Threats to Human Health: Use of Chlorine, an Obsolete Treatment Technology - Global Drinking Water Management and Conservation - page 193

Table 9.2 Individual-based studies related to chlorination byproduct exposure (Cantor 1997 )
Author (Refer-
ence) Year

Cancer sites (years
of diagnosis)

Number
of cases

Study location Exposure

timing

Wilkins and Com-
stock ( 1981 ).

Bladder, liver, kid-
ney 1963 - 1975

81,45,31 Western Mary-

land (USA)

Years in
1963
domicile

Cantor et al.
( 1987 )

Bladder 1978 2,805 10 locations in

USA

Lifetime

Freedman et al.
( 1997 )

Bladder 1975 1992 294 Western Mary-

land (USA)

Years in
1975
domicile

-

McGeehin et al.
( 1993 )

Bladder 1988 1989 327 Colorado

(USA)

Age 20 to
interview

-

Cantor ( 1997 ) Bladder 1986 - 1989 1,452 Iowa (USA) Lifetime

1994 927 Ontario
(Canada)
Note In all studies, individual histories of water source and chlorine disinfection were developed by
combining residential information from the questionnaire with historical data from water utilities

Marrett et al.
( 1996 )

Bladder 1992

-

“

Iowa, Colorado, and Ontario (see Table 9.2 ). He pointed out that the evidence for

carcinogenicity of chlorination byproducts is strongest for bladder cancer, where
associations were found overall or in major subgroups in five case-control studies

and one population cohort study (Cantor 1997 ).

Villanueva et al. ( 2003 ) carried out a meta-analysis of the best available epidemi-
ological evidence on chlorinated drinking water and bladder cancer. This meta-
analysis included six case studies (including one from Ontario) and two cohort
(panel data) studies. We begin by restating the results of one of the panel data
studies, that of Wilkins and Comstock 1981 . They found that for both sexes the
probability of getting bladder cancer from drinking chlorinated water was 70 per-
cent higher than from drinking deep well waters

”

that is what an odds ratio of 1.7
means. The breakdown by gender is shown in Table 9.3 .
The results of the meta-analysis (summarized in Table 9.4 ) show that the odds
ratio increases with exposure to chlorinated drinking water, from 1.13 (or the
probability is 13 percent higher due to chlorinated water) after 20 years to 1.43 after
60 years, i.e. the probability of bladder cancer is 43 percent higher after 60 years of
exposure to chlorinated drinking water.

—

Table 9.3 Odds Ratios from
Wilkins and Comstock ( 1981 )

Deep well
users

Chlorinated surface water
users

Men 1.0 1.8 (CI: 0.8 4.75)

-

Women 1.0 1.60 (CI: 0.54 - 6.32)

Both
sexes

1.0 1.7 (CI: 0.8 3.5)

-

Odds Ratios (OR) and 95 percent Confidence Intervals
(95 percent CI)