Excerpts Taken From
Integrated Science Assessment for Ozone and
Related Photochemical Oxidants
U.S. Environmental Protection Agency
Excerpted by Michael H. Klein
Chapter 1: Executive Summary
Section 1-3 (no page number) (ARP 77)
Ozone is naturally present in the stratosphere, where it serves the beneficial role of blocking harmful ultraviolet radiation from the Sun and preventing the majority of this radiation from reaching the surface of the Earth. However, in the troposphere, O3 acts as a powerful oxidant and can harm living organisms and materials. Tropospheric O3 is present not only in polluted urban air, but throughout the globe.
Ozone in the troposphere originates from both anthropogenic (i.e., man-made) and natural source categories. Ozone attributed to anthropogenic sources is formed in the atmosphere by photochemical reactions involving sunlight and precursor pollutants including volatile organic compounds, nitrogen oxides, and carbon monoxide. Ozone attributed to natural sources is formed through the same photochemical reactions involving natural emissions of precursor pollutants from vegetation, microbes, animals, biomass burning, lightning, and geogenic sources. A schematic overview of the major photochemical cycles influencing O3 in the troposphere and the stratosphere is shown in the figure to the right.
Ozone in rural areas is produced from emissions of O3 precursors emitted directly within the rural areas and from emissions in urban areas that are processed during transport. Because O3 is produced downwind of urban source areas and O3 tends to persist longer in rural than in urban areas as a result of lower chemical scavenging, the result is substantial cumulative exposures for humans and vegetation in rural areas, that are often higher than cumulative exposures in urban areas.
On a smaller scale, O3 can be influenced by local meteorological conditions, circulation patterns, emissions, and topographic barriers, resulting in heterogeneous concentrations across an individual urban area. On a larger scale, O3 persists in the atmosphere long enough that it can be transported from continent to continent and around the globe. The degree of influence from intercontinental transport varies greatly by location and time.
Background concentrations of O3 have been given various definitions in the literature over time. In the context of a review of the NAAQS, it is useful to define background O3 concentrations in a way that distinguishes between concentrations that result from precursor emissions that are relatively less directly controllable from those that are relatively more directly controllable through U.S. policies. For this document, we have focused on the sum of those background concentrations from natural sources everywhere in the world and from anthropogenic sources outside the U.S., Canada and Mexico, i.e., North American background. Since North American background is a construct that cannot be measured, the range of North American background O3 concentrations is estimated using chemistry transport models. Model-predicted annual average North American background estimates are typically less than 50 ppb across the country with highest concentrations in the Intermountain West during the spring and the Southwest during the summer.
Section 1-5 (no page number) (ARP 80)
On a breath-by-breath basis, humans at rest absorb between 80 and 95% of inhaled O3. The site of the greatest O3 dose to the lung tissue is the junction of the conducting airway and the gas exchange region, in the deeper portion of the respiratory tract. Additionally, the primary site of O3 uptake moves deeper into the respiratory tract during exercise when breathing becomes faster and the breathing route begins to move from the nose only to oronasal breathing (i.e., through the nose and mouth).
Table 1-1 Summary of ozone causal determinations by exposure, duration, and health outcome
|Health Outcome||Conclusions from Previous Review||Conclusions from 2011 2nd Draft ISA|
|Short-Term Exposure to O3|
|Respiratory effects||The overall evidence supports a causal relationship between acute ambient O3 exposures and increased respiratory morbidity outcomes.||Causal Relationship|
|Cardiovascular effects||The limited evidence is highly suggestive that O3 directly and/or indirectly contributes to cardiovascular-related morbidity, but much remains to be done to more fully substantiate the association.||Suggestive of a Causal Relationship|
|Central nervous system effects||Toxicological studies report that acute exposures to O3 are associated with alterations in neurotransmitters, motor activity, short and long term memory, sleep patterns, and histological signs of neurodegeneration.||Suggestive of a Causal Relationship|
|Mortality||The evidence is highly suggestive that O3 directly or indirectly contributes to non-accidental and cardiopulmonary-related mortality.||Likely to be a Causal Relationship|
|Long-term Exposure to O3|
|Respiratory effects||The current evidence is suggestive but inconclusive for respiratory health effects from long-term O3 exposure.||Likely to be a Causal Relationship|
|Cardiovascular Effects||No studies from previous review.||Suggestive of a Causal Relationship|
|Reproductive and developmental effects||Limited evidence for a relationship between air pollution and birth-related health outcomes, including mortality, premature births, low birth weights, and birth defects, with little evidence being found for O3 effects.||Suggestive of a Causal Relationship|
|Central nervous system effects||Evidence regarding chronic exposure and neurobehavioral effects was not available.||Suggestive of a Causal Relationship|
|Cancer||Little evidence for a relationship between chronic O3 exposure and increased risk of lung cancer.||Inadequate to infer a Causal Relationship|
|Mortality||There is little evidence to suggest a causal relationship between chronic exposure and increased risk for mortality in humans.||Suggestive of a Causal Relationship|
Section 1.6.5 (no page number) (ARP 85)
An important consideration in characterizing the association of O3 with morbidity and mortality is the shape of the concentration-response relationship across the O3 concentration range. In this ISA, studies have been identified that attempt to characterize the shape of the O3 concentration-response curve along with possible O3 “thresholds” (i.e., O3 levels which must be exceeded in order to elicit a physiological response). These studies have indicated a generally linear concentration-response function with no indication of a threshold for O3 concentrations greater than 30 or 40 ppb, thus if a threshold exists, it is likely at the lower end of the range of ambient O3 concentrations.
