More people commit suicide during heatwaves, a study has found.
The researchers assessed the relationship between daily temperature and daily suicide counts in England and Wales between January 1993 and December 2003. There were 53,623 suicides during this period, an average of 13.3 per day.
Their results published in the August issue of the British Journal of Psychiatry did not show that suicides went up during the spring and summer, but that for every degree above 18 degrees C, there was a 3.8%rise in all suicides and a 5.0% rise in violent suicide.
Suicide increased by 46.9% during the 1995 heat wave, although there was no change in suicide rates during the 2003 heat wave. This surprising finding might be explained by the fact that there were two periods of very hot weather in 2003 in mid-July and in August, the authors said.
Dr Lisa Page of the Department of Psychological Medicine, Institute of Psychiatry, London, suggested: "The earlier period of hot weather between 13 July and 17 July 2003 might have resulted in some excess deaths in those most susceptible to heat-related suicide, while also allowing some physiological or behavioural adaptation among other vulnerable individuals. The later heatwave in August 2003 may therefore have resulted in fewer deaths than would otherwise have occurred."
The researchers said that although the majority of European studies have shown a spring or summer peak in suicide, recent UK studies had not. One study investigating seasonality and suicide in 28 countries found that those with high levels of industrialisation and low numbers involved in agricultural work showed the least seasonality.
Deaths from all causes are known to increase as temperatures rise above 19 degrees C and this is the first to show that death from suicide is contributing to that effect.
Temperatures above 18 degrees C were recorded on 222 days over the 11-year study period. Real additional suicides, rather than suicides being 'brought forward', probably occur when temperatures are high, the researchers said.
Dr Page said: "Suicide is an important, preventable cause of premature mortality. This study has shown an effect of high temperatures on suicide counts that will probably become more important as global warming continues. It is possible that the population of England and Wales will adjust to higher ambient temperatures, although the speed of global warming may be too great for adaptation to occur."