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Hot weather and suicide; A real risk..or a statistical illusion?
- Emad Salib, Mario Cortina-Borja and Dan Anderson (7 September 2007)
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Authors' Reply
- Lisa A Page, Shakoor Hajat and R. Sari Kovats (25 September 2007)
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Emad Salib, Consultant Psychiatrist Peasley Cross Hospital, St Helens, Mario Cortina-Borja and Dan Anderson
Send letter to journal:
esalib{at}hotmail.com Emad Salib, et al.
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We read with interest the paper by Page et al (2007) reporting an association between increased risk of suicide and hot weather. We believe it is important that such finding is compared with similar associations reported in other countries and under similar conditions, particularly for countries with hotter climates but also for those moving through a period of climatic change. We are a little disappointed that despite the authors’ excellent statistical analyses and effective display of results, they determined the shape of their natural cubic splines “visually”, instead of using some model selection criterion, e.g. likelihood ratio tests, AIC..etc, . The authors also stated that Yip et al (2000) “failed to show any significant seasonality with recent U.K. data”. This may not be entirely accurate as we believe that what Yip et al (2000) did actually show is that there is a decreasing seasonal pattern, but it certainly has not vanished, e.g. “the assumption of even distribution of suicides was rejected (p<0.01); December had a trough for both males and females” and again “the effects of all seasonal harmonics are marginally statistically significant, with p ~ 0.05” The ‘unexpected’ reduction in suicide during the heat wave of 2003 (Page et al 2007) is difficult to explain on the basis on temperature alone particularly as there was 13.5% - 33% increase in general mortality during 2003 heat wave as reported by one of the authors in another study (Kovats et al 2006). Based on previous literature, it is clear that the association with high temperature is not suicide-specific. In an earlier study, Hajat et al (2002) reported an almost identical increase in all cause mortality of 3.34% (95% CI 2.47% to 4.23%) for every one degree increase in average temperature above 18 C, compared to 3.8% (no confidence interval provided) reported increase in suicide in their recent study (Page et al 2007). This raises the possibility of an unaccounted confounder linking suicide, total mortality and daily mean temperature above 18 C. Such factors include climatic, as well as non- climatic factors, whether acting independently or as interaction terms, for example number of sunshine hours (Salib & Gray 1997), relative humidity, rainfall, unusual weather conditions, stress in the hypothalamic -pituitary-adrenal axis or even changes in the solar wind as measured by satellites (Richardson et al 1994). Chronomics of suicides (Halberg et al 2005) which do not rely on calendar year but on periodicity of solar wind (Richardson 1994) may provide a plausible and alternative explanation to the study findings. Perhaps the only conclusion that can be made reading Page et al (2007) paper is that high temperature may be associated with increased mortality due all causes. Given the uncanny similarity in the rate of increase in all-cause mortality and that of suicide, the mechanism of effect of high temperature on suicide should not be expected to differ from any other cause of death. Although high daily mean temperature may increase suicide risk, this is not an independent risk factor and may not have the degree of influence upon public health policy in relation to global warming that the authors indicated. References: Halberg F, Cornélissen G, Panksepp J, Otsuka K, Johnson D. (2005) Chronomics of autism and suicide. Biomedicine & Pharmacotherapy 2005; 59 (Suppl 1): S100-S108. Hajat S. Kovats RS, Atkinson RW., Hains A (2002), Impact of hot temperatures on death in London: a time series approach J Epidemiol Community Health 56(5):367-72. Kovats RS, Johnson H., Griffith C (2006) Mortality in southern England during 2003 heat wave by place of death. Health Stat Q 29, 6-8 Page L., Hajat S., Kovats S (2007) Relationship between daily suicide counts and temperature in England & Wales British Journal of Psychiatry 191, 106-112 Richardson JD, Paularena KI, Belcher J et al (1994) Solar wind oscillation with 1.3 pear period Geographical research letters 21, 1559- 1560 Salib E, Gray N (1997) Weather conditions and fatal self harm in North Cheshire 1989-1993 British Journal of Psychiatry 171, 473-477 Yip P, Chao A. Chiu C (2000) Seasonal variations in suicide; diminished or vanished. Experience from England & Wales 1982-1996. British Journal of Psychiatry 177, 366-369 Conflict of interest: none Emad Salib: Consultant Psychiatrist, Peasley Cross Hospital, St Helens WA9 3DA esalib@hotmail.com Mario Cortina-Borja Senior Lecturer in Statistics, Institute of Child Health, UCL Daniel Anderson Specialist Registrar in Old Age Psychiatry Peasley Cross Hospital, St Helens |
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Lisa A Page, Research Fellow Kings College London, Institute of Psychiatry, Shakoor Hajat and R. Sari Kovats
Send letter to journal:
l.page{at}iop.kcl.ac.uk Lisa A Page, et al.
