(introduction...)

The health effects of ingesting arsenic-contaminated drinking-water appear slowly (Box 2). For this reason, a more important issue than the number of patients who currently have arsenic-caused diseases is the number who will develop these diseases in the future as a result of past and continuing exposure to arsenic. Large numbers of tube-wells were installed in Bangladesh approximately 5 to 20 years ago. If the population continues to drink arsenic-contaminated water, then a major increase in the number of cases of diseases caused by arsenic may be predicted.

Fig. 2. Children near a tube-well disconnected due to contamination of water with arsenic (WHO 00230)

Skin lesions

The latency for arsenic-caused skin lesions (i.e., the time from first exposure to manifestation of disease), in particular keratoses, is typically about 10 years (22). In the 1997 consultancy, it was found that the youngest individuals with skin lesions caused by arsenic were about 10 years old. Other studies have shown that skin lesions also occur in children younger than 10 years (23). It was also found that in adults, exposures commenced approximately 10 years before they stated the skin lesions began to appear. In some instances, the apparent latency for the appearance of skin lesions from the time of first exposure to contaminated water from the tube-well currently in use was much shorter, but as no measurements were available for water from previously used tube-wells, a short latency from first exposure could not be inferred. However, latency that is shorter or longer than 10 years may occur, and the rapidity of the appearance of skin lesions appears to be dose dependent (22). Further studies of the latency and patterns of occurrence of skin lesions are needed and these will require careful interviewing of participants about their current and past exposures.

Box 1. Magnitude of arsenic poisoning in Bangladesh

Box 2. Long-term health effects of exposure to arsenic

Cancer

Skin cancer. Small numbers of cases of skin cancer have started to appear. Since the typical latency is more than 20 years after the beginning of exposure, the fact that only a small number have been found provides little reassurance about the future incidence of skin cancer. A study of a large population in Taiwan found a clear dose-response relationship between arsenic concentrations in drinking-water and the prevalence of skin cancer (24). In this study, the average concentration of arsenic in water was about 500 µg/l, and by age 60 more than 1 in 10 had developed skin cancer. The lifetime risk of developing skin cancer from the intake of 1 µg · kg body weight ^-1 · day ^-1 (roughly equivalent to 1 litre per day at concentrations of 50 µg/l) of arsenic in water ranges from 1 per 1000 to 2 per 1000 (25). Though large numbers of skin cancers have been reported in Taiwan, the future burden of arsenic-caused skin cancer in Bangladesh is uncertain. Differences in susceptibility between the populations of Taiwan and Bangladesh may exist that only time and further study will identify. However, as yet there is no evidence to indicate that the long-term risks of skin cancer would be any lower in Bangladesh than in Taiwan.

Mortality from internal cancers. In other countries, the main causes of death associated with chronic ingestion of arsenic in drinking-water are internal cancers; skin cancers are not usually fatal if treated appropriately. Dramatic increases in mortality from internal cancers have been reported in Taiwan (26–28) and Chile (29). In Taiwan, populations exposed to high concentrations of arsenic in their drinking-water, containing an average of 800 µg/l of arsenic, had estimates of their relative risk of bladder cancer in the order of 30–60 (27, 30). In Region II of northern Chile, 5–10% of all deaths occurring among those over the age of 30 were attributable to arsenic-caused internal cancers, in particular bladder cancer and lung cancer (29). Average exposures were in the order of 500 µg/l (0.5 mg/l) over 10–20 years; exposure decreased in subsequent years after remediation efforts were introduced (29). Long latency was apparent, and increases in mortality continued for 40 years after the highest exposures began (29). In Argentina, a mortality study in the arsenic-exposed region of Cordoba found increased risks of bladder and lung cancer among men and women from 1986 to 1991, although concentrations were lower (average 178 µg/l) than in Taiwan and Chile (31, 32).

Using the current US Environmental Protection Agency standard of 50 µg/l, it has been estimated that the lifetime risk of dying from cancer of the liver, lung, kidney or bladder while drinking 1 litre a day of water containing arsenic at this concentration could be as high as 13 per 1000 persons exposed (30). Using the same methods, the risk estimate for 500 µg/l of arsenic in drinking-water would be 13 per 100 people (33). In its latest document on arsenic in drinking-water, the US National Research Council concluded that exposure to 50 µg/l could easily result in a combined cancer risk of 1 in 100 (34).

Although specific estimates of the current and future health effects of arsenic exposure are uncertain, in the case of Bangladesh it can be inferred that since there are many people who currently have skin lesions caused by ingesting arsenic, many more cases will occur if exposure continues; based on what is known about the relationship between ingestion and the development of internal cancers, it is reasonable to expect marked increases in mortality from internal cancers once sufficient latency has been reached. It is also reasonable to expect marked increases in the incidence of the other health effects listed in Box 2.