1.1 What are the 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:
Potential publication bias.
For example, studies that have found
no association between a pollutant
and a particular effect may not have
been published (see question
1.2).
Diverging evidence. For example,
data suggesting either the existence
or non-existence of a threshold
for ozone
(see question 1.3).
1.2 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...
1.3 Consistency of epidemiological
and toxicological evidence in defining
thresholds
WHO
states:
1.3.1
"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...
1.3.2Ozone:
"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...
1.3.3Particulate
matter: "Most epidemiological
studies on large populations have been
unable to identify a threshold
concentration below which ambientPM
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...
1.4 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...
1.5 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...
1.6 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...