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We thank Salib et al for their interest and agree our findings need to be replicated in other populations and climates (Page, Hajat & Kovats 2007). Salib et al may have misinterpreted the results of our analysis of suicides during the 2003 heat wave, as our finding of -1.8% (95%CI -17.8%, +18.4%) change in suicides from expected is consistent with no change rather than a reduction. We discuss reasons for this lack of effect in the paper, and point out that the lack of power in this calculation leads to an imprecise estimate. We disagree with Salib et al’s assertion that the effect of high temperature on all-cause mortality (rather than suicide specifically) is a reasonable explanation for our findings. We only examined deaths from suicide and undetermined intent, so it is not possible for other causes of death to have ‘confounded’ our results. We considered carefully which confounders to include in our models. Individual-level confounders, for example the effect of individual stress on the hypothalamic-pituitary- adrenal axis, are irrelevant in a time-series analysis as they do not vary day to day across a population. Sunshine hours were sufficiently accounted for by including a term for hours of daylight. Regarding Salib et al’s list of other potential confounders, we think it unlikely that any of these could be sufficiently associated with both temperature and suicide to explain our findings. Also, humidity, rainfall and unusual weather conditions (e.g. thunderstorms) tend to vary regionally more than temperature, meaning that exposure mis-classification would be a problem in a country-wide analysis. The role of solar winds in the aetiology of suicide is highly speculative. Higher temperatures affect mortality through a range of mechanisms (Bouchama and Knochel 2002). Cardiovascular and respiratory deaths during periods of high temperature are caused by physiological changes including increased coagulation, dehydration and increased cardiovascular output – particularly important in the elderly or those with pre-existing disease. A range of anti-psychotic drugs are known to inhibit sweating and therefore thermoregulation. Recent work has shown that deaths from respiratory and external causes are particularly increased at high temperatures (Hajat, Kovats & Lachowycz 2007). Further research is needed in the pathophysiology of heat but it is clear that persons with mental illness remain a high risk group for heat wave mortality (Kovats and Ebi 2006). References Bouchama, A. and Knochel, J. (2002). “Heat stroke.” The New England Journal of Medicine 346(25):1978-1988. Hajat, S., Kovats R.S. and Lachowycz K. (2007) “Heat-related and cold -related deaths in England and Wales: who is at risk?” Occupational & Environmental Medicine 64: 93-100 Kovats, R.S. and Ebi, K.L. (2006). “Heatwaves and public health in Europe.” European Journal of Public Health 16: 592-599. Page, L. A., S. Hajat and Kovats, R.S. (2007). "Relationship between daily suicide counts and temperature in England and Wales." British Journal of Psychiatry 191: 106-112. Declaration of Interest Lisa Page is supported by the National Institute of Environmental Health Sciences (NIEHS), NIH as a Ruth L Kirschstein National Research Fellow (F32 ES013690). The contents of this paper are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS, NIH. Shakoor Hajat is supported by a Wellcome Trust Research Career Development Fellowship. R Sari Kovats is funded by the European Commission DG SANCO for the EuroHEAT project [agreement no. 2004322]. |
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