National Ambient Air Quality Objectives For Ground-Level Ozone- Summary - Science Assessment Document

5. Material Effects

5.1 Effects Summary

Although sulphur dioxide remains the most important pollutant in the degradation of materials, other atmospheric pollutants are gaining in importance as a result of declining emissions of sulphur dioxide. Ozone is one of these. Ozone damages many different types of materials, both functionally and aesthetically, alone and in combination with other pollutants and environmental factors. Impacts of ozone alone are most significant for organic materials.

Among the stretched elastomer materials tested, natural rubber, general diene rubber, polyisoprene, polybutadiene, acrylonitrile-butadiene and styrene-butadiene were most affected by ozone. Indeed, one of the earliest techniques used to assess ambient ozone concentrations was based on the consistent and rapid cracking of stressed rubber strips when exposed to ambient air. Either the time until cracking or the depth of cracking after a specified time can be related to ambient ozone concentrations. Neoprene, silicones, ethylene, butyl rubber and propylene have not been shown to be affected by ozone. The difference in susceptibility of different elastomers is linked to their organic structure, and in particular, the relative proportion of unsaturated carbon molecules, which are most susceptible to attack by ozone. Protection of elastomers can be increased by the use of antiozonants and waxes. Degradation of elastomers also occurs as a result of exposure to natural weathering processes; in particular, to sunlight.

Ozone has the ability to damage textiles by reducing tensile and break through strength. Synthetic fibers tend to be less affected than natural fibres. Factors such as sunlight, heat, moisture and the presence of micro-organisms can also contribute to reductions in tensile strength, and may be much more important factors than exposure to ozone. However, low levels of ozone can degrade fabrics if they are sufficiently moist. Ozone also causes fading and/or discolouration of dyes. In fact, the primary causal agent of fading is ozone, although significant fading may only occur from exposure to ozone in combination with other factors (e.g. humidity). Lower molecular weight dyes appear to fade most quickly.

Ozone has the ability to embrittle and fade surface coatings by reacting with the organic binder and/or the pigment. Oil based house paints were most susceptible to ozone damage while automotive finishes and paints that contain carbonate filters were the most ozone resistant. Other factors that contribute to paint erosion are temperature, moisture, sunlight, and the presence of other ambient air pollutants. It is likely that the combined effect of these other factors will be larger than the degradation caused by ozone alone.

Damage to organic materials is caused at the molecular level by chain scissioning and cross-linking mechanisms. In some cases there is an added synergistic degradation of materials due to the presence of other ambient pollutants, specifically sulphur dioxide and nitrogen oxides, and/or high humidity levels.

The impact of ozone on metallic materials is primarily a result of synergistic effects with sulphur dioxide. In combination with sulphur dioxide, ozone accelerates the corrosive action of sulphur dioxide on metals. At typical ambient levels, the presence of ozone increases the deposition rate of sulphur dioxide to the metal surface and increases the rate of oxidation of this dioxide. Beyond this mechanism, minimal other information exists for describing potential synergistic effects. These synergistic effects have been noted for a variety of metals, such as copper, zinc, silver, aluminum, nickel and iron. Corrosion of these metals will also occur in the presence of other pollutants such as nitrogen oxides and organic acids.

Ozone on its own has little ability to affect other inorganic materials either. Corrosion of stone materials, such as marbles, sandstone, limestones, bricks, concrete, and gravel, does occur but as a result of the synergy between ozone and sulphur dioxide. Other environmental factors can also influence the effect of ozone on building materials.

5.2 Quantification of Impacts

In the majority of studies reviewed related to organic materials, effects were reported qualitatively (e.g., fading/cracking). Effect levels and corresponding exposure periods were assessed for elastomers, textiles, and dyes and surface coatings; however, no concentration-response relationships were developed. Where concentration-response relationships were identified (e.g. for paints, metals and stones), the ways in which the effects of ozone were quantified was diverse. Also, differences in mathematical expression of the relationship hampered the merging of the results into an overall concentration-response relationship for any given material.

Therefore, it is not yet possible to define concentration-response relationships or effect levels to describe the impact of ozone on materials. However, it should be recognized that chronic exposures in an ambient environment, in the order of weeks at concentrations in the range of 20 - 50 ppb, have the potential to adversely impact elastomers, textiles, paints and dyes. Erosion rates measured during field exposures for non organic materials (metals and stone) in atmospheres containing ozone in combination with sulphur dioxide, are smaller but nevertheless significant.

The most important pollutant causing material damage remains sulphur dioxide, the effects of which are well quantified. With the decline in emissions of sulphur dioxide, effects associated with exposure to nitrogen oxides and ozone are becoming more apparent. Concentrations of nitrogen oxides and ozone are highly correlated, however, and therefore their effects are not easily separated. These effects are known to be smaller than those of sulphur dioxide but beyond that, much remains to be done to characterize and quantify them.