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                                  International Journal of Environmental
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                                  Difficulties of defining the term, “heat
                                  wave”, in public health
                                                       a                    b                    c
                                  Juan Carlos Montero , Isidro Juan Miron , Juan José Criado ,
                                                  d               e
                                  Cristina Linares & Julio Díaz
                                  a
                                   Health Sciences Institute, Castile-La Mancha Regional Health &
                                  Social Welfare Authority , Talavera de la Reina, Toledo , Spain
                                  b
                                   Torrijos Health District, Castile-La Mancha Regional Health &
                                  Social Welfare Authority , Torrijos , Spain
                                  c
                                   Castile-La Mancha Health Service (Servicio de Salud de Castilla-
                                  La Mancha - SESCAM), Talavera de la Reina , Toledo , Spain
                                  d
                                   Centro Nacional de Epidemiologia, Area de epidemiología
                                  ambiental y cáncer , Madrid , 28029 , Spain
                                  e
                                   Instituto de Salud Carlos III. Escuela Nacional de Sanidad, Sinesio
                                  Delgado 8 , Madrid , 28029 , Spain
                                  Published online: 16 Oct 2012.

To cite this article: Juan Carlos Montero , Isidro Juan Miron , Juan José Criado , Cristina Linares
& Julio Díaz (2013) Difficulties of defining the term, “heat wave”, in public health, International
Journal of Environmental Health Research, 23:5, 377-379

To link to this article: http://dx.doi.org/10.1080/09603123.2012.733941

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International Journal of Environmental Health Research, 2013
                                                                  Vol. 23, No. 5, 377–379, http://dx.doi.org/10.1080/09603123.2012.733941

                                                                  TECHNICAL NOTES
                                                                  Difficulties of defining the term, ‘‘heat wave’’, in public health
                                                                  Juan Carlos Monteroa, Isidro Juan Mironb, Juan José Criadoc, Cristina Linaresd
                                                                  and Julio Dı́aze*
                                                                  a
                                                                   Health Sciences Institute, Castile-La Mancha Regional Health & Social Welfare Authority,
                                                                  Talavera de la Reina (Toledo), Spain; bTorrijos Health District, Castile-La Mancha Regional
                                                                  Health & Social Welfare Authority, Torrijos, Spain; cCastile-La Mancha Health Service
                                                                  (Servicio de Salud de Castilla-La Mancha - SESCAM), Talavera de la Reina (Toledo), Spain;
                                                                  d
                                                                   Centro Nacional de Epidemiologia, Area de epidemiologı´a ambiental y cáncer, Madrid 28029,
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                                                                  Spain; eInstituto de Salud Carlos III. Escuela Nacional de Sanidad, Sinesio Delgado 8, Madrid
                                                                  28029, Spain
                                                                  (Received 16 July 2012; final version received 22 July 2012)

                                                                  There can be no question that in any area of science it is important to establish
                                                                  methodologies and criteria which enable universally comparable and applicable
                                                                  results to be obtained. In this regard, while the effort made by the EuroHEAT
                                                                  project to furnish a single definition of ‘‘heat wave’’ for comparison of this
                                                                  phenomenon’s impact on different European cities is doubtless praiseworthy
                                                                  (D’Ippoliti et al. 2010), it would also be true to say that an effort of this nature is
                                                                  confronted by series of limitations which must be considered.
                                                                      In the first place, the use of any given parameter that synthesizes diverse variables
                                                                  in a single mathematical algorithm is only advisable if its efficacy has previously been
                                                                  tested under all possible scenarios. Reliance on apparent temperature implies
                                                                  assuming that, during any heat wave, there is a positive quadratic relationship
                                                                  between relative humidity (as measured by a wet-bulb temperature) and mortality,
                                                                  namely, that an increase in humidity is always associated with an increase in
                                                                  mortality. Nevertheless, a number of papers report exceptions to this: in the USA,
                                                                  for instance, the above relationship varies among the different cities analyzed (Braga
                                                                  et al. 2002); and in Spain, studies which have examined humidity independently show
                                                                  that the highest mortality is associated with low humidities (Dı́az et al. 2002;
                                                                  Montero et al. 2012). Failure to take into account the direction of the association
                                                                  may give rise to erroneous interpretation of results. There can be no doubt that the
                                                                  extent to which air temperature influences the definition of apparent temperature
                                                                  makes for this strong association between apparent temperature and mortality,
                                                                  regardless of any possible impact of relative humidity.
                                                                      Furthermore, to define heat waves for the purpose of implementing health
                                                                  actions, using a parameter based solely on the climatological conditions of one city
                                                                  or region means that a public health decision would be made without any population

                                                                  *Corresponding author. Email: j.diaz@isciii.es

                                                                  Ó 2012 Taylor & Francis
378
                                                                  2     J.C. Montero et al.

