American Journal of Epidemiology
Copyright O 1999 by The Johns Hopkins University School of Hygiene and Public Health
ADrightsreserved
Vol. 150, No. 5
Printed In USA.
Mutagenic Drinking Water and Risk of Male Esophageal Cancer: A
Population-based Case-Control Study
Xuguang Tao,1-2 Huigang Zhu,1 and Genevieve M. Matanoski2
Drinking mutagenic downstream water from the Huangpu River was hypothesized to have increased the risk
for male esophageal cancer in Shanghai, China. The authors conducted a population-based case-control study
of a total of 71 esophageal cancer deaths and 1,122 controls collected during a 5-year follow-up period,
1984-1988, from four male cohorts born before January 1, 1944, living in four communities consuming water
with different mutagenicities in the Shanghai area. The controls represented a 1% random sample of the defined
living cohorts selected at the end of each of the 5 years of follow-up. Logistic regression showed an odds ratio
of 2.77 (95% confidence interval: 1.52, 5.03) for drinking mutagenic downstream water from the river versus
drinking nonmutagenic upstream water after controlling for possible confounders including age, disease history
(hepatitis, cirrhosis, schistosomiasis, digestive tract ulcer), hazardous occupational history, pesticide exposure,
lifestyle factors (cigarette smoking, tea intake, and alcohol intake), dietary habits (intake of pickled vegetables,
maize, peanuts, and cured meat), education, poverty, urban environment, and water chlorination. Am J
Epidemiol 1999; 150:443-52.
case-control studies; chlorine compounds; esophageal neoplasms; mutagenicity tests; pesticides; risk; smoking;
water pollution
Drinking mutagenic downstream water from the
Huangpu River was hypothesized to have increased
the risk for male esophageal cancer in Shanghai,
China. Shanghai has one of the busiest harbors and is
one of the largest industrial and commercial cities in
China. The total metropolitan area is 6,100 km2 with
about 12 million people. It is composed of a centralized city area of 12 districts and 10 counties at its outskirts. Until 1987, about 98 percent of the drinking
water consumed in the city was taken from the
Huangpu River, and 90 percent of it was from the
downstream part of the river, where the water was
heavily polluted. This river is 88 km in length, with an
average runoff of 300 m3 per second. In 1987, all water
intakes at the downstream section were moved to
Linjiang at the midstream part of the river to improve
water quality (1).
In Shanghai, chlorination, in conjunction with coagulation, sedimentation, and filtration, has historically
been used against bacteria contamination for tap water.
Concentrations of halogenated hydrocarbons, such as
trihalomethanes and other chlorination by-products,
were apparently elevated because of the action of chlorine on the high levels of natural organic substances
present in the untreated, downstream water of the river.
A study showed that the average chloroform concentration, a useful indicator for the presence of halogenated hydrocarbons, in the chlorinated downstream
water was 45.6 jig/liter, over 40 times higher than the
value of 1.1 (Xg/liter found in upstream water (2). Prior
to the relocation of the water supply in 1987, Shanghai
water supplies were also tested for mutagenicity, as
both tap water and source water, using the Ames
Salmonella typhimurium/mammalian
microsome
mutagenicity test on strains TA100 and TA98 with and
without enzymatic activation. The test was considered
positive if the number of reversions per liter of water
was two or more times the blank and if the standard
error was less than 20 percent of the mean (3).
Between 1983 and 1985, chlorinated tap water samples from a downstream water plant, the largest tap
water plant in Shanghai, were 100 percent positive.
The raw water samples taken from the same river section were 96.9 percent positive. However, both raw
and chlorinated tap water samples taken from an
upstream water plant were negative (4). The results of
micronucleus tests on peripheral erythrocytes of cru-
Received for publication June 26, 1998, and accepted for publication January 22, 1999.
Abbreviation: Cl, confidence interval.
1
Department of Environmental Health, School of Public Health,
Shanghai Medical University, Shanghai, China.
2
Department of Epidemiology, School of Hygiene and Public
Hearth, The Johns Hopkins University, Baltimore, MD.
Reprint requests to Dr. Xuguang Tao, Department of
Epidemiology, The Johns Hopkins School of Hygiene and Public
Hearth, 111 Market Place, Suite 850, Baltimore, MD 21202-6709.
443
444 Tao et al.
cian carp caught along the river and unscheduled DNA
synthesis tests on water samples from different sections of the river also showed that downstream water
samples had much stronger mutagenicity than did
upstream samples (5, 6).