(no page number) (ARP 86)
Table 1-2 Summary of ozone causal determination for welfare effects, Vegetation, and Ecosystem Effects
|Vegetation and Ecosystem Effects||Conclusions from Previous Review||Conclusions from 2011 2nd Draft ISA|
|Visible Foliar Injury Effects on Vegetation||Data published since the 1996 O3 AQCD strengthen previous conclusions that there is strong evidence that current ambient O3 concentrations cause impaired aesthetic quality of many native plants and trees by increasing foliar injury.||Causal Relationship|
|Reduced Vegetation Growth||Data published since the 1996 O3 AQCD strengthen previous conclusions that there is strong evidence that current ambient O3 concentrations cause decreased growth and biomass accumulation in annual, perennial and woody plants, including agronomic crops, annuals, shrubs, grasses, and trees.||Causal Relationship|
|Reduced Productivity in Terrestrial Ecosystems||There is evidence that O3 is an important stressor of ecosystems and that the effects of O3 on individual plants and processes are scaled up through the ecosystem, affecting net primary productivity.||Causal Relationship|
|Reduced Carbon (C) Sequestration in Terrestrial Ecosystems||Limited studies from previous review||Likely to be a Causal Relationship|
|Reduced Yield and Quality of Agricultural Crops||Data published since the 1996 O3 AQCD strengthen previous conclusions that there is strong evidence that current ambient O3 concentrations cause decreased yield and/or nutritive quality in a large number of agronomic and forage crops.||Causal Relationship|
|Alteration of Terrestrial Ecosystem Water Cycling||Ecosystem water quantity may be affected by O3 exposure at the landscape level.||Likely to be a Causal Relationship|
|Alteration of Below-ground Biogeochemical Cycles||Ozone-sensitive species have well known responses to O3 exposure, including altered C allocation to below-ground tissues, and altered rates of leaf and root production, turnover, and decomposition. These shifts can affect overall C and N loss from the ecosystem in terms of respired C, and leached aqueous dissolved organic and inorganic C and N.||Causal Relationship|
|Alteration of Terrestrial Community Composition||Ozone may be affecting above- and below -ground community composition through impacts on both growth and reproduction. Significant changes in plant community composition resulting directly from O3 exposure have been demonstrated.||Likely to be a Causal Relationship|
Section 1.8 (no page number) (ARP 89)
Ozone is an important greenhouse gas, and increases in its abundance in the troposphere may contribute to climate change. Models calculate that the global burden of tropospheric O3 has doubled since the preindustrial era, while observations indicate that in some regions O3 may have increased by factors as great as 4 or 5. These increases are tied to the rise in emissions of O3 precursors from human activity, mainly fossil fuel consumption and agricultural processes.
(no page number) (ARP 90)
Ozone in the stratosphere is responsible for the majority of this shielding, but O3 in the troposphere provides supplemental shielding of UV radiation in the mid-wavelength range (UV-B), thereby influencing human and ecosystem health.
(no page number) (ARP 91)
Table 1-3 Summary of ozone causal determination for climate change and UV-B effects Effects
|Effects||Conclusions from Previous Review||Conclusions from 2011 2nd Draft ISA|
|Radiative Forcing||Climate forcing by O3 at the regional scale may be its most important impact on climate.||Causal Relationship|
|Climate Change||While more certain estimates of the overall importance of global-scale forcing due to tropospheric O3 await further advances in monitoring and chemical transport modeling, the overall body of scientific evidence suggests that high concentrations of O3 on the regional scale could have a discernable influence on climate, leading to surface temperature and hydrological cycle changes.||Likely to be a Causal Relationship|
|UV-B Related Health and Welfare Effects||UV-B has not been studied in sufficient detail to allow for a credible health benefits assessment. In conclusion, the effect of changes in surface-level O3 concentrations on UV-induced health outcomes cannot yet be critically assessed within reasonable uncertainty.||Inadequate to Determine if a Causal Relationship Exists|
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