Disinfection by-products have been regulated in finished drinking waters in the United States since 1979. The original regulations dealt only with trihalomethanes, but more recent rules also address haloacetic acids. Compliance with these regulations has traditionally been based on a running annual average of quarterly measurements of disinfection by-products in the distribution system and on concerns about adverse health effects associated with chronic exposure to these by-products. More recently, concerns have also been focused on short-term exposure and acute reproductive and developmental adverse health effects. Additionally, while compliance has previously been based on disinfection by-product levels in samples collected primarily at locations with average distribution system residence times, forthcoming regulations will base compliance on disinfection by-product levels in samples collected at locations with maximum trihalomethane and haloacetic acid concentrations. Moreover, instead of the maximum contaminant levels applying to the annual average of all distribution system samples, the new rules will require that the annual average of the regulated disinfection by-product concentrations at each monitoring location be less than their respective maximum contaminant levels. This paper reviews the evolution of disinfection by-product regulations in the United States and on the rationale behind these regulations.

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