The carcinogenic effects on humans from drinking
mutagenic downstream water from the Huangpu River
in the early years are unclear. Previous studies showed
that drinking the mutagenic downstream water from
the Huangpu River had a positive association with the
risk for stomach and liver cancers in Shanghai males.
The odds ratio was 2.02 (95 percent confidence interval (CI): 1.27, 3.21) for male stomach cancer and, for
liver cancer, the odds ratio was 1.85 (95 percent CI:
1.45, 2.36) controlling for possible confounders (7, 8).
Polluted water had been reported to be related to the
risk for esophageal cancer elsewhere in China and
other countries (9-13). Less attention has been paid to
the possible association between drinking the polluted
water of Huangpu River and esophageal cancer in
Shanghai, probably because of the declining incidence
of this cancer (14, 15). This trend is different from
those reported in many other countries where
esophageal cancer rates have changed little or actually
increased (16-23). A recent population-based casecontrol study identified cigarette smoking and heavy
alcohol intake as major determinants of esophageal
cancer risk, accounting for nearly half of all the cancer
cases among men in urban Shanghai (24, 25). Other
risk factors that might account for the other half of
esophageal cases are still unclear.
The purpose of this study was to explore the possible association between drinking mutagenic downstream water from the Huangpu River and male
esophageal cancer deaths after controlling for possible
confounders.
MATERIALS AND METHODS
Definition of cohorts
A stratified random sampling method was used in
the selection of study communities. To control for the
effects of urbanization-related factors such as chlorinated tap water, air pollution, and population density,
the community selection was stratified by urban or
rural locations. In Shanghai, all urban areas used chlorinated tap water, and most of the rural areas used raw
water before 1980. The sampling goal was to identify
the following four communities:
• two urban communities
• used nonmutagenic, upstream, chlorinated tap
water (community A)
• used mutagenic, downstream, chlorinated tap
water (community B)
• two rural communities
• used nonmutagenic, upstream, raw water (community C)
• used mutagenic, downstream, raw water (community D).
The following criteria were used in selecting the
communities. 1) The total population base was big
enough to have at least 100,000 person-years' observation (20,000 persons for 5 years) of males aged 40
years or over, with the expectation of collecting
enough cases to have about 80 percent power to detect
an odds ratio of 2 or above. Each of the communities
should have about 5,000 males aged 40 years or over
at the beginning of the follow-up. 2) The distance
between the two communities in each stratum (communities A and B or communities C and D) should be
far enough (more than 30 miles or 48 km) to reduce the
possibility of subject exchange and to have big differences in the mutagenicity of their drinking water. 3)
All four communities should have used their defined
water sources for at least 40 years before the start of
follow-up, and the two rural communities should have
used raw water; those communities that changed their
water source to tap water no more than 5 years before
the beginning of follow-up were to be included, however, because many rural communities changed thenwater source to tap water in the late 1970s and early
1980s. 4) It was decided a priori that community A
should be selected from the urban area that used chlorinated tap water from the water plant that located its
water intake at the far end of the Huangpu River
upstream and had negative Ames test results for all of
its water samples during 1983-1985. It was further
decided that community B should be selected from the
urban area that used chlorinated tap water from the
water plant that located its water Intake at the far end
of the river downstream and had 100 percent positive
Ames test results for all of its water samples collected
during the same period. 5) All communities should be
in historically stable residential areas with few newly
developed blocks and in nonindustriallzed regions.
Based on the above criteria, four groups of communities were listed as candidates. Each of the four study
communities was randomly selected from the group of
candidate communities that met the above criteria.
Figure 1 shows the location of the four residential
communities selected. Communities A and B were
located in urban areas, and communities C and D were
located in rural areas. All males born before January 1,
1944, and living in the four communities on January 1,
1984, comprised the study cohorts and were followed
from January 1,1984, to December 31,1988. The cohort
definition was based on data collected from the
Residence Registration and Administration agencies of
Am J Epidemiol
Vol. 150, No. 5, 1999
Mutagenic Water and Esophageal Cancer
445
5 years of follow-up. Table 1 shows detailed information
on cohort size, cases, and controls collected during the 5
years of follow-up on each cohort. Controls were not
required to match cases with any characteristics.
