APHEA - A pan-European
study on the effect of air
pollution

Analysis of reported
data

Evidence that high levels of air pollution have short-term effects on
human health was provided by the well-known episodes of severe air
pollution (smogs) in Europe and the USA prior to the 1960s. Subsequently,
air quality has improved, mainly as a result of the introduction of
national air quality standards and the controls necessary to ensure
compliance with them. Nevertheless, recent publications have indicated
that short-term health effects still occur even at levels of air pollution
which do not exceed current national standards or limits.

The European Commission agreed to support a programme to determine if
there is a relationship between pollutant levels and increases in
mortality and/or morbidity in 15 European cities. The programme, known as
the APHEA project (Air Pollution and Health,
European Approach)
consisted of a study of the short-term effects of air pollution on health.
The method of data collection and subsequent analysis had been agreed
prior to starting the study, which ensured that the data from the various
cities could be compared and also that a subsequent overall statistical
analysis (meta analysis) could be conducted. It entailed a comparison of
the incidence of mortality and also hospital admissions with various air
pollutant levels which had occurred during the periods and at the cities
shown in the table below.

The methods used (1) and
the findings for each of the cities studied have been published separately
(2) and
although an overall analysis (meta analysis) is being undertaken, it has
not yet been completed or published. CONCAWE has reviewed the overall
findings from the APHEA project and published its conclusions recently
(Report No. 96/61).

A risk ratio (RR) was calculated for the various pollutants in each of
the cities in the study. The RR gives an indication of the strength of the
association between a specific health effect and the air pollutant of
interest. An RR of 1.0 indicates that there is no association between
health effect and pollutant. An RR of greater than 1.0 indicates that
exposure to a pollutant is associated with a health effect, and an RR of
less than 1.0 indicates that there is a negative association between
health effect and pollutant (i.e. a higher pollutant level is associated
in a decrease in health effect). Total mortality, respiratory mortality
and mortality from cardiovascular disease was evaluated in 11 cities and
correlated with ambient concentrations of particulate matter, SO2, NO2,
ozone and carbon monoxide. Hospital admissions were evaluated in only six
of the cities and were correlated with the same air pollutants.

The APHEA project was a large study and a large database of information
has been assembled from a wide geographic area. Furthermore, the study was
conducted in a controlled way to allow comparisons between each of the
individual components of the study. Thus the database is valuable for
subsequent assessment. It makes a significant contribution to the
available knowledge in this area.

A simplified diagrammatic representation of the results is shown in
Figure 1, in which the RRs are plotted as average values for all the
cities for which there were data on the number of deaths from respiratory
disease and the number of deaths from heart disease for the two
pollutants, particulate matter and SO2.

Figure 1
A simplified representation of the
results of the APHEA study indicates the marginal effects of PM and
SO2.

A risk ratio of 1.0 indicates no effect of pollutant on mortality. An
RR of 1.2 indicates a marginal effect that can be due to random variation
or statistical coincidence, rather than indicating causality; an RR of 1.6
or greater is indicative of a causal effect. The diagram shows that, for
particulates and SO2, the effects were marginal. It also shows that there
was a wide variation of RRs between the various countries.

Other information given in the APHEA reports show that the relationship
between mortality and pollutant levels was not consistent. Although
increasing levels of pollutant would be expected to be associated with an
increase in effect, this was found not to be the case since the dose
response curve was not monotonic for any of the pollutants.

The study had some limitations and these should be borne in mind when
evaluating the results. The study was an ecological time series study
rather than an epidemiology study. Such time series studies are especially
difficult to interpret because exposures of the general population are
based on information derived from one or two monitors of pollutant levels.
The exposure to pollutants for those individuals who die or are admitted
to hospital are based on the single exposure measurements and these may
not necessarily apply to the individuals. Epidemiology studies on the
other hand compare the effects of populations known to be exposed with
those not exposed to a specific agent under investigation.

Confounding factors such as the influence of other pollutants and
weather on mortality or hospital admissions could not be excluded, and
weaken the power of the study to discriminate between chance and causal
effects. Other confounders could include factors such as day of the week,
hospital strikes, deprivation etc., and these weaken the power of the
study further.

(1) Katsouyanni K. et al. (1995) Eur.
Respir. J. 8,1030-1038
(2) J. Epid and Community Health (1996), 50,
(supplement 1),S2