Tolerance, to ozone

Menser, H. A., and G. H. Hodges. Tolerance to ozone of flue-cured tobacco cultivars in field and fumigation chamber tests. Tobacco Sci. 13 176-179, 1%9. 

Tolerance to ozone, 336-38, 375 Toluene, 60, 68, 69, 71, 78 Total respiratory resistance measuremoit, 395... 

Stokinger, Wagner, and Wright 10) who reported that a 4- to 6-week tolerance to ozone was developed from a single 6-hour exposure to 1 p.p.m. of ozone. 

Perhaps the most striking and unusual feature of ozone toxicity is the marked tolerance to multilethal doses of ozone that develops in rodents after brief exposures to ozone. Increased tolerance to ozone was first observed in rats that had been previously exposed for many days to 1 p.p.m. of ozone and then challenged in the rotating... 

Table VIII. Development of Tolerance to Ozone as Shown by Pre-exposure to Ozone Followed by Exercise during Ozone Exposure...

In an effort to determine the minimal time of ozone exposure required for the rats to develop significant tolerance, successively shorter periods of ozone pre-exposure were tested, from 18 hours to 1 hour. Tolerance to ozone is successively decreased in degree when pre-exposure is thus shortened, although some tolerance is demonstrated with as little as 1-hour pre-exposure to 1 p.pjn. of ozone (data lines 9 and 10). Tests for tolerance were made 24 hours after the start of the initial exposure, thus demonstrating the very rapid development of ozone tolerance. Because the 6-hour exposure to 1 p.p.m. of ozone developed a reproducible tolerance of significant degree, 6 hours was taken as a convenient pre-exposure time for subsequent studies to determine the duration of tolerance to ozone from a single exposure. 

In preliminary experiments no "spread of tolerance to ozone appeared to occur among rats that had previously been exposed to either nitrogen pentoxide or hydrogen peroxide and then challenged in the usual way to ozone combined with exercise. Further work will have to be done, however, to confirm these preliminary observations. 

Lung lesions observed after one week of exposure to 0.2, 0.5, 0.8 ppm ozone were more expressed than those observed after 3 months of continuous exposure. Some researchers [297,298] suggested that the enzyme levels in the lung are increased which may account for the development of adaptation and tolerance to ozone. 

Epithelial cels type I and squamous cells appear to be most sensitive to ozone [299]. An exposure to 0.5 ppm ozone increases the volume but reduces the area of epithelial cell types I [300]. These cells are regenerated to the normal condition after a week in fresh air and develop certain tolerance to ozone reexposure [301, 302]. The authors have studied tracheal cells (volume, density) by electron microscopy. They have found that all changes due to chronic ozone exposure were reversible, the normal values being regained if test animals are exposed to fresh air. 

Shaaltiel, Y., A. Glazer, P.F. Bocion, and J. Gressel. 1988. Cross tolerance to herbicidal and environmental oxidants of plant biotypes tolerant to paraquat, sulfur dioxide, and ozone. Pestic. Biochem. Physiol. 31 13-23. 

According to Stokinger, at least three effects of long-term exposure to ozone have been recc ized effects on morphology and function of the lung, lung-tumor acceleration, and aging. An additional effect, the development of tolerance after exposure to low concentrations of ozone, may also be related to chronic toxicity. 

Stokinger et reported that chronic bronchitis, bronchiolitis, and emphysematous and fibrotic changes in the lung tissues occur in mice, hamsters, and guinea pigs exposed daily to ozone at a concentration slightly above 1 ppm. These irreversible changes also develop in animals that have developed tolerance to acute inflammatory effects. 

One feature of the response to oxidants (in particular, ozone) that has stimulated considerable interest is the apparent development of tolerance to the acute effects of short-term exposure to these agents in laboratory animals. Fairchild reviewed possible mechanisms of this phenomenon. Tolerance has been defined as the increased capacity of an organism that has been pre-exposed to oxidant to resist the effects of later exposures to ordinarily lethal (or otherwise injurious) doses of the same agent or of different agents (cross-tolerance) with similar toxicologic properties. 

Of interest is the experimental approach whereby ozone is delivered solely to one lung. The observation of pulmonary effects in the unexposed lun indicates that there are extrapulmonary effects of ozone at edema-togenic concentrations. However, only the exposed lung appears to develop tolerance to later ozone exposure and to exhibit impairment of bacterial defense mechanisms. ... 