Jlangsu
Provinc*
Data collection
Yangtze Rlw
/
t
Suzhou Rlwr
Shanghai
Huangpu Rlvw
East China Saa
Zhoflang
Province
FIGURE 1. Locations of the four residential communities selected
for the case-control study of mutagenic drinking water and male
esophageal cancer, Shanghai, 1984-1988. A, urban community that
used nonmutagenic, upstream, chlorinated tap water; B, urban community that used mutagenic, downstream, chlorinated tap water C,
rural community that used nonmutagenic, upstream, raw water; D,
rural community that used mutagenic, downstream, raw water. Solid
black line, area boundary.
the Public Security Bureau. By law, residents of all ages
are registered at the local Residence Registration and
Administration agencies at birth or upon entering a community as a resident The registration of a resident in a
community is canceled immediately upon that resident's
moving to a new community or at death. Demographic
data of the populations in the communities are available
from the local agencies at the end of each year. The
mutagenicity data of the drinking water were based on
the results of Ames tests on water samples collected
from 1983 to 1985. The percentages of positive tests for
the water sample from communities A, B, C, and D during 1983-1985 were 0, 100, 0, and 96.9 percent (4).
Cases and controls
During the 5-year follow-up period of 1984-1988, all
71 esophageal cancer deaths in the study cohort were
identified from death certificates collected from the local
Department of Vital Statistics, with a total of 112,176
observed person-years from four male cohorts born
before 1944. To get a representative sample of the cohort
population, we selected a 1 percent random sample of
the total living cohort (n = 1,122) excluding cancer
cases, stratified by community, at the end of each of the
Am J Epidemiol Vol. 150, No. 5, 1999
Water supply system data were obtained from the
local Environmental Protection Agency and the
Shanghai Water Supply Company. The type of drinking water for all subjects was determined according to
their residential addresses. Water sources for each
address were verified by the Shanghai Water Supply
Company. Demographic data, necessary for sampling
controls, were collected from the Vital Statistics
Department of the Anti-epidemic Station and from the
Residence Registration and Administration agencies of
the Public Security Bureau. Personal exposure data
were collected by questionnaire at home visits. Data
for the 71 cases were collected and verified by interviewing the immediate adult family member (spouse,
parent, son, daughter, brother, or sister) or other relative who had lived with the case. For the 1,122 controls, individual exposure data were collected directly
from the subject or from the immediate adult relative
who had lived with the subject, if the subject was not
available at the time of the home visit. The interview
included questions on birth date, duration of residence
in the current community, medical history, occupational history, lifestyle factors, dietary habits, education, and the food expense budget.
Data analysis
Crude incidences of male esophageal cancer death
in the four studied cohorts by type of drinking water
were calculated from the number of cases and the
observed person-years among the defined male
cohorts. Incidence ratios and 95 percent confidence
intervals were calculated for downstream cohorts versus upstream cohorts by tap water or raw water (26).
Stratified by urban tap water or rural raw water, a
Mantel-Haenszel incidence ratio (27) for drinking
mutagenic downstream water versus nonmutagenic
upstream water from the Huangpu River was also calculated. An unconditional logistic regression was used
to estimate the odds ratio and confidence interval for
drinking mutagenic water against male esophageal
cancer, controlling for possible confounders including
age, medical history (hepatitis, cirrhosis, schistosomiasis, and digestive tract ulcer), occupational history
(with emphasis on selected hazardous occupations),
pesticide exposure, lifestyle factors (cigarette smoking, tea intake, and alcohol intake), dietary habits
(intake of pickled vegetables, maize, peanuts, and
446 Tao et al.
TABLE 1. Male esophageal cancer cases and controls collected during 5 years of follow-up In four
cohorts born before January 1,1944, with different drinking water sources, Shanghai, 1984-1988
Year
Two urban cohorts
Cohort A (upstream tap water)
Cohort size
No. of controls
No. of cases
Cohort B (downstream tap water)
Cohort size
No. of controls
No. of cases
Two rural cohorts
Cohort C (upstream raw water)
Cohort size
No. of controls
No. of cases
Cohort D (downstream raw water)
Cohort size
No. of controls
No. of cases
Total
Cohort size
No. of controls
No. of cases
1984
1985
1986
1987
1988
Total
6,413
64
4
6.386
64
2
6,291
63
6,250
63
2
6,167
62
4
31,507
316
16
6,531
65
5
6,491
65
6
6,451
65
4
6,406
64
6
6,349
64
3
32,228
323
24
5,901
59
0
5,842
58
4
5,811
58
3
5,744
57
1
5,621
56
4
28,919
288
12
4,178
42
5
4,038
40
5
3,918
39
6
3,724
37
2
3,664
37
1
19,522
195
19
23,023
230
14
22,757
227
17
22,471
225
17
22,124
221
11
21,801
219
12
112,176
1,122
71
cured meat), education, and monthly food expenses.
