Introduction
Record high temperatures were observed across Europe during the
summer of 2003. In Italy, the highest monthly mean was registered in many
cities in August, with record maximum temperatures above 35°C for several
consecutive days. There is debate among experts as to whether extreme temperatures,
as observed in summer 2003, are a normal fluctuation in the climate or
a sign of global warming attributable to human influences on the climate
system [1].
The full impact of climate change on health still remains unclear, and
an accurate analysis and quantification of the possible effects, both in
the short and long term, has still to be defined [2,3]. The effect of extreme
temperatures (often referred to as heat waves) on health is well documented
throughout the literature and is known to enhance mortality from cardiovascular,
cerebrovascular, and respiratory conditions [4-6].
One of the first documented episodes of Italian urban populations affected
by heat was the heat wave of summer 1983 in Rome, which was associated with
a 35% increase in mortality [4]. An evaluation of heat-related mortality
in 21 Italian provinces was carried out by the Istituto Superiore di Sanità (Italian
National Institute of Health) and over 4000 excess deaths (14% increase)
were estimated among the elderly [7]. The study also reported a large heterogeneity
of the effect among the Italian cities, with the highest increase in mortality
observed in the provinces of the north west, followed by the cities of
the south, and with the lowest effect observed in the central provinces
and the north east [7]. These differences are mainly due to the different
exposure levels during summer 2003, but may also be attributable to a different
vulnerability of the populations related to individual, social and environmental
factors.
This article presents a more detailed evaluation of the impact of heat waves
on mortality during the summer of 2003 (1 June – 31 August) in four
major Italian cities: Bologna, Milan, Rome and Turin. The aim is to analyse
the impact of heat waves on cause-specific mortality and to analyse the
role of demographic characteristics and socioeconomic conditions that may
increase the risk of mortality.
Data and methods
Daily mortality counts, for the resident population by age, sex and cause
of death were obtained from the local mortality information systems
in each city. The impact of heat on health is measured in terms of
maximum apparent temperature (Tappmax) which is an index of human discomfort
based on air temperature and dew point temperature [8]. The latter
combines two meteorological variables (temperature and humidity) that
have been shown to have an impact on human health.
Expected daily mortality was computed as the mean daily value from a
selected reference period (1995-2002 for Rome, Milan and Bologna and
1998-2002 for Turin). The daily mean expected value was smoothed using
a smoothing spline. Daily excess mortality was calculated as the difference
between the number of deaths observed on a given day and the smoothed
daily average. Confidence limits were determined assuming a Poisson distribution.
In Rome, excess mortality by socioeconomic level was evaluated for the
census tract of residence using a deprivation index based on a series
of components, namely education, occupation, unemployment, number of
household members, overcrowding and household ownership data [9]. The
indicator includes four classes: high, medium-high, medium-low and low.
In Turin, a socioeconomic indicator for the over-65 years age group was
developed based on the level of education subdivided into three classes
(high, medium, low).
Results
During the summer of 2003, Tappmax was higher than the mean for the reference
period in all cities; the greatest increase was observed in Milan (+4.4°C),
followed by Rome (+4.1°C), Turin (+3.4°C) and Bologna (1.6°C)
[TABLE 1].
A city-specific definition of heat wave was developed in order to better
reflect local conditions. heat waves have been identified as days with
Tappmax above the 90th annual centile and for the first day, an increase
of 2ºC compared with the previous day. This definition was made
on the basis of the literature reviewed and on the relationship observed
between temperature and mortality. Three major heat wave periods occurred
in Rome, and two major heat waves occurred in the north of Italy: a
minor one at the beginning of the summer (mid-June) and a major one
in August [FIGURE].
The results of the analyses indicate a strong association between daily
mortality and temperature; with peaks in mortality corresponding to
peaks in temperature or with a lag of 1-2 days [FIGURE].
The heat waves recorded between June and August 2003 are associated with
significant health effects; a total of 944 excess deaths were observed
in Rome (+19%), 577 (+33%) in Turin, 559 (+23%) in Milan and 175 (+14%)
in Bologna [TABLE 2].
In Rome, excess mortality was observed throughout the summer, but predominantly
during the three heat waves observed [8]. The first heat wave (9 June–2
July) was associated with an increase in mortality of 352 deaths; a
total of 319 excess deaths occurred during the second heat wave period
(10–30 July) and 180 excess deaths during the third (3–13
August) [FIGURE]. In the northern cities, although temperatures above
the reference mean were observed throughout the summer, excess mortality
was mainly concentrated in the first part of August, when weather conditions
became more extreme [FIGURE]. In Milan, 380 excess deaths were recorded
during the August heat wave (5-18 August), while in Turin, 257 excess
deaths were recorded during the heat wave from 3-14 August [FIGURE].
In Bologna, temperatures were less extreme throughout the summer and
heat wave periods were shorter, with less impact on mortality, and 62
excess deaths during the August heat wave (3-17 August).
When subdividing by age group, excess mortality increased dramatically
with age; the greatest impact observed in the old (75-84 years) and the
very old (85+ years) age groups. In the latter group, mortality increased
by 50% in Turin, 40% in Milan, 38% in Rome, and 33% in Bologna [TABLE
2]. When stratifying by age group, there is probably some residual confounding
related to sex, in that there is a larger proportion of females in the
older age groups. In fact, when stratifying by sex, the increase in mortality
seems to be greater among females, [TABLE 2] suggesting a possible higher
susceptibility.
The analyses of cause-specific mortality illustrated how the greatest
excess in mortality was observed for central nervous system, circulatory,
and respiratory diseases and metabolic/endocrine and psychological illnesses
[TABLE 3].
In Rome, the most significant excess was registered for diseases of the
central nervous system (+86%) and respiratory diseases (+38%). When
subdividing by age group, the excess, for both causes, was greatest
in the old (+123%, +52%) and the very old (+100% and +45%) age groups.
