Many water treatment systems use chlorine to disinfect drinking water. However, chlorine
reacts with dissolved organic matter in water to create trihalomethanes (THMs), which
have been associated with excess risk of bladder cancer in people who drink chlorinated
water. Now, in a large study conducted in Spain, an international team of scientists
has examined the role of tap water and total fluid intake in bladder cancer risk,
while also assessing the effect of exposure to THMs in water [EHP 115:1569–1572; Michaud
et al.]. The results suggest that higher water consumption is associated with lower
risk of bladder cancer, regardless of THM exposure.
Some studies have linked high consumption of fluids including tap water with a lower
risk of bladder cancer, perhaps because urinating more frequently allows more flushing
of the bladder. Other studies suggest that high tap water consumption could increase
bladder cancer risk if chlorination by-products or other water contaminants such as
arsenic are elevated in the water source. Adding to this complexity is that still
other studies have shown a THM-related excess risk of bladder cancer in men but not
women.
Between June 1998 and June 2001, researchers conducted a hospital-based case–control
study of bladder cancer in multiple centers in Spain. Male and female bladder cancer
patients aged 20–80 years were recruited from 18 participating hospitals. For the
397 bladder cancer cases available for this analysis, the team recruited 664 matched
controls who had been admitted to the same hospitals around the same time for hernias,
fractures, orthopedic problems, and other reasons.
Trained interviewers collected information during each subject’s hospitalization that
included sociodemographic characteristics, family history of cancer, smoking history,
occupational history, residential history, drinking water source at each residence,
beverage consumption (including water), and medical history. The researchers used
local government and water company data on annual average THM levels, water source
history since 1920, and chlorination history to calculate average year-by-year THM
exposure. These data were available for 78.5% of the total study person-years.
The researchers examined the association between total fluid and water consumption
and bladder cancer risk, while also examining the interaction between water intake
and THM exposure. The results suggest that drinking more water, even from chlorinated
sources with high THM levels, is beneficial in reducing risk of bladder cancer. The
authors found a 53% lower risk of bladder cancer in people who drank 1,400 mL or more
water per day compared with those who drank less than 400 mL per day after adjusting
for known and potential confounders. This inverse association held across all strata
of smoking status and THM exposure, and for both men and women.
The study was strengthened by high response rates from cases and controls, detailed
interview data on individual beverage consumption, detailed assessment of THM exposure,
and detailed data on smoking, which is believed to be the greatest risk factor for
bladder cancer.