Subjects with hazardous occupations were defined as
those ever occupationally exposed to radiation (alpha,
beta, gamma, X-ray), arsenic and its compounds,
asbestos, ethylene chloride, chromium and its compounds, cadmium and its compounds, chloromethyl
ether, formaldehyde, nickel and its compounds, coal
tar, asphalt, and acrylonitrile. Because of the small
number in each occupational subset, a combined indicator, ever exposed to any of the above hazardous
occupations, was used in the analysis. Likelihood
ratio tests and a backward method were used to finalize the regression model (28). The overall goal was to
obtain the best fitting model while minimizing the
number of parameters. As the first step, all independent variables were included in the initial model. The
next step was to fit a reduced model containing only
those variables found to be significant (p < 0.05) and
to compare it with the full model containing all the
variables. The likelihood ratio test comparing these
two models was obtained using a G statistic. G was
defined as the difference of a -2 log likelihood
between the two models.
RESULTS
The distribution of duration of residence in the
selected communities for cases and controls is shown in
4
table 2. All subjects were at least 40 years old at the
time of the study by cohort definition; 91.5 percent of
the cases and 93.5 percent of the controls had been living in their current communities for 20 years or more.
Cases tended to have longer times of residence than
controls, since cases on average were older than controls, even though controls on average had 6 months'
more follow-up time than cases, which was the result of
selecting controls at the end of each year of follow-up.
The crude incidences of male esophageal cancer
death in four cohorts for the 5 years of follow-up are
TABLE 2. Years of living In the current community for male
esophageal cancer cases and controls, selected from four
cohorts born before January 1,1944, Shanghai, 1984-1988
Years of Uvtng
Cases
Controls
No.
%
No.
£70
0
6
10
7
15
7
13
13
0.0
8.5
14.1
9.9
21.1
9.9
18.3
18.3
14
59
117
195
340
201
121
75
1.2
5.3
10.4
17.4
30.3
17.9
10.8
6.7
Total
71
100.0
1,122
100.0
community
<10
10-19
20-29
30-39
40-49
50-59
60-69
%
Am J Epidemiol Vol. 150, No. 5, 1999
Mutagenic Water and Esophageal Cancer
shown in table 3. The crude incidences for the two
downstream cohorts are higher than those for the two
upstream cohorts. Crude incidence ratios for drinking
downstream water versus upstream water are 1.47 (95
percent CI: 0.78, 2.76) for rural chlorinated tap water
drinking cohorts and 2.35 (95 percent CI: 1.14, 4.83)
for rural raw water drinking cohorts, without controlling for confounding factors. Stratified by urban tap
water or rural raw water, the Mantel-Haenszel incidence ratio for drinking mutagenic downstream water
versus nonmutagenic upstream water from the
Huangpu River is 1.80 (95 percent CI: 1.12, 2.88).
Because the distributions of some characteristics of
the four cohorts were found to be dramatically different, the above crude incidence ratios may not represent the real risk. The distributions of possible risk or
confounding factors in the four cohorts are shown in
table 4. Age distributions are well balanced in the four
cohorts. The distributions of hepatitis, liver cirrhosis,
and digestive duct ulcer histories show no significant
differences either. However, the two cohorts using tap
water in urban areas have slightly higher rates of
heavy smokers (20 or more cigarettes a day) and
higher rates of heavy alcohol drinkers (one and more
times a week) than the two cohorts drinking raw
water in rural areas. The two upstream cohorts had
significantly higher schistosomiasis prevalence rates
than did the two downstream cohorts, because the
area upstream of the river was a schistosomiasis epidemic area. The two upstream cohorts also had higher
rates of smokers and tea drinkers but a lower rate of
alcohol users. In addition, the two rural cohorts had
higher rates of pickled vegetable intake, cured meat
intake, pesticide exposure, poverty, and poor education. Cohort C, the upstream rural cohort, was very
special, having the highest rates of schistosomiasis
prevalence, pesticide exposure, and poor education
and the lowest rate of hazardous occupation. The
upstream cohort, using chlorinated tap water from
upstream, had significantly higher intake rates of
maize products and peanut products. Some of these
factors were possible confounders, since they were
associated with exposures in this study and are
reported to have associations with esophageal cancer
in previous studies. However, it is important to point
out that all of these rates are group-based ecologic
data; to connect them to incidence data directly may
lead to ecologic fallacy. Individual-based multivariate
analysis is needed to adjust for these potential confounders.
