This study intends to calculate the lifetime
cancer risks resulting from intakes of
trihalomethanes (THMs) in
drinking water based on the presence of each THM species. The slope factors for each THM species are used, combined with exposure model and Monte Carlo simulations, to calculate the
cancer risks with consideration of different exposure routes (oral ingestion, inhalation and dermal absorption). The results revealed that the highest risk comes from the inhalation exposure to
chloroform during showers, which also dominates the total risk associated with
chloroform exposure. For
dichlorobromomethane and
chlorodibromomethane, inhalation exposure also plays an important role for total risks; however, contribution from the oral consumption cannot be ignored for these two compounds.
Bromoform contributes the least
cancer risk among the four THM species, with a risk factor two orders of magnitude smaller than the other three THM species. For all of the four THM species, exposure from dermal absorption is not significant when compared with oral ingestion and inhalation exposures. This study also uses the THMs data collected from Taiwan to calculate the
cancer risks associated with THM exposures in different areas of Taiwan. Due to the variations of the THMs compositions, it is observed that higher concentrations of total THMs do not necessarily lead to higher
cancer risks. Areas with higher
bromide concentration in raw water and often with higher total THM concentration may actually give lower
cancer risk if the THMs formed shift to
bromoform. However, this also leads to the violation of THM standards since
bromoform has much higher molecular weight than
chloroform. Based on the results of the
cancer risks calculated from each THM species, the regulatory issue of the THMs was also discussed.