Earlier toxicity studies had suggested that the hazard from repeated ozone exposure might be reduced, inasmuch as animals became tolerant to the acute pulmonary edematogenic action of ozone. However, more recent studies, which demonstrated that tolerance to the ordinarily ozone-induced increase in susceptibility to infectious microorganisms or to the effects on respiratory mechanics does not develop, suggest that the tolerance phenomenon would have little protective value with respect to repeated exposure to ambient oxidant smog. 

Hanson presented a list of 160 woody species, from observations at the Los Angeles State and County Arboretum, that were sensitive or tolerant to oxidants. Several recent investigations have considered susceptibility of tree species to ozone. Jensen studied 9 hardwood... 

Differential susceptibility of individual clones of eastern white pine to ozone and sulfur dioxide was shown by Berry and Heggestad and Costonis. When Dochinger et a/. determined that chlorotic dwarf could be caused by an interaction of ozone and sulfur dioxide, th used a chlorotic dwarf-susceptible clone to eliminate the genotype variable. Houston tested the response of tolerant and susceptible clones of eastern white pine (on the basis of symptom expression under ambient conditions) to ozone or sulfur dioxide. Injury caused by sulfur dioxide or sulfur dioxide plus ozone correlated well with the earlier field responses, but ozone did not produce a consistent response. They also found that a 6-h exposure to a mixture of sulfur dioxide and ozone caused a difference in needle elongation between clones within tolerant and sensitive groups. This suggests that tolerance may function over a wide range of responses. 

Houston and Houston and Stairs did clonal repeatability analyses to determine genetic control of tolerance in white pine with an ozone-sulfiir dioxide mixture and a 6-h exposure. Th used needle elongation and two injury estimates in assessing effects. The repeatability estimates indicated that tolerance to the pollutant mixture is under genetic control. The nature of the inheritance of tolerance is still not understood, but field selection of tolerant or susceptible individuals is possible. Demeritt et reported an evaluation system that used... 

Levy, R., D. P. Jouvenaz, and H. L. Cromroy. Tolerance of three species of insects to prolonged exposures to ozone. Environ. Entomol. 3 184-185, 1974. 

When epidemiological studies form the basis for the risk assessment of a single chemical or even complex mixtures, such as various combustion emissions, it may be stated that in those cases the effects of combined action of chemicals have been incorporated. Examples can, for instance, be found in the updated WHO Air Quality guidelines (WHO 2000). Thus, the guideline value for, e.g., ozone was derived from epidemiological studies of persons exposed to ozone as part of the total mixture of chemicals in polluted ambient air. In addition, the risk estimate for exposure to polycyclic aromatic hydrocarbons was derived from studies on coke-oven workers heavily exposed to benzo[fl]pyrene as a component of a mixture of PAH and possibly many other chemicals at the workplace. Therefore, in some instances the derivation of a tolerable intake for a single compound can be based on studies where the compound was part of a complex chemical mixture. 

If the ozone level is too low, nothing will happen. If the level is too high, elongation will cease and the tube will die. These events occur within a narrow dose range (5-35 ppm for 5-15 min) and can therefore be used to study the threshold levels of sensitivity for pollen of any variety. Other work has shown that relative sensitivity of pollen to ozone is positively correlated to the ozone sensitivity of the pollen parent (43). Sulzbach and Pack (46) have also demonstrated that pollen sensitivity to atmospheric pollutants may be a useful tool for screening plant materials to learn their relative tolerance to pollutants and to attempt to discover selection pressures that may be affecting plant pcpulations. 

During the late afternoon when the vapor pressure gradient declines, ponderosa pine stomata may open wider, resulting in greater oxidant uptake and simultaneous depression of carbon dioxide fixation. Some knowledge of stomatal function would be useful to see if there is any relationship between intraspecific oxidant tolerance and ability to close stomates in the presence of elevated ozone concentrations. This mechanism is an inherited characteristic of an ozone-resistant onion variety which closes its stomates when exposed to ozone (30). It is not known if this mechanism is involved in conditioning interspecific tolerance or sensitivity of the important conifer species. 

From the experimental and calculated results the tolerance limits of exposure of rats to ozone might be determined, as shown by the following two examples. 

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