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Air Pollution Nitrogen Dioxide

5. What are the uncertainties regarding this study?

  • 5.1 Uncertainties of the WHO answers, guidelines, and risk assessments
  • 5.2 Consideration of publication bias in the review
  • 5.3 Consistency of epidemiological and toxicological evidence in defining thresholds
  • 5.4 Contribution of different sources to PM-related health effects
  • 5.5 Impact of methods of analysis used in epidemiological studies
  • 5.6 Possible regional characteristics modifying the effects of air pollution

5.1 Uncertainties of the WHO answers, guidelines, and risk assessments

How could these influence the conclusions for policy-makers?

Uncertainties linked to gaps in knowledge exist and will continue to exist in the future. The expert group which wrote the reference documents for this Digest was aware of these uncertainties, and tried to take them into account – to the best of their knowledge – when drawing their conclusions.

Uncertainties were addressed in a systematic way, following the recommendations of a WHO guideline document. It was not feasible to quantify the uncertainties linked to all answers within this study.

It was stressed that, in accordance with the precautionary principle, uncertainties should not be taken as a cause for not acting, if the potential risks are high and measures to reduce the risks are available at a reasonable cost.

Examples of uncertainties related to this study are:


5.2 Consideration of publication bias in the review

WHO states: "Publication bias occurs when the publication process is influenced by the size of the effect or direction of results. The bias is usually towards statistical significant and larger effects. It can be detected and adjusted for using statistical techniques. Bias may also occur when literature is selectively ascertained and cited.

This review used a systematic approach to identify all short-term exposure studies, but it did not formally investigate publication bias. The reviewers were aware that evidence of publication bias has been identified in meta-analyses of single city time series studies, but when estimates were corrected for this bias, significant positive associations remained. Furthermore, the multi-city time series studies, which have published results from all participating cities and are free from publication bias, have reported significant positive associations.

Because of the size and experience of the review group and referees, it is unlikely that any important published long-term study has been missed. Formal assessment of a possible publication bias has not been undertaken. Every effort was made to systematically ascertain long-term exposure studies." More...

Source & ©: WHO Europe (2004)

5.3 Consistency of epidemiological and toxicological evidence in defining thresholds

5.3.1 WHO states:

"Multiple factors determine whether a threshold is seen [for effects due to exposure to air pollutants] and the level at which it can occur. Exposure-response curves depend on the age and gender of the subjects, their health status, their level of exercise (ventilation) and, especially the health effect selected. For highly uniform population groups, with a specific exposure pattern, a full range of concentrations, and a specific health outcome, one could identify a specific threshold. However, when there are different exposure-response curves for different groups, thresholds are harder to discern in population studies, and may ultimately disappear. Therefore, the evidence coming from the epidemiological and toxicological studies is not contradictory." More...

5.3.2 Ozone: "Chamber studies [(controlled exposure studies)] may show thresholds for mean effects of ozone on lung function and airway inflammation but a few individuals show these responses below these levels. As mentioned previously, a particular threshold in a particular experimental situation does not necessarily contradict a finding of effects below these levels in other situations. The time-series results often have insufficient data to distinguish between a linear and non-linear model with confidence. In addition, the statistical analyses applied to investigate thresholds in datasets on particles have not been applied to the same extent to datasets on ozone. There remain uncertainties in interpreting the shape of exposure-response relationships in epidemiological studies due to different patterns of confounding by other pollutants and correlations with personal exposure across the range of ozone concentrations. Although there is evidence that associations exist below the current [ozone] guideline value, our confidence in the existence of associations with health outcomes decreases as concentrations decrease. The answer and rationale [in question 2.3] refer to acute effects of ozone, as this is most important for health impact assessment of the effects of ozone." More...

5.3.3 Particulate matter: "Most epidemiological studies on large populations have been unable to identify a threshold concentration below which ambient PM has no effect on mortality and morbidity. It is likely that within any large human population, there is a wide range in susceptibility so that some subjects are at risk even at the low end of current concentrations" More...

Source & ©: WHO Europe (2004)

5.4 Contribution of different sources to PM-related health effects

WHO states: "Only a few epidemiological studies have addressed source contributions specifically. These studies have suggested that combustion sources are particularly important.

Toxicology, because of its simpler models and potential to tightly control exposures, provides an opportunity to determine the relative toxic potency of components of the PM mix, in contrast to epidemiology. Such toxicology studies have highlighted the primary, combustion-derived particles having a high toxic potency. These are often rich in transition metals and organics [organic compounds and matter], in addition to their relatively high surface area. By contrast, several other components of the PM mix are lower in toxic potency, e.g. ammonium salts, chlorides, sulphates, nitrates and wind- blown crustal dust such as silicate clays.

Despite these differences among constituents under laboratory conditions, it is currently not possible to precisely quantify the contributions from different sources and different PM components to health effects from exposure to ambient PM." More...

Source & ©: WHO Europe (2004)

5.5 Impact of methods of analysis used in epidemiological studies

WHO states: "This answer addresses matters relating to uncertainties in methods of analysis used. Epidemiological studies use statistical models of various types, including Poisson and logistic regression. The estimates of effect provided by air pollution studies are generally accompanied by confidence intervals. These convey the precision of the estimate or statistical uncertainty that arises because the analyses are subject to a degree of random error. To a varying degree, the results of these analyses are sensitive to the details of the model and the specification of confounding and interacting factors. Extensive sensitivity analyses have shown that associations between air pollution and health remain irrespective of the methods of analyses used." More...

Source & ©: WHO Europe (2004)

5.6 Possible regional characteristics modifying the effects of air pollution

WHO states: "Potentially this could be a very influential issue since the characteristics of populations, environments and pollution (including particle concentration, size distribution and composition) vary throughout Europe. However, at this stage there is not sufficient evidence to advocate different guidelines for particles or other priority pollutants in different parts of Europe.

Several studies on short and long-term effects of particulate matter have consistently reported an association between pollution levels and mortality; however, there are differences in the size of the estimated effects of PM according to geographical region or according to the levels of other variables (potential effect modifiers). For example, it has been reported that the short-term effects of PM10 are greater where long term average NO2 concentration is higher, when the proportion of the elderly is larger and in warmer climates. Modification by socioeconomic factors, such as the level of education, has also been reported. Plausible explanations for some of these observations have been proposed.

Effect modification, for example by the age distribution in a population and by climate should, if possible, be taken into account in sensitivity analysis of health impact assessments or risk assessments.

Possible effect modifiers of other criteria pollutants have not been investigated to any extent so far." More...

Source & ©: WHO Europe (2004)

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