The odds ratios and confidence intervals of the
independent variables in the initial and final logistic
regression models are shown in table 2. In the first
model that included all the independent variables, no
significant associations were found between the risk
of esophageal cancer death and the history of hepatitis, cirrhosis, schistosomiasis, and digestive tract
ulcer; intakes of alcohol, tea, peanut products, and
cured meat; and a monthly food expense less than the
median of controls (p > 0.05). After removing these
10 nonsignificant variables, we established the final
model using procedures described in Materials and
Methods. The difference of a - 2 log likelihood
between the initial model and the final model was
12.378, which was much smaller than 18.307, the
chi-square value at a = 0.05 and df = 10. This means
that, after exclusion of those nonsignificant variables,
the final model fit the data as well as the initial
model. The result shows that, adjusted for the other
independent variables in the model, the odds ratio for
drinking mutagenic downstream river water compared with drinking nonmutagenic upstream river
TABLE 3. Incidences and Incidence ratios of male esophageal cancer In four cohorts born before
January 1,1944, wtth different mutageniclty of drinking water sources, Shanghai, 1984-1988
Cohort
Two urban cohorts
Cohort A (upstream tap water)
Cohort B (downstream tap water)
Two rural cohorts
Cohort C (upstream raw water)
Cohort D (downstream raw water)
Total
Drinking
water
Ames test
positive,
19831985
0
100
0
96.9
Personyears
(no.)
Incidence
(1/100,000
personyears)
Incidence
ratio
24
31,507
32,228
50.78
74.47
1.00
1.47* (0.78, 2.76)t
12
19
28,919
19,522
41.50
97.33
1.00
2.35* (1.14,4.83)
112,176
63.29
1.80* (1.12,2.88)
Cases
(no.)
16
* Crude incidence ratio.
t Numbers in parentheses, 95% confidence interval.
% Mantel-Haenszel incidence ratio stratified by water type, that is, tap water or raw water.
Am J Epidemiol
Vol. 150, No. 5, 1999
447
448
Tao et al.
TABLE 4. Characteristics among population-based controls from four male cohorts bom before January 1,1944 (Shanghai,
1984-1988)
Rural raw water
Urban tap water
Characteristics
Cohort A
(upstream)
CohortB
(downstream)
CohortC
(upstream)
CohortD
(downstream)
Total
Ch t-square
p value
Numerical distributior l(no.)
Person-years
No. of controls
31,507
316
32,228
323
28,919
288
19,522
195
112,176
1,122
Percentage distribution (%)
Age (years)
40M9
50-59
60-69
270
History of disease
Hepatitis
Liver cirrhosis
Schistosomiasis
Digestive tract ulcer
Smoking (cigarette(s)/day)
1-4
5-19
220
Tea intake 21 cup/day*
Alcohol intake 21 time/week
Pickled vegetables intake 23 months/year
Maize products intake frequently
Peanut products intake
<1 time/week
21 time/week
Cured meat intake 23 months/year
Hazardous occupationst
Ever occupatjonally exposed to pesticide
Money for food/month < control median
Education < primary school graduate
37.4
28.8
19.9
13.9
32.8
26.0
21.4
19.8
34.7
29.9
19.1
16.3
36.9
28.7
19.0
15.4
35.2
28.3
20.0
16.5
0.73
7.0
0.9
16.5
11.1
7.4
0.6
0.0
13.3
4.5
1.4
25.0
10.8
6.2
0.5
2.6
6.2
6.3
0.9
11.5
10.8
0.48
0.70
0.00
0.09
15.8
32.7
21.1
65.5
22.8
1.9
83.5
10.2
30.1
26.6
45.2
34.1
4.6
32.3
22.9
43.4
18.4
52.1
21.9
47.2
24.7
11.8
39.0
25.1
27.7
30.3
48.2
27.1
15.3
35.8
22.7
49.6
27.1
22.4
43.9
0.00
0.00
0.00
0.00
0.00
89.6
4.7
3.8
8.2
3.5
45.9
44.3
67.5
9.3
2.8
10.8
3.1
42.4
44.8
75.3
2.8
8.0
0.7
69.8
56.6
88.2
50.8
2.6
5.1
8.2
12.3
59.5
49.7
72.8
5.2
4.8
7.0
21.9
50.0
55.8
0.00
0.02
0.00
0.00
0.00
0.00
• One cup is approximately 240 ml.
