The Worcester water incident, UK: bias in self reported symptoms to an emergency helpline.

In April 1994, chemical contamination of the River Severn aVected the water supply to a population of 160 000 in Worcestershire. During the incident the health authority free phone emergency helpline received 106 complaints of symptoms attributed to consumption of contaminated water. These complaints were important as they alerted the emergency incident team to the possibility of illness being associated with the chemical contamination, gave an early indication of the possible severity of illness, and also provisionally identified a register of cases. However, identification of cases from those presenting to medical services or prompted from media interest during an infectious disease or chemical incident may result in a biased selection of the most severely aVected or anxious. 3 Bias associated with self reported symptoms to an emergency helpline has not been previously reported. To investigate this possibility, we undertook a retrospective cohort study of complaints of symptoms to the helpline.