Introduction
Since the beginning of the 19th century, 33 spells of exceptionally
warm weather in the Netherlands have been officially labelled heat waves
by the Koninklijk Nederlands Meteorologisch Instituut (Royal Dutch Meteorological
Institute). For this purpose, Dutch meteorologists use a definition that
is, in view of the generally mild, maritime climate of the Netherlands,
less demanding than those of countries at lower latitudes. A warm spell
qualifies as heat wave if it consists of at least five days with a maximum
temperature of 25oC or above, including at least three ‘tropical’ days
with a maximum temperature of 30oC.
The summer 2003 heat wave amply satisfied these criteria: it lasted from
31 July to 13 August, a total of fourteen days, including seven tropical
days, and it was preceded by four tropical days in mid-July. This earlier
warm spell failed the heat wave requirements, as the tropical days were
interrupted by a single cool day.
The recent heat wave is neither the longest, nor the hottest on record.
Of the 33 heat waves on record, one lasted eighteen days (1975) and two
seventeen days (1947 and 1976, the latter including ten tropical days).
Nor did it break the record for the highest maximum temperature: the 7
August 2003 maximum of 35.0oC has twice been surpassed in the
past century, with an all time high of 36.8oC in 1947.
The relationship between climate and excess mortality is a complex
one. It can be represented in a V-shape, with the lowest all
cause mortality rate in the Netherlands at an average daily temperature
of 16.5oC [1]. As the average summer temperatures are much closer
to this optimum than average winter temperatures, the mortality
risk is normally below average in summer, and well above average
in winter. As shown in Figure 1, the effect of climate on mortality
has strongly decreased between the 1950s and 1970s, but does
not seem to have changed significantly since then. This trend
has only been influenced by climate change over the past century
to a very minor extent. The indices in Figure 1 show the degree
to which the monthly number of deaths in the relevant decade
is higher or lower than the number that would be expected if
deaths were spread evenly over the year (a value of 110 representing
a 10% higher mortality). The lower summer indices in earlier
periods are largely caused by the detrimental effect of cold
weather, inflating the average mortality risk. In the Netherlands,
as in all other countries with mild climates, annual cold-related
mortality is higher than heat-related mortality [2].
Data, method of estimation and results
The Centraal Bureau voor de Statistiek (Statistics Netherlands) collects
information on the cause of death for all persons who are considered
official residents of the Netherlands. By linking this information to
more detailed demographic data provided by the municipal population registers,
it is possible to determine the various relationships between personal
characteristics and cause of death. Unfortunately, this procedure is
inappropriate for the study of heat-related mortality, both in the Netherlands
and elsewhere. Even in unusually hot summers, very few deaths are directly
or indirectly attributed to these external causes. In 2003, only four
deaths were attributed to exposure to excessive natural heat (ICD-10
code X30, including hyperthermia, heatstroke and heat exhaustion) or
to the effects of heat and light (T67) as a primary or secondary cause
of death.
A common method for estimating the extent of excess mortality
during a certain period involves the comparison with mortality
rates in one or more earlier years. A drawback of this method
is the fact that unusually low or high temperatures during these
earlier periods affect the estimate and are difficult to adjust
for. We therefore followed a different indirect estimation procedure
that is independent of both mortality rates in earlier years
and the official Dutch definition of heat wave.
We first calculated the average maximum daily temperature per month in
the period June-September for the most recent 30-year period (June 19.8oC,
July 22.1oC, August 22.3oC and September 18.7oC).
The temperatures were those recorded by the Royal Dutch Meteorological
Institute at De Bilt, which is located in the centre of the Netherlands.
We then calculated the weekly averages of the daily maximum temperatures
and carried out a linear regression analysis between the weekly temperature
curve (independent variable) and the weekly mortality curve (dependent
variable). A linear regression was considered appropriate because the
period of observation is relatively short, and all temperatures were
higher than the optimum temperature of 16.5oC. For the purpose
of this analysis, weekly values were selected as the optimal trade-off
between daily values (with high random fluctuations) and monthly values
(in which variations in temperature tend to obscure the effect of hot
spells). As temperature does not usually have an immediate effect on
mortality, we estimated the average time lag between both variables by
determining the best fit between the mortality and temperature curves.
Allowing for the resulting time lag of three days, we obtained an estimate
of the regression coefficient of 33.5 deaths per week per degree Celsius
[Figure 2] (r2 = 0.57). The standard error (7.5 deaths per
week per degree Celsius) was used to obtain a low estimate of the absolute
number of heat-related deaths per degree Celsius (26.0) and a high estimate
(40.9), with a two thirds probability. The total excess mortality in
the period June-September 2003 was finally estimated at between 1400
and 2200 deaths, implying an increase of approximately 3% to 5% above
the number normally recorded during this period. The number of excess
deaths during the heat wave of 31 July – 13 August may have been
around 500.