t Including occupations exposed to radiation (alpha, beta, gamma, X-ray), arsenic and its compounds, asbestos, ethylene chloride,
chromium and its compounds, cadmium and its compounds, chloromethyl ether, formaldehyde, nickel and its compounds, coal tar, asphalt,
and acrylonitrile.
water was 2.77 (95 percent CI: 1.52, 5.03). The odds
ratios for other risk factors or confounders having a
significant positive association (p < 0.05) with
esophageal cancer death were 2.63 (95 percent CI:
1.20, 5.78) for drinking chlorinated tap water in
urban areas, 4.50 (95 percent CI: 2.09, 9.69) for age
50-59 years, 5.91 (95 percent CI: 2.61, 13.36) forage
70 years and older, 2.53 (95 percent CI: 1.16, 5.50)
for smoking 5-19 cigarettes per day, 3.25 (95 percent
CI: 1.47, 7.18) for smoking 20 cigarettes or more per
day, 3.19 (95 percent CI: 1.54, 6.59) for pickled vegetable intake 3 months or more a year, 2.02 (95 percent CI: 1.15, 3.56) for maize products intake 3
months or more a year, 2.47 (95 percent CI: 1.01,
6.01) for hazardous occupations, 3.64 (95 percent CI:
1.75, 7.57) for pesticide exposure, and 5.19 (95 percent CI: 2.17, 12.38) for education lower than primary school graduation.
DISCUSSION
Esophageal cancer remains a deadly disease with a
poor prognosis in China. The worldwide variation in
incidence suggests that environmental exposures contribute to the development of esophageal cancer. This
study found that males from communities drinking
heavily polluted, highly mutagenic, downstream water
from the Huangpu River had higher incidences of
esophageal cancer than did males from communities
drinking relatively clean, nonmutagenic, upstream
water. This difference did not disappear after controlling for possible confounding factors such as age, disease history, occupational history, lifestyle factors,
dietary habits, education, water chlorination, and
urban location. This suggests that drinking mutagenic
water appears to be a risk factor for the disease. This
study is consistent with many previous studies, which
Am J Epidemiol
Vol. 150, No. 5, 1999
Mutagenic Water and Esophageal Cancer
showed that possible risk factors of esophageal cancer
are age, smoking, alcohol, pickled vegetables, corn
and wheat products, pesticides, and drinking polluted
water (9-12, 24, 25, 29-37). However, the risk from
alcohol intake was significant in this study (odds ratio =
1.54, 95 percent CI: 0.86, 2.76). The frequency indicator used for recording alcohol intake in the investigation might not be as sensitive as a quality indicator.
This may partly explain why alcohol intake was elevated but not significant in the study. However, the
subjects who used alcohol more frequently would most
likely have consumed more alcohol. If poor data on
alcohol led to residual confounding, it probably was
449
not very great. Another issue related to alcohol intake
was that the two downstream cohorts had higher alcohol intake rates than did the two upstream cohorts,
which suggested the possibility of correlation between
alcohol and water mutagenicity. Including or excluding the alcohol intake and some other variables in the
model (table 5), however, did not change the odds ratio
of drinking mutagenic water very much. The test result
of -2 log likelihood showed that excluding those variables did not cause significant change of the goodnessof-fit of the model either. Thus, although the risk estimate for alcohol intake may not be accurate, for
controlling purposes, it may not bias the water muta-
TABLE 5. Odds ratios for independent variables In the first and the final unconditional multlvarlate logistic regression models
based on 71 male esophageal cancer cases and 1,122 controls, Shanghai, 1984-1988
Variables
Drinking water mutagenicity, mutagenic
vs. nonmutagenic
Drinking water type, tap water vs. raw
water
Age (years)
50-59 vs. 40-49
60-69 vs. 45-49
£70 vs. 40-49
History of disease
Hepatitis (yes vs. no)
LJver cirrhosis (yes vs. no)
Schlstosomiasis (yes vs. no)
Digestive tract ulcer (yes vs. no)
Smoking (clgarette(s)/day)
1-4 vs. <1
5-19 vs. <1
£20 vs. <1
Tea intake (cups/day), £1 vs. <1$
Alcohol intake (times/week), £1 vs. <1
Pickled vegetables intake (months/year),
£3 vs. <3
Maize products intake (months/year),
£3 vs. <3
Peanut products intake
<1 time/week vs. no
£1 time/week vs. no
Cured meat intake (months/year), £3
vs. <3
Hazardous occupation,§ ever vs. never
Exposure to pesticide, ever vs. never
Monthly food expenses < control median,
yes vs. no
Education < primary school graduate,
yes vs. no
Case/control
nos. in two groups*
Odds ratio
Initial model
- 2 ln(Bke)ihood) = 413.713
Final model
- 2 In(likelihood) = 426.091
43/518 vs. 28/604
2.70 (1.38, 5.28)t
2.77(1.52,5.03)
40/639 vs. 31/483
2.64(1.17,5.99)
2.63(1.20,5.78)
11/317 vs. 3/396
32/224 vs. 3/396
25/185 vs. 3/396
0.91 (0.42, 1.96)
4.99 (2.29, 10.88)
6.77(2.95, 15.51)
0.93 (0.44, 1.98)
4.50 (2.09, 9.69)
5.91 (2.61, 13.36)
5/71
1/10
7/129
7/121
vs. 66/1,051
vs. 70/1,112
vs. 64/993
vs. 64/1,001
2.46 (0.85, 7.07)
2.77(0.29,26.13)
0.98 (0.37, 2.56)
0.80(0.32, 1.97)
vs.
