Spatial variability of air pollutants in the Basel area and carcinogenic and non-carcinogenic health risk

In environmental epidemiology, exposure assessment is a central topic as its
quality determines in large part the validity of a study. Further, it is used in
risk assessment to quantify the impact of environmental risk factors. Usually
in cross-sectional studies the air pollution measurements, performed at one
fixed site monitoring station, are used to assign exposure levels to all study
subjects living in the corresponding city. In order to prove the validity of this
technique, spatial and temporal variability of air pollutants in the Basel area
was systematically investigated. Based on these findings a risk assessment was
performed to quantify the carcinogenic and non-carcinogenic health risk that
is associated with the occurring pollution levels in the cantons Basel-Stadt and
Landschaft (Switzerland). During 1997 a broad palette of air pollutants was measured at six temporary
sites using a mobile monitoring station changing location every two weeks.
The six sites differed mainly with respect to the road traffic density and
represented typical residential place in the city of Basel. It was measured
particulate matter (PM4, PM10 and TSP), classical pollutants (NOx, SO2, CO,
O3, black carbon) and 61 different compounds belonging to VOCs (volatile
organic compounds), PAHs (polycyclic aromatic carbons) and nitro-PAHs,
thereof many carcinogens. Additional, air pollutant data from further urban
and rural monitoring stations as well as from the following year were taken
into account in the data analysis. Concurrently performed reference measurements
allowed a statistical analysis of the spatial variability of air pollutants at
the six temporary sites. To quantify cancer risk from air pollution, two methodological approaches
were performed. On the one hand cancer risk attributable to air pollution was
quantified based on the indicator concept, taking PM10 as a surrogate of the
cancerogenicity of air pollution. This integrative approach was based purely
on human epidemiological data. On the other hand unit risk factors were used
to quantify the cancer risk with an analytical approach. Unit risk factors
express the cancer risk of each single agent and have been traditionally applied in cancer risk assessment. They are based mainly on animal, partly
also on human data. The analyses of the air pollutant measurements yielded generally a homogeneous
distribution of PM4, PM10, and TSP in the city of Basel. With the
exception of one site in a street canyon next to a traffic light, proximity to road
traffic had only a weak tendency to inc rease the levels of PM. Mean PM10
concentration at the six temporary sites with different traffic densities was in
the range of less than ±10% of the average urban PM10 level. Higher spatial
variability in the urban environment and a stronger association to the traffic
density, respectively, was found for the particle number, some traffic related
components in PM, and for most carcinogens. Differences in the PM levels between urban and more elevated rural sites were
larger during the cold season than during the warm season due to days with
persistent surface inversion in winter. Generally, largest urban-rural-differences
were observed for primary traffic related substances such as carbonaceous
compounds and Pb, smallest differences for the secondary produced
sulfate and ammonium. Comparing mean PM values by day of the week showed a clear trend with
highest values on Wednesday and Thursday and lowest values on weekend.
The workday/weekend was most pronounced for mineralogical elements and
elemental carbon. The quantification of the cancer risk attributable to air pollution yielded 0.8
(range: 0.2-11.4) annual lung cancer cases per 100,000 persons in the cantons
Basel-Stadt and Basel-Land when an unit risk-based approach was used. By
means of an indicator-based approach resulted 6.7 (95%-CI: -0.8 to 14.2)
cases. Generally estimates based on human data yielded a higher risk.
Estimating the cancer risk from diesel exhaust in ambient air based only on
human occupational studies without considering animal studies resulted in
11.4 (95%-CI: 3.1-19.7) lung cancer cases per 100,000 person years in the
study area. In total 59 (95%-CI: 36-81) premature deaths per 100,000 person years were
estimated to be associated with air pollution in the both cantons. Further a significant number of respiratory and cardiovascular hospitality admissions,
chronic bronchitis, acute bronchitis, restricted activity days, and asthma
attacks had been quantified. The remarkable spatial homogeneity of long-term mean PM levels reduces the
error of assigning data from one fixed monitoring site to all study subjects
living in one city as usually done in cross-sectional studies. The homogeneous
distribution is mainly caused by the long residence time of particle in the
atmosphere and the plurality of emissions source types for PM. The difference
between mean PM concentration on workday as compared to the one on
weekend can be interpreted as an indicator of the influence from regional
anthropogenic emission sources. Difference between long-term PM levels at
urban and rural sites seems more to be affected by differences in the altitude
than by the distance to the cit y, due to occurring persistent surface inversions
in the cold season. The health risk assessment generated evidence that a large part of the cancerogenicity
of air pollution is caused by synergistic effects of various pollutants
in the ambient air mixture and cannot be ascribed to single agents. Thus, it is
propagated that in future an efficient air pollution regulatory strategy is rather
focussed on the whole air quality than on single agents. This can mean that
setting and controlling of air pollutant standards is done based on the philosophy
of the 'indicator-perspective' of the environmental epidemiology. If so, air
pollutants are mainly comprehended as a proxy of certain emission sources.
The causal acting on human health of a single agent is of minor importance.
However, reducing the indicator pollutants, decrease strategically all
pollutants so that implicitly synergistic effects of the air pollution mixture are
diminished. The planned sanctions to reduce PM10 levels in Switzerland show
that regulatory policy has developed in this direction, yet.