western European countries is worst in the Federal Republic of Germany (FRG), where 55% of the trees are injured, followed by Switzerland, where 33% of the stands are affected by "Waldsterben" [Schutt and Cowling 1985]. The situation in Eastern Europe is not well documented but conceivably could be much worse.

There are reliable reports of extensive ozone damage to yellow pine, white pine, red spruce, fraser fir, sugar and yellow maple, beech, birch, red maple and a wide variety of other tree species throughout eastem North America [AFA 1987, Bruck 1987, Knight 1987, Zedaker et al. 1987, Wang and Borman 1986]. Field and other experiments have confirmed that levels of ozone commonly encountered throughout the eastern United States can cause significant reductions in growth and net photosynthesis [Wang and Borman 1986]. One study showed that in areas where known anthropogenic air pollution takes place, trees from polluted areas have been significantly more suppressed in terms of their growth rates over the past 25-30 years as compared to non-pollutant sites, and surface soils have in fact accumulated lead, often an order of magnitude above that of soils in non-polluted regions [Reich and Amundson 1985].

Photochemical oxidants accelerate the deterioration processes of plastics and rubber and significantly contribute to visibility degradation due to the formulation of aerosols in the light scattering range [OECD 1988a].

Tropospheric ozone is a greenhouse gas. Ozone absorbs infrared radiation and increased ozone concentrations in the troposphere may eventually contribute to global warming and climate modification.

# **Carbon Monoxide**

Over 90% of the carbon monoxide in many urban areas is emitted by motor vehicles. Because the affinity of hemoglobin in the blood is 200 times greater for carbon monoxide than for oxygen, carbon monoxide hinders oxygen transport from blood into the tissues. Therefore, more blood must be pumped to deliver the same amount of oxygen. Scientific studies in humans and animals have demonstrated that subjects with weak hearts are placed under additional strain by the presence of excess CO in the blood. For example, an assessment by the Health Effects Institute indicated that low levels of COHb produce significant effects on cardiac function during exercise in subjects with coronary artery disease [Allred et al. 1989]. In addition, fetuses, sickle cell anemics and young children may also be especially susceptible to low exposure levels of CO.