The effect of heat on mortality shows a strong increase with age (0-64
years r2 = 0.16; 65-79 years r2 = 0.43; 80+ years
r2 = 0.65).
A numerical example illustrates the estimation procedure. Week
33 (11-17 August) had an average daily maximum temperature of
30.6oC, 8.3oC higher than the 30-year average
for August (22.3oC). The estimated number of excess
deaths is therefore 8.3 times 33.5 (the estimated regression
coefficient), hence 278 deaths. The actual number of deaths during
this week was 2826. This would imply that almost 10% of all deaths
during this hot spell were heat-related.
Discussion
The effect of extreme temperatures on mortality has been demonstrated
in numerous studies. Although the media regularly inform the public
of this fact during episodes of exceptionally warm weather, public
calls for action to prevent heat-related deaths have been rare in the
Netherlands. Interest in the possible excess mortality in the Netherlands
was largely fuelled by reports from France, where much higher temperatures
resulted in about 15 000 heat-related deaths in August 2003 [3]. Considering
the difference in population size between the two countries, heat-related
mortality in France may therefore have been about three times higher
than in the Netherlands.
As far as we are aware, the publication of this estimate prompted only
a little activity to investigate the specific conditions that led to
the excess mortality, and no large-scale public actions to prevent heat-related
deaths in the future. The general feeling that the only victims of the
extreme summer temperatures were very elderly and frail people who would
anyway have died within a few weeks (this is sometimes referred to as ‘harvesting’)
may partly account for this equanimity.
The Netherlands should not be complacent about taking actions
to protect those at increased risk of heat-related mortality
simply because excess mortality among the elderly was much lower
than in a number of other countries, particularly France. Even
if very high temperatures, unlike very low temperatures, result
in ‘harvesting’, this cannot account for all the
excess deaths. Some researchers have demonstrated a temporary
fall in the number of deaths following a heat wave [4-7], but
the findings of recent research on heat-related excess mortality
in the Netherlands are less conclusive: a forward shift of mortality
was found in some heat waves, but not in others [2]. The French
heat wave mortality peak of 2003 was not counterbalanced by a
trough in the remaining months of the year [3]. Figure 3, representing
average and observed mortality among persons aged 80 years or
above in the period May-November 2003, suggests that some forward
shift may have taken place in the Netherlands, but this shift
does not fully compensate the heat-related excess mortality.
Also, above-average mortality during hot weather is most pronounced among
the elderly, but not restricted to them. The observed number of deaths
in August 2003 among persons aged 40-59 years was 11% higher than the
expected number calculated on basis of data for the period 1995-2002.
Mortality did not increase in the younger age groups.
The mortality figures of the Netherlands were less dramatic
than those of France, but this may be because a smaller proportion
of the population were exposed to extreme temperatures, and not
because the Netherlands provides superior care for people at
high risk. While the heat wave in France strongly affected the
metropolitan areas (Centre and Ile-de-France), the maximum temperatures
in the Netherlands were far lower than in France, and occurred
in the relatively less densely populated regions. The absolute
maximum temperature in the Netherlands was registered at the
weather station of Maastricht, in the far southeast (36.2oC,
as opposed to 42.6oC in Orange, France). In the far
northwest, the highest value only once surpassed the 30-degree
barrier (30.2oC in Den Helder). As the western coastal
provinces are more densely populated than the eastern landlocked
provinces, the lower temperatures in these provinces may have
had a substantial downward effect on the overall mortality rates
for the Netherlands. Compared to the average number of deaths
in August for the period 1995-2002, the number of deaths in August
2003 was indeed 13% higher in all eastern provinces taken together
(Groningen, Drenthe, Overijssel, Gelderland and Limburg). In
the western coastal provinces (Noord-Holland, Zuid-Holland and
Zeeland) the August 2003 mortality rate was 2% lower.
Therefore, even in a small country like the Netherlands, regional
differences in climate contribute to evening out the harmful
effect, measured at the national level, of hot weather on health.
This effect, in absolute terms, is furthermore less noticeable,
as the upward influence of heat waves takes place when the general
mortality risk is lower than the yearly average. Therefore, even
during the heat wave of 2003, the number of deaths in August was
hardly above the number that would be expected if deaths were
spread evenly over the year [Figure 4].
This does not mean that there is no need for policy actions, however,
or for keeping a more watchful eye on particular risk groups. Among
these risk groups are people suffering from dementia, who need to be
prompted by others to take preventive measures in order to avoid dehydration
and hyperthermia [8]. The lowest curve in Figure 3 shows that the effect
of the August 2003 heat wave was more marked in the elderly in nursing
homes than in the non-institutionalised elderly population. This institutionalised
population has a much higher share of frail and demented persons than
the non-institutionalised population.
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