This paper summarizes the epidemiological evidence for adverse health effects associated
with disinfection by-products (DBPs) in drinking water and describes the potential
mechanism of action.

There appears to be good epidemiological evidence for a relationship between exposure
to DBPs, as measured by trihalomethanes (THMs), in drinking water and bladder
cancer, but the evidence for other cancers including colorectal cancer is inconclusive and
inconsistent. There appears to be some evidence for an association between exposure to
DBPs, specifically THMs, and little for gestational age/intrauterine growth retardation
and, to a lesser extent, pre-term delivery, but evidence for relationships with other
outcomes such as low birth weight, stillbirth, congenital anomalies and semen quality
is inconclusive and inconsistent. Major limitations in exposure assessment, small sample
sizes and potential biases may account for the inconclusive and inconsistent results in
epidemiological studies. Moreover, most studies have focused on total THMs as the
exposure metric, whereas other DBPs appear to be more toxic than the THMs, albeit
generally occurring at lower levels in the water.

The mechanisms through which DBPs may cause adverse health effects including
cancer and adverse reproductive effects have not been well investigated. Several
mechanisms have been suggested, including genotoxicity, oxidative stress, disruption
of folate metabolism, disruption of the synthesis and/or secretion of placental
syncytiotrophoblast-derived chorionic gonadotropin and lowering of testosterone levels,
but further work is required in this area.