A hypothetical study such as this useful but can be very alarming. Keep in mind that actually contaminating a distribution system is much easier said than done. The threat is considered low because the contaminant will be diluted during hyrdaulic transport through the piping system and may not reach the tap. Factors such as chlorine residual disinfectant in the distribution system will inactivate microbial pathogens. Shower exposures are important not only here but also in the case of other drinking water contaminants such as radon.
Jack Schijven, Jean Marie Forêt, Jurgen Chardon, Peter Teunis, Martijn Bouwknegt, Ben Tangena. Evaluation of exposure scenarios on intentional microbiological contamination in a drinking water distribution network. Water Research, Volume 96, 1 June 2016, Pages 148-154.
Drinking water distribution networks are vulnerable to accidental or intentional contamination events. The objective of this study was to investigate the effects of seeding duration and concentration, exposure pathway (ingestion via drinking of water and tooth brushing and inhalation by taking a shower) and pathogen infectivity on exposure and infection risk in the case of an intentional pathogenic contamination in a drinking water distribution network. Seeding of a pathogen for 10 min and 120 min, and subsequent spreading through a drinking water distribution network were simulated. For exposure via drinking, actual data on drinking events and volumes were used. Ingestion of a small volume of water by tooth brushing twice a day by every person in the network was assumed. Inhalation of contaminated aerosol droplets took place when taking a shower. Infection risks were estimated for pathogens with low (r = 0.0001) and high (r = 0.1) infectivity. In the served population (48 000 persons) and within 24 h, about 1400 persons were exposed to the pathogen by ingestion of water in the 10-min seeding scenario and about 3400 persons in the 120-min scenario. The numbers of exposed persons via tooth brushing were about the same as via drinking of water. Showering caused (inhalation) exposure in about 450 persons in the 10-min scenario and about 1500 in the 120-min scenario. Regardless of pathogen infectivity, if the seeding concentration is 106 pathogens per litre or more, infection risks are close to one. Exposure by taking a shower is of relevance if the pathogen is highly infectious via inhalation. A longer duration of the seeding of a pathogen increases the probability of exposure.