In Turin, a statistically significant increase in mortality was observed
for metabolic/endocrine disorders (+145%), diseases of the central
nervous system 124%) and respiratory diseases (+57%). Cardiovascular
disease registered the greatest excess in Turin (+41%) [TABLE 3]. In
Milan, the most significant excess in mortality was associated with
metabolic/endocrine disorders (+68%), respiratory diseases (+82%) and
disorders of the central nervous system (+118%).
Analysis by socioeconomic level illustrates the greatest excess among
the lower levels in both Turin (+43% increase in deaths of those with
a low level of education) and Rome (+17.8% increase in deaths of those
with low socioeconomic level), suggesting that this could be an important
risk factor.
Discussion
The unusual heat waves of summer 2003 had a strong impact on
the population in terms of mortality, especially in the north west,
where peak temperatures reached record values. Daily mortality trends
and peaks in mortality showed a temporal variation associated with
temperature trends [9,11]. Furthermore, prolonged periods of high temperatures
may have a stronger effect on health compared with periods with extreme
peak values but a lower mean. During summer 2003, the persistent high
temperatures were a strong determinant of the increase in observed
mortality.
This study gives a valid insight into the effects of heat waves on health
in medical terms, confirming previous results of increases in heat-related
mortality by respiratory and cardiovascular diseases [5,13,14] and showing
that extreme heat can worsen the conditions of people suffering from
chronic disease. The most interesting result that arises form this study
is the increase in deaths caused by diseases of the central nervous system
in all cities which include different illnesses associated with the elderly
(e.g., Parkinson’s disease, Alzheimer’s disease) [13] and
other illnesses which require constant medication which may enhance the
susceptibility of these subjects [15].
These results may be an important tool for identifying susceptible populations,
and developing effective warning systems and prevention programmes. In
Italy, as in other countries, the possible effects of global warming
could make susceptible subgroups more vulnerable [2,3] and together with
the increasing proportion of elderly people, may enhance heat-related
mortality. It is important to recall the heterogeneous nature of the
health impact of heat waves in terms of characteristics, such as the intensity
and temporal variation in relation to the meteorological conditions between
the different cities. Demographic and social factors, as well as the
level of urbanisation, air pollution and the efficiency of social services
and healthcare units, represent important local modifiers of the impact
of heat waves on health. Results from 2003 highlight the necessity of
implementing further preventive actions targeting the groups of susceptible
people involved (over 75+, especially females) as well as deprived urban
areas and low income populations.
Concerning the latency between the peak in temperature and the increase
in the mortality, the data showed that peaks in mortality were observed
1-2 days following the heat wave. These results are consistent with results
of previous time series studies that reported temperature lags at 0-3
days as having the maximum effect on mortality, and demonstrate that
heat related-mortality is a very acute event requiring timely intervention.
Some methodological aspects need to be discussed. Firstly, a limited
time window of three months was used as a more complete time series was
not available. The choice of reference period is a controversial topic
throughout the literature, as by using different reference periods, different
estimates of excess mortality are produced. Summer 2003 mortality was
compared with that of a reference period which was selected to be long
enough to account for the variability of the exposure variable and of
the observed effect, and, on the other hand, not too long, in order to
account for long-term variation of mortality due to variations in the
denominator and of mortality rates. However, the limited time window
analysis did not permit an evaluation of a possible harvesting effect
(displacement of mortality), but lower excess mortality during the third
heat wave period in Rome, for example, could be attributed to a reduction
in the susceptible population, as observed in other cities [10].
During the summer months, many Italian cities are affected by seasonal
migration, and populations in urban areas are reduced (e.g., Milan, Rome)
[12]. It is important to note that the migratory pattern differs from
year to year, depending on when heat waves occur, and may be unequally
distributed among the population. Susceptible groups, such as the elderly
and ill people with lower socioeconomic status, often remain in the city,
creating a bias in predicted excess death. The high number of excess
deaths in these subgroups might reflect the higher proportion of elderly
people of low socioeconomic status who remain in the city during the
summer. Several socioeconomic factors might have an impact on health,
including poor housing quality, lack of air conditioning, lack of access
to health and social services, and individual behaviours (e.g., alcohol
consumption and taking medication).
The evaluation of the heat waves of 2003 emphasise the importance of introducing
further preventive measures, both for the general population and for
susceptible groups, to reduce heat-related deaths during summer. Heat
stress conditions may be predictable, and appropriate prevention measures
may reduce heat-related mortality. This is achievable if efficient and
effective warning systems are introduced to alert residents in urban
areas to the oppressive weather conditions. In 2004, the Italian Department
for Civil Protection implemented a national programme for the evaluation
and prevention of the health effects of heat waves during summer. A Heat/Health
Watch/Warning System (HHWWS) [6,16,17] and city-specific prevention programmes
were activated during summer 2004 in Bologna, Milan, Rome and Turin,
while in four other cities (Brescia, Genoa, Palermo and Florence) warning
systems were run experimentally for the first time. Warning systems and
prevention programmes will be extended to other cities in summer 2005
as part of the national plan.
The implementation of warning systems and prevention programmes at both
the national and local level and the monitoring and surveillance of mortality
during heat waves may represent a valid tool for the reduction of heat-related
deaths. Furthermore, the national plan includes the identification of
susceptible subgroups, such as the elderly aged 75+ and people with specific
illnesses who are at higher risk during heat waves. Health guidelines
developed by the Ministry of Health have been put in place for the implementation
of appropriate prevention programmes. On the basis of the data collected
during summer 2004, it will be possible to assess and compare the performance
of intervention programmes implemented in each city and to evaluate the
reduction of heat-related deaths.
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