                                                                  health indicator being taken into account. Most experts recommend using a
                                                                  definition based on ‘‘a robust understanding of the cause-and-effect relationships
                                                                  between the thermal environment and health outcomes at the population level’’
                                                                  (Kovats and Ebi 2006).
                                                                      It must be borne in mind that heat waves occur at lower temperatures in colder
                                                                  places (Curriero et al. 2002), and hence it is recommended to use the percentile of the
                                                                  temperature series rather than absolute temperatures to define a thermal extreme.
                                                                  This feature must doubtless be considered when interpreting the higher mortality in
                                                                  Europe’s northernmost countries during the 2003 heat wave (D’Ippoliti et al. 2010).
                                                                  Based on this, the 90th percentile of the monthly distribution could be assumed to be
                                                                  the point at which the health effects of high temperatures were always triggered, and
                                                                  the requirements described in the preceding paragraph for defining a heat wave
                                                                  would thus be met.
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                                                                      The relationship between health and temperature is not immutable, however. On
                                                                  the contrary, it is regulated by a complex number of economic, social, cultural, and
                                                                  health variables (Basu 2009). Several studies which have analyzed the trend in the
                                                                  association between mortality and temperature, in sufficiently long series in the USA
                                                                  (Davis et al. 2003), UK (Carson et al. 2006) and Spain (Mirón et al. 2010), have
                                                                  detected that temperature evolves differently, adapting itself to the changes in the
                                                                  characteristics of each society. There is one variable that has special importance in
                                                                  the trend in this relationship, i.e. the ageing index. One recent study in Spain
                                                                  reported that it is in populations that have a larger elderly segment where an increase
                                                                  in mortality at lower temperatures is detected (Montero et al. 2012). In such a case, it
                                                                  would seem that, from a health standpoint, it could be said that the greater the
                                                                  ageing of the population, the lower the temperature at which the effects of a heat
                                                                  wave appear.
                                                                      In our opinion, these results amount to a change of paradigm when it comes to
                                                                  defining a heat wave. Such a definition cannot be a fixed one: not only is it impossible
                                                                  to establish a temperature that would serve for all latitudes, but it is also impossible
                                                                  to fix a percentile of the temperature series to which a population would be exposed,
                                                                  since the temperature–mortality relationship varies over time. In contrast, efforts – in
                                                                  the form of consistent epidemiological studies – should be directed at searching for
                                                                  the threshold temperature for defining a heat wave, i.e. for defining, by reference to
                                                                  the target area, from which temperature the effects of heat on the health variables
                                                                  considered are to be deemed unacceptable.
                                                                      Needless to say, a threshold temperature determined in this way will be a
                                                                  consequence of the social, economic, and demographic characteristics of a specific
                                                                  society. One would thus be sure that implementation of prevention plans based on
                                                                  the surpassing of these types of thresholds would result in a reduction of the impact
                                                                  of high temperatures on the health of the population.

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                                                                  Basu R. 2009. High ambient temperature and mortality: a review of epidemiologic studies
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                                                                  Braga AI, Zanobetti A, Schwartz J. 2002. The effect of weather on respiratory and
                                                                     cardiovascular deaths in 12 U.S. cities. Environ Health Perspect. 110:859–863.
                                                                  Carson C, Hajat Sh, Armstrong B, Wilkinson P. 2006. Declining vulnerability to temperature-
                                                                     related mortality in London over the 20th century. Am J Epidemiol. 164:77–84.
                                                                  Curriero FC, Heiner KS, Samet JM, Zeger SL, Strug L, Patz JA. 2002. Temperature and
                                                                     mortality in 11 cities of the Eastern United States. Am J Epidemiol. 155:80–87.
International Journal of Environmental Health Research            3793

                                                                  Davis RE, Knappenberger PC, Novicoff WM, Michaels PJ. 2003. Decadal changes in summer
                                                                      mortality in US cities. Int J Biometeorol. 47:166–175.
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