Health Stream Literature Summary - Issue 50 - June 2008

Is Colon Cancer Mortality Related to Arsenic Exposure?
Yang, C.-Y., Chang, C.-C., Ho, S.-C. and Chiu, H.-F. (2008) Journal of Toxicology & Environmental Health, Part A, 71(8); 533-38.

Endemic blackfoot disease (BFD), a peripheral vascular disease, has been found to be associated with the consumption of drinking water from artesian wells containing high levels of arsenic along the southwestern coast of Taiwan. Long-term ingestion of high levels of arsenic has also been associated with cancers of skin and various internal organs including bladder, kidney, lung, and liver. There has been less attention given to the association between long-term exposure to arsenic and colon cancer. In the early 1960s a tap water supply system was implemented in the BFD-endemic areas and between 1966 and 1975 all of these areas were almost completely supplied with municipal water. Artesian well water was therefore no longer used for drinking and cooking after the mid-1970s. This study was undertaken to examine whether mortality rates attributed to colon cancer had decreased amongst residents living in the BFD-endemic areas after arsenic exposure ceased.

The study area included four townships located on the southwest coast of Taiwan where BFD was endemic. Residents from the 84 villages in the study area had consumed high arsenic artesian well water since 1910. A study in the early 1960s found an arsenic concentration of artesian well water in the study area ranging from 0.35 to 1.14 ppm with a median of 0.78 ppm. Use of artesian well water for drinking and cooking ceased in the mid-1970s. The current tap-water supply for the study area comes from the Tzeng-Wen reservoir where arsenic concentration of water is less than 0.01 ppm (the U.S. EPA standard limit for arsenic in drinking water).

Information on the number of colon cancer deaths and the mid-year population by gender, age and calendar years during the years 1971-2006 was obtained from the Bureau of Vital Statistics of the Taiwan Provincial Department of Health. Standardised mortality ratios (SMRs) for colon cancer were calculated, which is the ratio of the number of expected colon cancer deaths occurring in the study area to the number expected given the standard rates in each gender and age group.

Between 1971 and 2006 there were 365 deaths attributed to colon cancer in the BFD-endemic areas of which 189 were males and 176 were females. Time trends of SMR for male colon cancer showed decreasing mortality rates. The estimated slope for male SMR (rate of decrease per year) in the linear time trend analysis was -1.48 (p=0.02). A linear time trend analysis was performed for female SMR and the estimated slope was -0.54 (p=0.36). No significant association with the slope of the time trend was detected for females.

This study showed that mortality attributed to male colon cancer gradually declined after consumption of artesian well water containing high amounts of arsenic ceased. The finding suggests that the association between arsenic exposure and colon cancer is likely to be causal for males but not females on the basis of the reversibility criterion. Gender differences in the metabolism of arsenic have been observed, and these may explain the results found here. However given that this was an ecological study, a chance association cannot be ruled out.

Comment Colon cancer is more common in affluent countries and known risk factors include poor diet (high fat, low fibre, low vegetable intake), obesity and lack of physical activity. The pattern for female colon cancer mortality shown in the graphs in this paper is perhaps even suggestive of an increasing trend since the mid-1980s.