What proportion of chemical leaks provoke mass hysteria?

Mass hysteria and not leaked chemicals was the likely cause of the symptoms experienced by those exposed in 16 per cent of hundreds of chemical leaks recorded in England and Wales between January 2007 and April 2008.

That's according to an analysis by Lisa Page and colleagues at the Institute of Psychiatry of 280 chemical leaks recorded by the Centre for Radiation, Chemical and Environmental Hazards based at Chilton in Oxfordshire.

Otherwise known as 'mass psychogenic illness', mass hysteria is the occurrence of physical symptoms such as dizziness and nausea in more than one person, with no identifiable organic cause.

Page's team presented expert toxicologists and medics with vignettes of the incidents (plus further information where necessary) and had them rate the possibility that the documented symptoms, when present, of those exposed could have been caused by the chemicals involved. Among the incidents were a spillage of phosphoric and hydrochloric acid outside a domestic residence, and the opening of a container from South East Asia at a distribution centre (further examples).

In total, the experts' verdict was that 19 of the incidents involved physical symptoms that were most likely caused not by the suspected leak but by mass psychogenic illness - that equates to 7 per cent of all incidents analysed and 16 per cent of those in which physical symptoms were reported.

Incidents at schools and hospitals and those involving reports of an odour were more likely to trigger mass psychogenic illness. By contrast, factors related to emergency response such as the presence of police or paramedics were not relevant.

This is the first ever attempt to provide a formal estimate of the prevalence of mass psychogenic illness within a given context. 'Our findings suggest that mass psychogenic illness is an important differential diagnosis in a substantial minority of chemical incidents,' the researchers concluded.

'The importance of early diagnosis rests in the considerable difference in management [of mass psychogenic illness] compared with other chemical incidents,' the researchers added. 'Mass psychogenic illness is best managed by reassurance, separating symptomatic from non-symptomatic psychogenic persons, minimising unnecessary medical procedures and providing a credible explanation for symptoms. In contrast, casualties from mass toxic incidents may require decontamination, antidotes, and invasive medical care.'
_________________________________

Page, L., Keshishian, C., Leonardi, G., Murray, V., Rubin, G., & Wessely, S. (2010). Frequency and Predictors of Mass Psychogenic Illness. Epidemiology DOI: 10.1097/EDE.0b013e3181e9edc4

Link to Psychologist magazine feature article on dancing plagues and mass hysteria.