vs.
vs.
vs.
vs.
11/294
11/294
11/294
35/565
42/818
1.55(0.51,4.68)
2.51 (1.11,5.68)
3.20(1.35,7.59)
0.70(0.39, 1.25)
1.54(0.86,2.76)
1.42 (0.48, 4.20)
2.53(1.16,5.50)
3.25(1.47,7.18)
27/251 vs. 44/871
3.11 (1.46,6.63)
3.19(1.54,6.59)
31/492 vs. 40/630
1.81 (1.00,3.28)
2.02(1.15,3.56)
7/58 vs. 10/247
54/817 vs. 10/247
1.92(0.88,4.18)
1.56 (0.49, 4.94)
6/54 vs. 65/1,068
8/79 vs. 63/1,043
22/246 vs. 49/876
2.63 (0.94, 7.32)
2.64(1.07, 6.55)
3.99(1.89, 8.43)
43/561 vs. 28/561
1.29(0.74,2.24)
64/626 vs. 7/496
5.51 (2.28, 13.31)
6/172
29/401
25/255
36/557
29/304
2.47(1.01,6.01)
3.64(1.75,7.57)
5.19(2.17, 12.38)
* This column gives the distribution of case and control numbers in two comparison groups. However, the odds ratios and confidence intervals in the table are the results of multivariate logistic regression and thus do not match the crude odds ratio and confidence interval calculated based on the numbers from this column.
t Numbers in parentheses, 95% confidence interval.
t One cup is approximately 240 ml.
§ Including occupations exposed to radiation (alpha, beta, gamma, X-ray), arsenic and its compounds, asbestos, ethylene chloride,
chromium and its compounds, cadmium and its compounds, chloromethyl ether, formaldehyde, nickel and its compounds, coal tar, asphalt,
and acrylonitrile.
Am J Epidemiol Vol. 150, No. 5, 1999
450
Tao et al.
genicity analysis result. Another interesting thing in
this study is that drinking chlorinated tap water in
urban areas was found to be significantly associated
with the risk for esophageal cancer in multivariate
analysis (odds ratio = 2.63, 95 percent CI: 1.20, 5.78).
This finding was inconsistent with the univariate incidence comparison outcomes, in which the difference
between urban and rural cohorts was very small. One
of the important reasons might be the skewed distribution of other confounders. For instance, the two rural
cohorts had much higher rates of pickled vegetable
intake and pesticide exposure than did the two urban
cohorts. These two factors are significant risk factors
in the study (table 5). So the existing higher risk attributed to urban chlorinated water than to rural raw water
found in multivariate analysis was masked by the high
rates of pickled vegetable intake and pesticide exposure that occurred in rural cohorts when doing a univariate comparison. It should be pointed out that drinking chlorinated tap water could not be separated from
urban residence, since all chlorinated tap water users
in the study lived in urban communities. The risk
found among chlorinated tap water drinkers is attributed to both chlorinated tap water and other urban factors, such as population density and air pollution. The
possible association of chlorinated drinking water with
cancer has been widely investigated. Although one
study showed an odds ratio of 2.12 (95 percent CI:
1.10, 4.08) associated with drinking chlorinated tap
water and the risk of esophageal cancer, other studies
did not show any significant risk, with odds ratios
ranging from 0.97 to 1.76 (13, 38-42).
