Page, L., Kesheshian, C., Leonardi, G., et al. (2010). Frequency and Predictors of Mass Psychogenic Illness. (Abstract only.) Epidemiology. 21(5): 744-747.
The authors reviewed a random sample of 280 chemical incidents and found that 19 “were probable episodes of mass psychogenic illness.” They shared that odor was a strong predictor of this phenomenon and suggest that “a substantial minority” of chemical incidents may be a result of mass psychogenic illness.