In this study, a new control sampling method, stepwise population-based proportional sampling, was
used to collect controls. Usually there are three control
sampling strategies in nested case-control studies (43,
44). The first strategy is that, at the end of follow-up,
controls are selected from subjects who do not develop
the disease of interest (cumulative incidence sample)
or are alive and disease free (cumulative survival sample). The problem with this strategy is that a biased
estimation of risk could result from competing causes
of death and withdrawal, especially when the followup period is long and the mortality for other diseases is
high. The second strategy is that, at the beginning of
follow-up, controls are selected from subjects who are
alive and disease free (case base sample). The problem
with this strategy is that biased underestimation could
result from the failure to exclude subjects who later
become cases or die from other diseases. The third
strategy is that, during the follow-up period, controls
are selected from subjects who are alive and disease
free at the time the case is diagnosed (density sampling). This method is considered to yield an unbiased
estimate of risk. It is easy to conduct with matched
controls but difficult for population-based proportional
sampling. The control sampling strategy used in the
present study was that controls were selected in a stepwise fashion from persons who were alive and cancer
free at the end of each subperiod (1 year) during the
total 5-year follow-up period. There were three advantages to this method. 1) It could reduce the influence of
competing causes of death and withdrawal, since in the
new method the subperiod is short although the entire
follow-up period is long. 2) It resembles density sampling, since cases and controls are collected relatively
concurrently compared with sampling controls at the
beginning or the end of the 5-year follow-up. 3) It is
easy to draw population-based proportional control
samples.
Some limitations to this study should be considered.
This study was done only on male cohorts aged 40
years or more from only four communities, so the
result may not be truly representative of the risk for the
12 million persons in Shanghai. However, the study
can help to test the hypothesis that drinking mutagenic
downstream water from the Huangpu River could have
some association with male esophageal cancer. If the
hypothesis is correct, the information would be important to everyone who has consumed that mutagenic
water. Bias might exist when using deceased persons
as cases and living populations as controls.
Information about cases came from an adult firstdegree family member or relative who had lived with
the case, while information about controls was primarily self-reported. However, since some factors, such as
dietary intake, were usually similar among family
members, and since some personal behaviors such as
smoking were easily remembered by family members,
the response bias to these questions should be limited.
The water supply information was not collected
through the interview but from the water supply company, thus eliminating recall or family member bias. A
limiting issue is that drinking water sources were
determined according to residential addresses.
Misclassification might occur if a subject worked far
from his residential area and had a different quality of
drinking water there. Additionally, 8.5 percent of the
cases and 6.5 percent of the controls had lived less than
20 years in their current community or in neighboring
communities with similar water quality. This means
that their drinking water would be a mixture of mutagenic and nonmutagenic water but that it was classified in our study as either mutagenic or nonmutagenic
water. However, this type of misclassification could
only underestimate the risk rather than contribute to
the elevated risk found in the study. Drinking bottled
water or using home charcoal water cleaners has been
Am J Epidemiol
Vol. 150, No. 5, 1999
Mutagenic Water and Esophageal Cancer
popular since the mid 1980s in Shanghai. However,
considering the latency period of esophageal cancer,
the change should not affect the results of this study.
The variability of water mutagenicity within each
cohort was very small for the two urban tap water
cohorts, since subjects within each cohort had as their
water source the same water supply plant. More variability might exist among subjects in the two rural
cohorts. However, the variability within a rural cohort
should be much less than the difference between the
two rural cohorts, since they are located at two extreme
ends of the river.
In spite of these limitations, this study has identified
a significant association between drinking mutagenic
downstream water from the Huangpu River and the
risk for male esophageal cancer after controlling for
possible confounders. This finding, although not conclusive, may lead to a better understanding of the etiology of male esophageal cancer in that area and the
association of drinking water mutagenicity with
esophageal cancer.
ACKNOWLEDGMENTS
This work was supported by the Shanghai Environmental
Protection Agency, Shanghai, China, and was finished during the tenure of a Research Training Fellowship awarded to
Xuguang Tao (IARC/R.2170, 1993-1994) by the
International Agency of Research on Cancer, Lyon, France.
The authors thank Dr. S. Z. Yu and Dr. C. J. Hong for their
help in research design; Drs. Q. Y. Zhao, H. Jiang, J. R.
Wang, J. P. Bao, F. Y. Wang, Y. Lu, B. Q. Zhang, and J.
Cheng for help in data collection; G. D. Wu, X. F. You, W.
L. Zhai, C. Li, and Z. Q. Sun for their additional help in data
collection; and L. Schwartz, D. Lantry, and L. Swartz for
help in manuscript preparation.
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